Hamstrings Behaviour and Movement Expression: the Great Debate

Hamstrings function and behaviour during sprinting is an extremely interesting topic for researchers, coaches, and athletes alike, primarily because this muscle group is a critical contributor to athletic performance while remaining a primary concern for athletic injury as well. Despite decades of study and a copious literature, the scientific community is currently engaged in a significant debate regarding how these muscles actually behave when an athlete reaches maximum velocity. Understanding whether the hamstrings act as eccentric brakes or isometric anchors has profound implications for how we train athletes and rehabilitate injuries.

The Two Faces of Hamstring Function: Brake vs. Spring

Historically, the scientific literature has supported an eccentrically lengthening, energy-absorbing, "brake-driven" model of hamstring function during the late-swing phase of sprinting. In this model, the hamstring muscle-tendon unit (MTU) is thought to actively lengthen to absorb mechanical energy, acting as a damper or shock absorber to rapidly decelerate the knee joint. This phase is considered particularly hazardous because the hamstrings are stretched to maximum or near-maximum lengths while being subjected to forces as high as ten times body weight.

However, a contrasting theory has recently gained traction: the isometrically contracting, "spring-driven" model. Proponents of this view, such as Van Hooren and Bosch, suggest that while the overall MTU might lengthen, the muscle fascicles themselves behave isometrically. In this scenario, the muscle acts as a stable anchor point, allowing the series elastic elements (tendons and fascia) to do the work of stretching and recoiling like a spring. This "spring-like" behavior is theorized to be more metabolically efficient and allows for a more forceful elastic recoil prior to ground contact.

During the late-swing phase of sprinting the hamstrings are stretched to maximum or near-maximum lengths while being subjected to forces as high as ten times body weight (picture from Danielsson et al., 2020)

Navigating the Complexity of Muscle Contraction

To navigate this debate, it is essential to clarify what is the meaning of contraction within a dynamic environment. In sports science and physiology, the term contraction is often redefined to mean the active state of the muscle—the attempt to shorten—without necessarily implying a change in length.

The confusion often arises because the behavior of the whole MTU (seen as the movement of bony attachments) does not always reflect the behavior of the muscle fibers located within the muscle. For instance, during "isometric" MTU behavior, the muscle fibers can still shorten by up to 28% as they stretch the internal elastic structures. Furthermore, because muscle fibers and sarcomeres are viscoelastic, they are subject to strain and deformation whenever they experience tension, making a "strictly isometric" contraction under dynamic conditions such as sprinting almost impossible to achieve outside of the lab.

Anatomy Insights: Not All Hamstrings Are Equal

One of the most insightful findings in recent research is that the hamstrings are not to be considered as a single, uniform group of muscles; rather, each individual muscle—the biceps femoris long head (BFlh), short head (BFsh), semitendinosus (ST), and semimembranosus (SM)—possesses a unique architectural arrangement.

• Semitendinosus and BFsh: these muscles are characterized by relatively long fascicles, small physiological cross-sectional areas (PCSA), and low pennation angles. Such arrangement makes them well-suited for high-velocity contractions and large fascicular excursions.

• Semimembranosus and BFlh: these are more pennate in appearance, with short fascicles and larger PCSAs, suggesting a greater capacity for force generation and potential spring-driven behavior.

Research indicates that the BFlh and SM experience the highest peak forces during sprinting, with the BFlh specifically experiencing the largest peak strains (~12%), which may explain why it is the most common site for injury (Chumanov, Heiderscheit and Thelen, 2007; Thelen et al., 2005). Because of these differences, it is anticipated that the hamstring MTUs adopt a combination of spring, brake, and motor-driven functioning depending on their specific architectural role.

The Role of Variable Muscle Gearing

A fascinating automatic transmission system (Azizi, Brainerd and Roberts, 2008) exists within pennate muscles like the hamstrings, known as variable muscle gearing. Because muscle fibers are oriented at an angle to the tendon, they can rotate during contraction.

When a muscle shortens under low-force, high-velocity conditions, the fibers rotate to increase pennation, which heightens the muscle’s shortening velocity relative to the fiber’s velocity (high gear). During high-force eccentric contractions—like those occurring in the late-swing phase of a sprint—the muscle likely operates at a high gear to reduce the strains experienced by the fascicles, protecting them from excessive damage. This mechanism suggests that the hamstrings possess a sort of internal protection mechanism to mitigate the risks of high-speed lengthening.

Metabolic Efficiency: The Isometric Myth?

One of the primary arguments for the isometric spring model is that it saves metabolic energy by reducing muscle work. However, literature suggest this assumption may be flawed. In fact, research by Holt et al. (2014) demonstrated no detectable differences in metabolic cost between isometric work and muscular stretch-shorten cycles.

While shortening muscles consume significant energy, lengthening muscles produce higher forces at a lower metabolic cost. Therefore, the energy savings traditionally attributed to isometric work loops may actually be a result of the eccentrically induced force enhancement that occurs when active muscles are stretched.

The Challenge of Direct Assessment

The debate persists because accurate assessment of hamstrings behavior during sprinting is extremely difficult. Limitations of traditional methods are as follows:

• EMG: research into muscle activation is inconsistent, with some studies showing the hamstrings are active throughout the entire sprint cycle and others showing gaps in electrical signaling. Uncertainties in electromechanical delay (EMD)—the time between a signal and actual force production—make it difficult to pinpoint exactly when the muscle is "active".

• Computer Modelling: most models rely on the Hill muscle model, which often assumes uniform strain and fails to accurately predict forces under highly dynamic, fluctuating conditions.

• Ultrasound: frequently used in humans to estimate muscle and fascicle strains during different dynamic movements. However, current ultrasound methods for evaluating muscle–tendon function often rely heavily on extrapolation, and measuring the hamstrings during sprinting is particularly difficult due to both the intensity of movement and the muscle's location.

Implications for Training: Moving Beyond Replication

In the applied sports performance setting, exercises are often selected due to their potential ability to "replicate" sprinting mechanics. Proponents of the isometric theory argue for isometric loading (like the single-leg hamstring bridge), while traditionalists favor eccentric exercises (like the Nordic Hamstring Curl).

The sources suggest that "functional replication" should not be the exclusive goal for exercise selection. Isolated exercises—whether concentric, isometric, or eccentric—rarely reach the activation levels (~60% of max) or the extreme angular velocities (>1000°/s at the knee) seen in top-speed sprinting (Prince et al., 2020; Raiteri, Beller and Hahn, 2021).

Instead, a more rigorous approach is to select exercises through an evidence-based framework of stimulus and adaptation:

1. Eccentric Training: known to shift the force-length profile toward longer lengths and increase fascicle length, which may help the muscle function more isometrically at longer lengths and reduce injury risk.

2. Isometric Training: may strengthen the system's ability to act as a stable anchor for the tendon, potentially improving efficiency.

3. Sprinting: ultimately, the only way to perfectly replicate the unique mechanical demands of sprinting is to sprint.

Current evidence suggests that the hamstrings function primarily as an eccentric energy-absorbing brake during the late-swing phase, necessary to decelerate the lower limb and manage loads that exceed isometric capacity. However, the presence of long tendons and specific architectures in muscles like the BFlh ensures that tendon contributions and spring-like behavior are also vital.

The issue of hamstring behavior remains complex and multifactorial. Rather than searching for the Holy-Grail of the right contraction modality, coaches should recognize that the hamstrings are a diverse group of actuators requiring a variety of stimuli to optimize performance and mitigate the high risk of injury inherent in maximal sprinting.

References

1. 
   
2. 
   

3. Azizi, E., Brainerd, E.L. and Roberts, T.J. (2008) 'Variable gearing in pennate muscles', Proceedings of the National Academy of Sciences,105(5), pp. 1745 – 1750.

4. Chumanov, E.S., Heiderscheit, B.C. and Thelen, D. G. (2007) 'The effect of speed and influence of individual muscles on hamstring mechanics during the swing phase of sprinting', Journal of Biomechanics, 40(16), pp. 3555 – 3562.

5. Danielsson, A., Horvath, A., Senorski, C., Alentorn-Geli, E., Garrett, W. E., Cugat, R., Samuelsson, K. and Senorski, E. H. (2020) 'The mechanism of hamstring injuries - a systematic review', BMC Musculoskeletal Disorders, 21(1), p. 641.

6. Holt, N.C., Roberts, T.J. and Askew, G.N. (2015) 'The energetic benefits of tendon springs in running: is the reduction of muscle work important?', Journal of Experimental Biology, 217(Pt 24), pp. 4365 – 4371.

7. van Hooren, B., Bosch, F. (2017) 'Is there really an eccentric action of the hamstrings during the swing phase of high-speed running? Part I: a critical review of the literature', Journal of Sports Sciences, 35(23), pp. 2313 – 2321.

8. van Hooren, B., Bosch, F. (2017) 'Is there really an eccentric action of the hamstrings during the swing phase of high-speed running? Part II: implications for exercise', Journal of Sports Sciences, 35(23), pp. 2322 – 2333.

9. Kalkhoven, J. T., Lukauskis-Carvajal, M., Sides, D. L., McLean, B. D. and Watsford, M. L. (2023) 'A Conceptual Exploration of Hamstring Muscle-Tendon Functioning during the Late-Swing Phase of Sprinting: The Importance of Evidence-Based Hamstring Training Frameworks', Sports Medicine, 53(12), pp. 2321 - 2346.

10. Prince, C., Morin. J. B., Mendiguchia, J., Lahti, J., Guex, K., Edouard. P. and Samozino, P. (2020) 'Sprint specificity of isolated hamstring-strengthening exercises in terms of muscle activity and force production', Frontiers in Sports and Active Living, 2:609636.

11. Raiteri, B. J., Beller, R. and Hahn, D. (2021) 'Biceps femoris long head muscle fascicles actively lengthen during the nordic hamstring exercise', Frontiers in Sports and Active Living, 3:669813.

12. Roberts, T. J. (2002) 'The integrated function of muscles and tendons during locomotion', Comparative Biochemistry and Physiology, 133(4), pp. 1087-1099.

13. Thelen, D. G., Chumanov, E. S., Best, T. M., Swanson, S. C. and Heiderscheit, B. C. (2005) 'Simulation of biceps femoris musculotendon mechanics during the swing phase of sprinting', Medicine & Science in Sports & Exercise, 37(11), pp. 1931–1938.

Antonio Robustelli is the mastermind behind Omniathlete. He is an international high performance consultant and sought-after speaker in the area of Sport Science and Sports Medicine, working all over the world with individual athletes (including participation in the last 5 Olympics) as well as professional teams in soccer, basketball, rugby, baseball since 23 years. Currently serving as Faculty Member and Programme Leader at the National Institute of Sports in India (SAI-NSNIS).