Stretching, Jumping, and the Stretch Shortening Cycle

Stretching has become a routine part of almost all athletes’ pre-training warm up to improve performance and decrease the risk of injury (Perrier et al., 2011; Young and Behm, 2003). The believed benefits of stretching are thought to be achieved by increasing muscle temperature, increasing neural activation, and reducing musculotendinous stiffness (Young and Behm, 2003). Most warm-ups will include a low-intensity aerobic component, stretching of relevant muscles, and some rehearsal of the activities that will be performed. While the use of a warm-up prior to maximum effort explosive exercise (sprinting, jumping etc.) is rarely questioned, the protocol leading to optimum performance remains a topic of debate. 

Most research indicates that a comprehensive warm-up should include both general and specific preparation. The general warm-up should increase core and muscle temperature and range of motion. The specific portion of a warm-up should reinforce the movement and motor patterns of the activity that follows, as explosive and dynamic movements require high levels of neuromuscular activation and optimal musculotendinous stiffness (Sotiropoulos et al., 2010). The stretch-shortening cycle (SSC) is the “pre-stretch” or countermovement that can be seen in movements like walking, running, and jumping. This pre-stretch allows the athlete to produce more force move quicker.

Therefore, it may be beneficial to include more dynamic movements in the general and specific warm-up to prepare the neuromuscular system for the intense activity utilizing the SSC rather than static stretching which is less transferrable to the requirements of the explosive movements in the activity that will follow.  

Static Stretching

Why You Should Do Stretches Every Day (And The Right Way To Do It)

Static stretching has been shown to increase acute range of motion at the joint being stretched, however research has shown it decreases the strength and power outputs of these muscles (Simic et al., 2013). Further, stretching alone has not been shown to reduce injury risk (Shrier, 2001). Because it seems that static stretching decreases maximal force production, jump height, and sprint speed, while increasing reaction time and impairing balance (Perrier et al., 2013), there has been a shift away from pre-training static stretching in favor of a more functional dynamic warm-up. 

The exact mechanism that leads to reduced performance after static stretching remains unclear, but it is likely due to both mechanical and neural factors. After stretching, contractile units must contract more rapidly over a greater distance, likely increasing the time needed to produce a given amount of force. Neurologically, static stretching seems to decrease motor unit activation as a result of inhibitory mechanisms involved with lengthening the muscle point (Perrier et al., 2013). 

Dynamic Stretching

Static vs. Dynamic Stretching | RISE Physical Therapy

Dynamic stretching incorporates low-intensity aerobic activity such as skipping, jogging, or shuffling of increasing intensity to replicate the movement patterns about to be performed. Dynamic stretching is as effective in increasing joint range of motion but because it is more active in nature, it could improve performance by rehearsing the movement patterns to be used and increasing blood flow to the muscles (resulting in increased oxygen delivery and waste removal) (Perrier et al., 2013). Dynamic stretching increases the number of firing motor units and may cause the facilitation of the stretch reflex which could help prime the nervous system for more intense activity (Kirmizigil et al., 2014). 

Because successful performance in sport or maximum effort jumping, throwing, and sprinting, the activity and specificity of dynamic warm-ups are thought to be more effective at improving performance in such activities (Kirmizigil et al., 2014). Young and Behm found that a dynamic style warm-up including general, submaximal aerobic activity had a positive influence on muscle performance in jump height compared to static stretching alone. Jump height was further increased after a protocol of general submaximal aerobic activity was followed by the skill that was about to be performed (jumping) at gradual increasing intensity (Young and Behm, 2003). Therefore, a well-designed warm-up should include both general preparation as well as movement- or sport-specific rehearsal to optimally prepare the athlete for peak performance. 

The Stretch-Shortening Cycle (SSC)

The stretch-shortening cycle (SSC) is the countermovement action that can be seen in movements such as walking, running, and jumping. Athletes have been shown to jump 2-4 cm higher using a countermovement when jumping than during a squat jump with no countermovement (Bobbert and Casius, 2005). There is some debate as to the mechanisms responsible for the performance improvements using the SSC, however ensuring the body’s neuromuscular system is primed using a thorough dynamic warm-up that incorporates elements specific to the activity that will follow can contribute to optimized performance and increased jump height. Some of the neurophysiological mechanisms that contribute to the SSC include the storage of elastic energy, involuntary nervous processes, active state, length-tendon characteristics, and motor coordination (Walker, 2016). 

Stretch-Shortening Cycle in Countermovement Jump: Exclusive Review of  Force-Time Curve Variables in Eccentric and Concentric Pha

The SSC is a cyclical muscle action composed of an eccentric, amortization (transitional period), and concentric phase and can be described as a spring-like mechanism (Lloyd et al., 2012). Compressing the spring will cause the coil to rebound and jump off the surface and increasing the speed at which it is pressed, or the amount of force applied will result in a jump of greater magnitude. Therefore, a jump using a countermovement will often allow the athlete to jump higher or farther than a jump from a static position (McBride et al., 2008). 

Electromechanical Delay (EMD)

Electromechanical delay (EMD) refers to the neural and physiological delay in the production of force (Walker, 2016). This means the muscle cannot transmit force instantaneously but this delay in force production can lead to a reduction in performance. Some factors that may contribute to this delay include:

  • Finite rate in muscle stimulation by the CNS
  • Propagation of the action potential on the muscle membrane
  • Time-constraints of calcium release and cross-bridge formation
  • Interaction between contractile filaments

Optimizing muscular pre-activity has been shown to reduce or counteract the effects of EMD by exciting the muscle and creating musculotendinous stiffness prior to the start of the SSC (Turner and Jeffreys, 2010). Because of the detrimental effects of EMD on force production, the completion of a proper warm-up will optimize the SSC, reduce the effects of EMD, and allow the athlete to perform optimally.

Key Takeaways

  • Static stretching has been shown to increase acute joint range of motion but decrease strength and power output. 
  • Dynamic stretching increases range of motion, rehearses movement patterns, increases blood flow and prepares the nervous system for more intense activity. 
  • Performance increases when the warm-up is specific to the activity to be performed. 
  • The stretch-shortening cycle (SSC) is the countermovement action that can be seen in movements such as jumping. 
  • Electromechanical delay (EMD) is detrimental to force production but can be minimized or negated by properly preparing the body with a specific warm-up for the optimization of the SSC. 


  • Perrier ET, Pavol MJ, Hoffman MA. The acute effects of a warm-up including static or dynamic stretching on countermovement jump height, reaction time, and flexibility. J Strength Cond Res. 2011 Jul;25(7):1925-31. doi: 10.1519/JSC.0b013e3181e73959. PMID: 21701282.
  • Young WB, Behm DG. Effects of running, static stretching and practice jumps on explosive force production and jumping performance. J Sports Med Phys Fitness. 2003 Mar;43(1):21-7. PMID: 12629458.
  • Sotiropoulos, K., Smilios, I., Christou, M., Barzouka, K., Spaias, A., Douda, H., & Tokmakidis, S. P. (2010). Effects of warm-up on vertical jump performance and muscle electrical activity using half-squats at low and moderate intensity. Journal of sports science & medicine, 9(2), 326–331.
  • Simic, L., Sarabon, N. and Markovic, G. (2013), Acute static stretching and performance. Scand J Med Sci Sports, 23: 131-148. doi:10.1111/j.1600-0838.2012.01444.x
  • Shrier I. Should people stretch before exercise?. West J Med. 2001;174(4):282-283. doi:10.1136/ewjm.174.4.282
  • Kirmizigil B, Ozcaldiran B, Colakoglu M. Effects of three different stretching techniques on vertical jumping performance. J Strength Cond Res. 2014 May;28(5):1263-71. doi: 10.1519/JSC.0000000000000268. PMID: 24755866.
  • Bobbert MF and Casius LJ. Is the countermovement on jump height due to active state development? Med Sci Sport Exerc 37: 440–446, 2005.
  • Walker, O. (2016, January 23). Stretch-Shortening Cycle. Retrieved November 01, 2020, from
  • Lloyd, R.S., Oliver, J.L., Hughes, M.G., and Williams, C.A. (2012). The effects of 4-weeks of plyometric training on reactive strength index and leg stiffness in male youths. Journal of Strength and Conditioning Research, 26(10), pp.2812–2819. 
  • McBride JM, McCaulleyGO, and Cormie P. Influence of preactivity and eccentric muscle activity on concentric performance during vertical jumping. J Strength Cond Res 23: 750–757, 2008.
  • Turner, A.N., Jeffreys, I. (2010). The stretch-shortening cycle: proposed mechanisms and methods for enhancement. Journal of Strength and Conditioning Research, 17, 60-67.

[Via Athletic Lab] Mechanics of Forefoot Running by Gaby Smith

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[This is a guest blog by Gaby Smith. Gaby Smith completed her MS in Exercise Science at Northeastern University and is participating in the Athletic Lab Mentorship Program. Gaby is a Certified Strength and Conditioning Specialist and holds certifications with U.S. Soccer, USAW, and USTFCCCA.]

Today’s market is full of many different types of running footwear claiming to help runners run more efficiently, improve performance, and reduce the likelihood of injury. Minimalist footwear claims to help runners strike more on the forefoot, while traditional footwear provides more cushioning and promotes a heel first landing. Even with the technological advancements in running shoes, up to 79% of endurance runners are injured in a given year, with almost half (46%) of these injuries being recurrences (Davis et al., 2017). While the causes of these injuries are multifactorial, one of the hypothesized reasons for the high injury rate is the altered rear foot strike pattern promoted by cushioned running shoes. This change in loading and mechanics may not provide the optimal mechanical environment for the foot and ankle compared to the forefoot striking pattern commonly seen when barefoot or wearing minimalist running shoes.


Humans have been running for millions of years. The modern running shoe was not invented until the 1970s, so for most of evolutionary history runners were barefoot or wore very minimalist footwear like sandals or moccasins (Lieberman et al. 2010). Runners were able to cope with the impact of the foot colliding with the ground using a predominately forefoot strike before bringing down the heel. Today, habitually shod runners rear-foot or heel strike as a result of the elevated and cushioned heel of modern shoes. Kinematic analysis shows that forefoot striking generates smaller collision forces because of the plantarflexed foot and increased ankle compliance during impact (Lieberman et al., 2010). This type of striking pattern and gait may protect the feet and lower limb from impact related injuries experienced by many runners.

Modern Running Shoes and Heel Striking

Most modern running shoes feature large cushioned heels for a comfortable and stable landing which cushions some of the force of impact and distributes the force over a larger area of the rear-foot (Liberman et al. 2010). They also typically have arch support and stiffening elements to prevent overpronation (rolling in of the foot) and prevent the flattening of the arch.

Hasegawa et al. (2007) estimate that 75% of shod runners heel strike. This may be because it is comfortable to have the shock-absorbing features cushion the impact, there is more cushioning in the rear-foot than the forefoot, and it is stable. Most likely, the higher heel makes it easier to heel strike because the sole below the heel is about twice as thick as the sole beneath the forefoot (Hasegawa, 2007). While there is nothing wrong with heel striking, some runners may get repetitive stress injuries each year using a rear-foot strike (Lieberman et al., 2010).

At impact, the foot and lower leg come to a dead stop, leading to a high impact transient of about 1.5 to 3 times bodyweight within 50 milliseconds of striking the ground (Lieberman et al. 2010). While many running shoes make heel strikes more comfortable because they slow the rate of impact and spread the force over a greater area of the foot, they do not eliminate the high impact nature of the strike.

Forefoot Striking

Humans have been running for millions of years with a primarily forefoot or midfoot strike pattern. These strikes lead to lower force impacts which likely lead to lower rates of injury than heel striking. Forefoot striking has also been found to strengthen the muscles in the foot and arch, which may also help prevent injury. Forefoot striking may also be more efficient because it uses the natural “springs” in the foot and calf to store and release energy (Lieberman et al. 2010). Running barefoot or in minimalist shoes, which are typically lighter than traditional running shoes, mean that there is less mass to accelerate at with each stride and has been shown to use up to 5% less energy than traditional shod running (Divert et al., 2005; Squadrone & Gallozzi, 2009).

In a forefoot strike, the forefoot will come to a dead stop upon ground contact, but the heel and lower leg continue to fall as the ankle flexes. This type of impact produces essentially no impact transient impact forces up to seven times lower than those experienced by shod runners who heel strike (Lieberman et al. 2010).

Transitioning to Forefoot Striking

While there is nothing wrong with rear-foot striking, transitioning to forefoot striking may reduce the risk of repetitive-use injuries and offer other benefits. Barefoot or minimalist running is most often associated with a forefoot striking pattern; however, most modern footwear alters runners’ striking pattern to a predominately rear-foot striking pattern (Lieberman et al., 2010). Forefoot running loads the plantar fascia, Achilles tendon, and gastrocnemius more efficiently and may optimize their adaptation to the force of impact (Sinclair et al., 2015). In addition to strengthening the muscles of the feet and calves, the force of impact is not as violent as that of a rear-foot strike (Davis et al., 2017).

Converting to a forefoot striking pattern should be done slowly and should be accompanied by foot and lower leg strengthening exercises to minimize injury during the transition of gait. As a result of wearing modern shoes, the muscles of the foot and calf are probably weak and will require great strength to contract eccentrically to lower the heel following landing (Lieberman et al., 2010). If transitioning, build up slowly and stretch the calves, hamstrings, and arches frequently to help repair the muscles. Thick soled shoes are also more forgiving when running on uneven surfaces, so minimalist running may be more comfortable on a hard but smooth surface such as a tennis court, track, or smoothly paved road. While forefoot striking is associated with lower impact forces and potentially lowers the rate of impact-related repetitive use injuries, the transition from rear-foot to forefoot striking must be done slowly and with caution as to prevent overusing weak muscles potential injury.

Key Takeaways

  • Modern footwear promotes a rear-foot strike pattern which may be associated with the high rates of repetitive use injuries runners experience
  • Forefoot striking lessens the force of impact by allowing the heel to continue to lower towards the ground and lengthen as the ankle flexes
  • Forefoot striking strengthens the muscles of the foot, arch, and calf


  • Divert C, Mornieux G, Baur H, Mayer F, Belli A. Mechanical comparison of barefoot and shod running. Int J Sports Med. 2005 Sep;26(7):593-8. doi: 10.1055/s-2004-821327. PMID: 16195994.
  • Hasegawa H, Yamauchi T, Kraemer WJ. Foot strike patterns of runners at the 15-km point during an elite-level half marathon. J Strength Cond Res. 2007 Aug;21(3):888-93. doi: 10.1519/R-22096.1. PMID: 17685722.
  • Lieberman, D., Venkadesan, M., Werbel, W. et al. Foot strike patterns and collision forces in habitually barefoot versus shod runners. Nature 463, 531–535 (2010).
  • Moody D, Hunter I, Ridge S, Myrer JW. Comparison of Varying Heel to Toe Differences and Cushion to Barefoot Running in Novice Minimalist Runners. Int J Exerc Sci. 2018 May 1;11(1):13-19. PMID: 29795721; PMCID: PMC5955330.
  • Sinclair J, Atkins S, Richards J, Vincent H. Modelling of Muscle Force Distributions During Barefoot and Shod Running. J Hum Kinet. 2015 Oct 14;47:9-17. doi: 10.1515/hukin-2015-0057. PMID: 26557186; PMCID: PMC4633245.
  • Squadrone R, Gallozzi C. Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners. J Sports Med Phys Fitness. 2009 Mar;49(1):6-13. PMID: 19188889.

[Via Athletic Lab] Can Isometric Training Increase Strength? by Gaby Smith

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Without a doubt, strength training will improve strength. Resistance training adaptations include increased muscle mass, tendon quality, strength, power, range of motion, and improved voluntary activation (Oranchuk et al., 2019). Dynamic movements that incorporate the stretch-shortening cycle make up the majority of most strength training programs; however, isometric training also has many reported benefits.

Types of Muscle Contractions

Concentric muscle contractions occur when the muscle shortens while generating force to overcome resistance. An example of a concentric contraction would be a bicep curl: as the bicep contracts, the arm bends at the elbow, and the weight moves towards the shoulder.

Eccentric contractions result in the elongation of a muscle while the muscle is still generating force. Eccentric contractions occur when resistance is greater than the force generated by the muscle. Voluntary eccentric contractions include the lowering of a heavy weight raised during the concentric contraction, while an involuntary eccentric contraction may occur when a weight is greater than the force generated by the muscle.

Isometric contractions generate force without changing the length of the muscle. Although there is no net movement, the forces generated during isometric contractions are potentially greater during concentric contractions (Reed and Bowen, 2008). It should be noted that although isometric contractions are characterized by the lack of change in muscle length, the muscle actually slowly shortens, while the tendon lengthens. This muscle shortening coupled with tendon lengthening results in no net movement (Smith).

Types of Isometrics

Yielding Isometrics

Yielding isometrics are defined as holding a weight or position and resisting the urge to move. Examples include holding a weight at 90 degrees, causing the biceps to contract isometrically. Bodyweight exercises, such as forearm planks, are also yielding isometrics as the muscles are working to hold the body in place and resist movement. This type of isometric can be done with or without external load and place less stress on the joints because there is no movement. They can also be used to teach proper positioning by incorporating pauses to create bodily awareness and familiarity with the given position. Yielding isometrics also eliminate the use of momentum to “cheat” a rep by bouncing out of the bottom.

Overcoming Isometrics

Overcoming isometrics involve trying to move an immoveable object. Unlike yielding isometrics in which a position is being maintained, overcoming isometrics actively try to move an external object that will not move. These contractions should be done with maximum intent and are very taxing on the nervous system. However, this type of isometric recruits the maximum amount of muscle fibers and motor units to try to move the object which teaches the nervous system how to engage as quickly and forcefully as possible. An example of an overcoming isometric would be pressing a barbell into pins while bench pressing or squatting.

Benefits of Isometric Training

One benefit of isometric training is the controlled application of force within a pain-free range of motion. This type of contraction has also been shown to be a reliable means of testing and tracking changes in force production (Oranchuk et al., 2019). Isometric training is especially beneficial in rehabilitative settings where joint movement may be limited or painful.

Isometric training provides a force overload because maximal isometric force is greater than that of a concentric contraction (Oranchuk et al., 2019). Additionally, isometric training can be used in a sport-specific capacity by targeting a specific weak point in a range of motion which can positively transfer to performance and injury prevention by strengthening the given joint angle (McArdle et al., 2015). Isometric training is more effective in maximum force development at a specific angle compared to a dynamic movement and can be used to target a biomechanically disadvantaged position of a specific movement (Lum, 2019).

Another benefit of isometric training is the ability to activate nearly all available motor units (Read, 2020). Motor units are recruited during increased voluntary contraction according to the size of the contraction they produce (according to the size principle), so increasing the time of contraction may allow for more motor unit activation (Milner-Brown et al., 1973).

Other potential benefits include improved tendon and joint health, minimal muscle soreness, increased neural drive and efficiency, increased work capacity, and strength through sticking points (Smith). A study conducted in 2001 found that isometric resistance training caused tendon structures to stiffen and found a correlation between the duration of contraction and tendon stiffness (Kubo et al., 2001).

Limitations of Isometric Training

While isometric training has been shown to increase strength, dynamic strength training is more beneficial and transferrable to dynamic movements (Lum, 2019). Therefore, isometric training should be included as part of a training program that also includes dynamic movements.

Some coaches have been wary of including plyometrics in their resistance training programs citing decreases in coordination and speed of movement or decreased muscle elasticity (Kubo et al., 2017). In 2001, Kubo et al. found that isometrics resulted in increased tendon stiffness, but their study in 2017 found plyometrics to be a superior means of improving performance in stretch-shortening cycle exercises (Kubo et al., 2017).

Another concern of coaches is that isometrics will only be beneficial at the specific joint angle. Verkhoshansky and Siff found that isometric training could produce strength gains in a range of 15 degrees on either side of the training angle, however given the specificity of adaptation at the trained joint angle, improving strength through the entire range of motion may require training at multiple angles which may be impractical and time-consuming (McArdle et al., 2015).
Although isometric training may not result in soreness, it is taxing on the central nervous system (CNS) and will take the nervous system longer to recover than the muscular system (Read, 2020). This fatigue may not be as obvious as muscle fatigue or soreness but may impact performance if followed by other CNS-demanding activities.

Incorporating Isometrics into Training

While isometrics should not make up the majority of a training program, they can be beneficial in motor unit recruitment and improve strength without causing much stress on the joints. While they are very useful in rehabilitative settings to control force production at a given joint angle, isometrics are not as transferrable to dynamic performance as dynamic movements (Lum, 2019). If using isometrics, the current literature suggests performing a contraction of 1-5 seconds at 80-100% maximum voluntary contraction (MVC) for a total of 30-90 seconds per session to increase maximum strength. To increase hypertrophy, the contraction should be performed at 70-75% of MVC for 3-30 seconds for greater than 80-150 seconds per session (Lum, 2019).

Key Takeaways

  • Isometric muscle contractions result in no movement; however, during the contraction the muscle shortens while the tendon lengthens
  • Isometric training is effective in increasing strength, but may not be as beneficial to dynamic movement as a dynamic resistance training
  • Isometric training is more effective in improving maximum force at a specific angle compared to dynamic training
  • Isometric training is effective in rehabilitative settings as force can be controlled in a pain-free range of motion


  1. Kubo, K., Ishigaki, T., & Ikebukuro, T. (2017). Effects of plyometric and isometric training on muscle and tendon stiffness in vivo. Physiological reports, 5(15), e13374.
  2. Kubo, K., Kanehisa, H., & Fukunaga, T. (2001). Effects of different duration isometric contractions on tendon elasticity in human quadriceps muscles. The Journal of physiology, 536(Pt 2), 649–655.
  3. Lum D, Barbosa TM. Brief Review: Effects of Isometric Strength Training on Strength and Dynamic Performance. Int J Sports Med. 2019 May;40(6):363-375. doi: 10.1055/a-0863-4539. Epub 2019 Apr 3. PMID: 30943568.
  4. McArdle, W. D., Katch, F. I., & Katch, V. L. (2015). Chapter 22 Muscular Strength: Training Muscles to Become Stronger. In Exercise physiology: Nutrition, energy, and human performance (pp. 512-513). Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins.
  5. Milner-Brown, H. S., Stein, R. B., & Yemm, R. (1973). The orderly recruitment of human motor units during voluntary isometric contractions. The Journal of physiology, 230(2), 359–370.
  6. North, D. (2020, January 09). 2 Types of Isometrics for Maximal Strength and Muscle. Retrieved September 25, 2020, from
  7. Read, A. (2020, June 03). Isometric Training: What It Is and How to Do It Correctly. Retrieved September 25, 2020, from
  8. Smith, J. (n.d.). Isometric Exercises for Athletes: The Complete Guide. Retrieved September 25, 2020, from
  9. Verkhoshansky, Y., & Siff, M. C. (2009). Isometrics. In Supertraining. Rome, Italy: Verkhoshansky

[Via Athletic Lab] Obesity and COVID-19: A Deadly Combination by Gaby Smith

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The United States has inarguably been one the hardest hit countries in the world by COVID-19. While the reasons for this is undoubtedly multi-factorial, an often overlooked confounding issue is the state of our nationwide health. In this guest post, Gaby Smith presents the compelling science on why COVID-19 and obesity are such a deadly combination.

[This is a guest blog by Gaby Smith. Gaby Smith completed her MS in Exercise Science at Northeastern University and is participating in the Athletic Lab Mentorship Program. Gaby is a Certified Strength and Conditioning Specialist and holds certifications with U.S. Soccer, USAW, and USTFCCCA.]

In the United States, we find ourselves in the midst of two epidemics: COVID-19 and obesity. While initially the two may seem to be only tangentially related, numerous studies have found a significant association between BMI and risk for death among COVID-19 patients independent of related health conditions (diabetes, hypertension, coronary artery disease) (Tartof et al., 2020). This association is of particular concern in the United States, as over 40% of the population is considered obese (BMI >30 kg/m2) (Kass, 2020).

To this point, the two primary strategies to slow the spread of COVID-19 include mitigation (handwashing, wearing a face covering, social distancing) and the adoption of practices consistent with good overall health. COVID-19 is primarily transmitted through airborne droplets when infected individuals cough, sneeze, talk, or breathe deeply. Inhaled droplets infect cells lining the airway and cause modest symptoms including coughing, fever, shortness of breath, fatigue, and loss of taste or smell, among others (CDC, 2020).

Why is obesity such a significant risk factor?

Obesity seems to increase the severity of COVID-19 cases as a result of the chronic inflammation that characterizes obesity and causes metabolic and immune derangement (Korakas et al., 2020). Obese populations have chronically higher levels of leptin (which has inflammatory effects) and lower levels of adiponectin (which has anti-inflammatory effects) (Luzi and Radaelli, 2020). Higher levels of adipose tissue also lead to defective immune function and altered immune activation in the presence of antigens. Obesity not only inhibits the cells’ response to viral agents, but the dysregulated pro-inflammatory response also contributes to the increased severity of illness observed in this population (Luzi and Radaelli, 2020).

Another reason this population tends to experience more severe respiratory illness is their lack of physical inactivity. Both physical activity and exercise have been shown to increase the immune response and help facilitate positive outcomes in both metabolic and immunological health (Luzi and Radaelli, 2020). These individuals may also have decreased respiratory capacity or difficulty breathing, further worsening the respiratory symptoms associated with the illness.

Obese individuals also tend to be more contagious than leaner populations. These patients shed the virus for up to 104% longer than leaner subjects, potentially increasing their likelihood of spreading the virus (Luzi and Radaelli, 2020). Because of the dysregulated and delayed immune response, these individuals also tend to produce more virulent strains of the virus, as the virus is able to replicate for a longer period of time. Further, BMI has been found to be positively correlated with increased infectious virus exhaled per breath. This is likely due to the combination of higher ventilation volumes and viral loads observed in this population (Luzi and Radaelli, 2020).

In short, obesity and physical inactivity lead to increased systemic inflammation, decreased immune function, decreased viral defense, and potentially inefficient ventilatory capacity (Nieman, 2020).

So can I reduce my risk?

Diet and physical activity are essential preventative measures to improve defense against severe COVID-19 infection and are important aspects in maintaining good overall health. Not only do diet and exercise aid in weight management, but they also improve immune function. Regular moderate-intensity physical activity has been shown to improve immune function and reduce morbidity and mortality from viral infection (Nieman, 2020). Physical activity not only improves the early immune response, but also affects energy balance, leptin response, and antibody production which all are essential in protecting from infection (Luzi and Radaelli, 2020).

Regular aerobic exercise improves immune function by stimulating the exchange of white blood cells and infection-fighting cells, while also keeping stress hormones and inflammatory responses low both during and in the hours following exercise (Nieman, 2020). When performed regularly, exercise increases antipathogenic cells which enhance immunity and reduce illness risk. Clinical studies have shown that engaging in near-daily aerobic activity can reduce illness days stemming from respiratory infections by 40-45% when compared to sedentary lifestyles in both younger and older adults (Nieman, 2020). The severity and symptomatology of the illness were also reduced by 32-41% in those engaging in aerobic activity, indicating that physical activity can both reduce the duration and severity of respiratory illnesses. In addition to having positive effects on the immune system, regular physical activity improves cardiovascular function and protects against cellular stress, as well as improving the endurance, strength, and efficiency of respiratory muscles (Jakobsson et al., 2020).

But my gym is closed … how can I remain physically active?

Mandatory stay at home orders, the closures of gyms, and enforcement of other mitigation strategies have made physical activity difficult for many. While remaining physically active may be more challenging in the midst of the COVID-19 pandemic, it may be more important than ever. In addition to enhanced immunity, exercise may prevent weight gain, increase metabolic rate, reduce stress and anxiety, and improve sleep. The American Heart Association recommends adults perform at least 150 minutes of moderate intensity or at least 75 minutes of vigorous intensity exercise every week.

While going to the gym is one of the more obvious ways to remain physically active, there are many “socially distant” means of physical activity that do not require a gym and are not affected by the forced closure of gyms or other fitness facilities. Getting outside and walking, biking, jogging, or hiking are all great ways to stay safe, get fresh air, and safely distancing oneself from others. There are also online videos, training sessions, and live virtual classes offering many different types of exercise classes that are easily accessible and can be performed from home. Also, simply remaining active by doing chores like mowing the lawn, washing the car, or cleaning the house are low-intensity and simple ways to remain active while at home.

Key Takeaways:

Obesity is a health risk under the best of circumstances. But in the presence of a global pandemic the risks are even greater. The effects of this deadly combination are especially stark in the United States where our population already suffers from high lifestyle related health issues.

  • In the United States, over 40% of the population is considered obese (BMI >30 kg/m2)
  • Obesity increases the risk of severe COVID-19 symptoms and complications independent of other comorbidities or conditions
  • Obesity and physical inactivity decrease immune function, create systemic inflammation, decrease ventilatory capacity, and decrease viral defense leading to an increased viral load in these subjects
  • Physical activity is a critically important preventative measure (even if your gym is still closed!) to maintain and improve immune function and cardiorespiratory capacity
  • All physical activity is beneficial (walking, running, biking, hiking, virtual classes/videos)
  • Aim for at least 150 minutes of moderate intensity or 75 minutes of vigorous activity per week.


  1. Nieman, D. C. (2020). COVID-19: A tocsin to our aging, unfit, corpulent, and immunodeficient society. Journal of Sport and Health Science.
  2. Kass, D. A. (2020). COVID-19 and Severe Obesity: A Big Problem? Annals of Internal Medicine. doi:10.7326/m20-5677
  3. Tartof, S. Y., Qian, L., Hong, V., Wei, R., Nadjafi, R. F., Fischer, H., . . . Murali, S. B. (2020). Obesity and Mortality Among Patients Diagnosed With COVID-19: Results From an Integrated Health Care Organization. Annals of Internal Medicine. doi:10.7326/m20-3742
  4. Korakas, E., Ikonomidis, I., Kousathana, F., Balampanis, K., Kountouri, A., Raptis, A., . . . Lambadiari, V. (2020). Obesity and COVID-19: Immune and metabolic derangement as a possible link to adverse clinical outcomes. American Journal of Physiology-Endocrinology and Metabolism, 319(1). doi:10.1152/ajpendo.00198.2020
  5. Jakobsson, J., Malm, C., Furberg, M., Ekelund, U., & Svensson, M. (2020). Physical Activity During the Coronavirus (COVID-19) Pandemic: Prevention of a Decline in Metabolic and Immunological Functions. Frontiers in Sports and Active Living, 2. doi:10.3389/fspor.2020.00057
  6. Luzi, L., & Radaelli, M. G. (2020). Influenza and obesity: its odd relationship and the lessons for COVID-19 pandemic. Acta diabetologica, 57(6), 759–764.

[Via Athletic Lab] Unilateral vs. Bilateral Training by Gaby Smith

This content was originally posted on

[This is a guest blog by Gaby Smith. Gaby Smith completed her MS in Exercise Science at Northeastern University and is participating in the Athletic Lab Mentorship Program. Gaby is a Certified Strength and Conditioning Specialist and holds certifications with U.S. Soccer, USAW, and USTFCCCA.]

When it comes to resistance training, you may automatically think of traditional bilateral exercises like the back squat, bench press, or deadlift which use both arms or both legs simultaneously. While these bilateral exercises have been effective in improving strength and power, there is increasing interest in the benefits of unilateral exercises – those that use only one arm or leg at a time – to improve athletic performance. Unilateral exercises are typically used as assistance exercises to bilateral exercises to add volume or variation within a strength program. Given that many sport-specific movements are unilateral in nature, many argue that unilateral resistance training provides a more specific and transferable stimulus to sport performance. Sprinting, jumping, and changing direction are all performed unilaterally, so training in a predominately unilateral manner may provide greater specificity and ultimately provide better transfer from training to competition given the similarity of the movement patterns. There have been many studies investigating the effects of both bilateral and unilateral training and determining which is best for improving speed and power.

Advantages of Bilateral Training

Improvements in both strength and power using bilateral exercises, such as the back squat, have been established and are commonly used as a primary exercise for this reason (Spiers, 2016). Training the back squat has been shown to significantly improve running velocity and jump height and may be correlated with improvements in change of direction performance (Spiers, 2016). Bilateral exercises allow for greater loading; therefore, the production of greater absolute force. This peak force production helps improve the athlete’s strength which is a key indicator of athletic performance and a main goal in most training programs. Bilateral exercises also allow for greater stability because the center of mass can be balanced between both limbs. These exercises require the coordination between both sides of the body and can be used to train both sides of the body simultaneously. Additionally, bilateral exercises may be easier to teach and perform, especially for novice athletes, and can be used as a safe option for even inexperienced athletes.

Advantages of Unilateral Training

The main argument for the use of unilateral exercises (reverse lunge, box step-up, single-arm row or press) is their greater specificity and potential transfer to sport-specific movements like running, bounding, jumping, and changing direction which are entirely or predominately unilateral. Specificity is a key principle in program design to maximize the transfer between training and competitive performance. Therefore, to most effectively improve performance, it is argued that resistance exercises must closely resemble the forces and mechanics required for the specific sport. The smaller base of support of unilateral exercises requires neuromuscular coordination and increased stability; however, this comes at the cost of external loading (Appleby, 2019). Some research also suggests that unilateral training may reduce muscular imbalances and the overall bilateral deficit (Jansson, 2013).

The Bilateral Deficit

Another theory supporting the incorporation of unilateral exercises in a training program is known as the bilateral deficit. This refers to the difference between the maximal strength of a bilateral contraction and the sum of the strength of the right and left limbs when contracting alone (Costa, 2015). This phenomenon has been observed in both dynamic and isometric contractions, in both the upper and lower limbs, large and small muscle groups, and in populations including athletes, non-athletes, elderly, and adolescents (Jansson, 2013; Costa, 2015). There are many theories regarding the cause of the increased force capacity of unilateral contractions compared to the two limbs contracting in tandem. One possible explanation as to the origin of the deficit is the difference in antagonist muscle activation between unilateral and bilateral contractions (Kuruganti, 2011). It may also be more complicated for the body to fully activate the largest and strongest motor units during bilateral contractions, as this requires coordinated neural impulses from both of the brain’s hemispheres. Therefore, it is theorized that unilateral training may be a more efficient means of training if maximal speed and strength production presents a challenge to the neuromuscular system (Jansson, 2013).

The Bottom Line

Both bilateral and unilateral exercises demonstrate improvements in their trained movement and proved to be equally effective in increasing strength (Appleby, 2019; McCurdy, 2005; Spiers, 2016). While one modality did not prove to be significantly more effective in increasing strength than the other, there was a noted reciprocal benefit: bilateral training also improved performance on unilateral tests and unilateral training also improved performance on bilateral tests (Appleby, 2019; McCurdy, 2005; Spiers, 2016). Although the exact mechanism for this finding is unclear, the improvements in strength likely have a carryover effect between tests. Even with the carryover effect, greater improvements in strength were seen on the specific test modality the athletes trained for, likely due to the mechanical specificity between the training stimulus and test (Appleby, 2019).

Because both bilateral and unilateral exercises can elicit similar relative improvements in strength, both can and should be incorporated in an athlete’s training program. Bilateral exercises will allow for the use of greater absolute loads and greater overall force production. While unilateral exercises will require lighter absolute loads than a bilateral exercise, the relative intensity may be greater than the bilateral movement and could enhance force development in a way that better corresponds to sport-specific strength gains (Jones, 2012). Another reason unilateral exercises require lighter loads is because of the increased stability required to perform the exercise. Therefore, they may be less effective for the development of maximal strength, but can be used to complement and transfer strength gains to bilateral exercises (Appleby, 2019). Also, using unilateral exercise alters the training stimulus which may also enhance recovery and reduce the risk of overuse injury while still providing a similar training result to a bilateral exercise (Jones, 2012). Practically, research suggests that both unilateral and bilateral exercises should be used as complimentary components of a training program as a means of increasing strength.


  • Costa EC, Moreira A, Cavalcanti B, Krinski K, Aoki MS. Effect of unilateral and bilateral resistance exercise on maximal voluntary strength, total volume of load lifted, and perceptual and metabolic responses. Biol Sport. 2015;32(1):35–40.
  • Jansson, D. (2014). Effects of Unilateral versus Bilateral Complex Training and High Intensity Interval Training on the Development of Strength, Power and Athletic Performance : An experimental study on elite male and female handball players during preseason training.
  • Jones, Margaret T; Ambegaonkar, Jatin P; Nindl, Bradley C; Smith, Jeffrey A; Headley, Samuel A Effects of Unilateral and Bilateral Lower-Body Heavy Resistance Exercise on Muscle Activity and Testosterone Responses, Journal of Strength and Conditioning Research: April 2012 – Volume 26 – Issue 4 – p 1094-1100. doi: 10.1519/JSC.0b013e318248ab3b
  • Kuruganti U, Murphy T, Pardy T. Bilateral deficit phenomenon and the role of antagonist muscle activity during maximal isometric knee extensions in young, athletic men. Eur J Appl Physiol. 2011;111(7):1533-1539. doi:10.1007/s00421-010-1752-8
  • McCurdy KW, Langford GA, Doscher MW, Wiley LP, Mallard KG. The effects of short-term unilateral and bilateral lower-body resistance training on measures of strength and power. J Strength Cond Res. 2005;19(1):9-15. doi:10.1519/14173.1
  • Speirs, Derrick E.1,2; Bennett, Mark A.3; Finn, Charlotte V.4; Turner, Anthony P.2 Unilateral vs. Bilateral Squat Training for Strength, Sprints, and Agility in Academy Rugby Players, Journal of Strength and Conditioning Research: February 2016 – Volume 30 – Issue 2 – p 386-392
    doi: 10.1519/JSC.0000000000001096

[Via Athletic Lab] A Closer Look at Heart Rate Variability

This content was originally posted on

[This is a guest blog by Gaby Smith. Gaby Smith completed her MS in Exercise Science at Northeastern University and is participating in the Athletic Lab Mentorship Program. Gaby is a Certified Strength and Conditioning Specialist and holds certifications with U.S. Soccer, USAW, and USTFCCCA.]

Resting, exercise, and recovery heart rates are common measures used to monitor fatigue, fitness, and performance responses and are often used to adjust training load. Recently, heart rate variability (HRV) has become a measure more commonly used to assess an athlete’s readiness to perform and ensure the appropriate dose of training (ie. preventing overtraining or detraining). Generally, training load is quantified by external and internal indicators of intensity along with training time. External indicators may include distance, power output, or number of repetitions, while internal indicators include oxygen uptake, heart rate, blood lactate, or RPE (Buchheit, 2014).

When it comes to monitoring and measuring athletes’ fatigue, heart rate (HR) measures are commonly used because they are inexpensive, time efficient, and can be applied routinely and simultaneously with many athletes. Resting HR, exercise HR, and HRV are all related to autonomic nervous system (ANS) activity and their use in combination may improve the monitoring of the training status of athletes. While there are a number of HR measures that can be used to assess an athlete’s training status, it is of utmost importance that whatever measure is used, it is standardized in order to isolate the training-induced effects (Buccheit, 2014).

Physiological determinants of resting HR include cardiac muscle morphology, ANS activity, body position, and plasma volume (Buccheit, 2014). Best practice recommends the athlete measure resting HR upon waking up in the morning from a supine, seated, or standing position – but the position should be consistent across measurements. Exercise HR is another commonly used and easy to measure assessment of training status. Because HR is closely related to oxygen uptake during continuous exercise, exercise HR can be used to measure relative exercise intensity. There are correlations between decreases in exercising HR and improvements in high-intensity exercise performance.

Recently, HRV has become a measure of increasing interest and has been more commonly used for tracking the adaptation or maladaptation of athletes to training. HRV measures beat-to-beat differences in HR and is thought to reflect the cardiac regulation by the ANS. While previously used to predict sudden cardiac death and disease progression, recent studies have demonstrated that HRV is also useful in exercise training. These studies have reflected that HRV can be used as a marker of the sympathetic or vagal component of the ANS and can track the course of training adaptation to assist in setting optimal training loads to improve performance (Dong, 2016).

What is HRV?

HRV refers to the variations in the time between successive heart beats (time between consecutive R-R intervals) and is a result of the two competing branches of the ANS: the sympathetic and parasympathetic nervous systems. HRV can be measured using an electrocardiogram (ECG), fitness tracker, smartphone app, or heart rate strap. Along with being used to assess training status, HRV has been used in a medical context where it has been shown to be a predictor of mortality and incidents of sudden cardiac death (Bigger, 1992).

High HRV means the body is responsive to both sympathetic and parasympathetic inputs and is capable of adapting and responding to training. Low HRV indicates that one branch of the ANS is dominant (usually the sympathetic) and is usually associated with stress, fatigue, dehydration, etc. (Dong, 2016).

HRV is affected by mental stressors, not just physical stressors such as competition or training. Work-related stress, having to make difficult decisions, public speaking, and performing tests or exams have all been shown to reduce HRV (Dong, 2016; Taelman, 2004).

What is a “Good” HRV?

While a higher HRV is generally a sign of better fitness, HRV is highly individualized and differs between individuals. Age, gender, fitness level, environment, and genetics must all be taken into account when looking at an individual’s HRV. Younger people tend to have higher HRV than older people, and males tend to have higher HRV than females. Athletes tend to have higher HRV than non-athletes or sedentary individuals (Buccheit, 2014).

When monitoring HRV, it is important to track individual trends, rather than comparing HRV between individuals. With training, HRV should increase, and a downward trend may be a sign of overtraining, fatigue, or stress (Dong, 2016).

Training and HRV

HRV has become one of the most used training and recovery monitoring tools because autonomic regulation is an important indicator of training adaptations and the body’s responsiveness to training (Aubert, 2003). Heart rate is primarily controlled through the activity of the ANS, which is comprised of the sympathetic and parasympathetic nerves. The sympathetic nervous system causes the body to respond in stressful environments (training, games, competitions, etc.) by secreting epinephrine and norepinephrine and increasing HR, contractility, and blood pressure to increase blood flow to the muscles. The parasympathetic nervous system does the opposite; it reduces heart rate and blood pressure in the absence of stress. The parasympathetic nervous system helps facilitate recovery after a stressful event by counteracting the effects of the sympathetic nervous system. Therefore, both the sympathetic and parasympathetic nervous systems are critical for performance and recovery. An imbalance between the two systems can lead to a reduction in performance.

Because HRV reflects ANS function and stress, it is frequently used to identify when training is optimal and monitor recovery status and the potential for overtraining. Research has suggested that HRV can (Aubert, 2003):

  • Reflect recovery status
  • Help determine overtraining
  • Identify an athlete’s ability to adapt and respond to training
  • Aid in training prescription
  • Potentially predict susceptibility to illness or injury

HRV-Guided Training

The concept of using HRV-guided training is perhaps one of the most useful applications when discussing HRV. This idea uses HRV to prescribe the intensity of a given training session. If the athlete had an average or above average HRV, they would be prescribed an intense training session. When the athlete’s HRV is below normal, they would be prescribed a lower-intensity session. Because there will be inter-athlete variability in baseline HRV, it is important to prescribe intensity based on each athlete’s HRV relative to their baseline rather than comparing absolute HRV between athletes.

One study found that HRV-guided training improved running performance and maximal running velocity more so than pre-planned training (Kiviniemi, 2007). Another study found a relationship between high HRV scores with improved VO2max, while those with low HRV scores were found to have reduced VO2max (Hedelin, 2001). These findings suggest HRV-guided training may be more beneficial in improving aerobic performance than traditional pre-planned training. While the effectiveness of HRV-guided training in regard to strength development is not as conclusive, it appears that athletes with above baseline HRV scores are more sensitive to performance gains than those with lower HRV scores.
HRV and Recovery

Many studies have cited a reduction in HRV following intense training. One study found a reduction in HRV in the 24-hours following a high intensity strength training session. HRV returned to baseline after 72 hours of recovery, indicating a relationship between HRV and recovery (Chen, 2011). Another study found that HRV increased during an intense training period and decreased over an overload training phase. After two weeks of recovery, HRV rebounded and increased above baseline, indicating that intense training may actually improve HRV (Pichot, 2002). There have also been studies to determine the relationship between HRV and injury. While this relationship requires further investigation, it is thought that the correlation between low HRV and fatigue or stress would lead the athlete to be more susceptible to injury.

Key Takeaways:

  • HRV is the variation in time between successive heart beats
  • High HRV scores are suggested to indicate greater ability to adapt to training stress and improve performance
  • HRV-guided training appears to induce greater performance gains than pre-planned training programs
  • HRV is correlated with recovery status


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  • Buchheit M. Monitoring training status with HR measures: do all roads lead to Rome?. Front Physiol. 2014;5:73. Published 2014 Feb 27. doi:10.3389/fphys.2014.00073
  • Chen, J-L, Yeh, D-P, Lee, J-P, Chen, C-Y, Huang, C-Y, Lee, S-D, Chen, C-C, Kuo, TBJ, Kao, C-L, and Kuo, C-H. Parasympathetic nervous activity mirrors recovery status in weightlifting performance after training. J Strength Cond Res 25(6): 1546–1552, 2011.
  • Dong GJ. The role of heart rate variability in sports physiology. Exp Ther Med. 2016 May; 11(5): 1531–1536.
  • Hedelin, R., Bjerle, P., & Henriksson-Larsen, K. (2001) Heart Rate Variability in athletes: relationship with central and peripheral performance. Medicine & Science in Sports & Exercise, 33(8), 1394-1398.
  • Kiviniemi, A.M., Hautala, A., Kinnumen, H., & Tulppo, M. (2007) Endurance training guided by daily heart rate variability measurements. European Journal of Applied Physiology, 101: 743-751.
  • Pichot, V., T. Busso, F. Roche, M. Garet, F. Costes, D. Duverney, J. R. Lacour, And J. C. Barthe´Le´My. Autonomic Adaptations To Intensive And Overload Training Periods: A Laboratory Study. Med. Sci. Sports Exerc., Vol. 34, No. 10, Pp. 1660–1666, 2002.
  • Taelman J, Vandeput S, Spaepen A, and Van Huffel S. Influence of Mental Stress on Heart Rate and Heart Rate Variability. Eur J Appl Physiol. 2004 Jun;92(1-2):84-9.