No eccentric hamstring action during running?

It is widely believed that the hamstrings undergo an eccentric muscle fiber action (they are lengthening while being activated) during the swing phase of high-speed running. Therefore, eccentric exercises are being incorporated into training programs to prevent hamstring injuries and improve sports performance as it is thought that they specifically replicate the hamstring functioning during running. However, in a recent review, we argued that there may actually be no eccentric, but rather an isometric hamstring muscle fiber action during the swing phase of high-speed running [1].

Evidence from animal studies

Animal studies directly measured muscle fascicle length changes using sonomicrometry and showed that the hamstrings first passively lengthen and when the muscle fibers are activated later during the swing phase, the tendinous tissues stretch and recoil, while the muscle fibers remain isometric (Figure 1). Therefore, the evidence from animal studies suggests that there is no eccentric muscle fiber action during running.

In humans it is not possible to directly measure muscle fascicle length changes using sonomicrometry. Therefore, fascicle length changes have been estimated using computational modelling studies. In contrast to the animal studies, these modelling studies found an eccentric hamstring muscle fiber action during running. However, these modelling studies did not correctly simulate several mechanical processes such as muscle slack and muscle gearing, which has probably lead to incorrect results [1, 2]. When these mechanical processes would have been correctly simulated, the modelling studies would probably also have found an isometric muscle fiber action (Figure 2).

There is currently no strong evidence for an eccentric muscle fiber action during running and the evidence from animal studies actually suggest that there is an isometric muscle fiber action [1, 2]. Based on this, it can be argued that high-intensity isomeric exercises (see Figure 3 for an example exercise) more specifically replicate hamstrings functioning during high-speed running than eccentric exercises and that these exercises are therefore also highly effective at preventing injuries and improving performance [4]. However, more research is required to investigate the effectiveness of these isomeric exercises.

References

1. Van Hooren B, Bosch F. 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. J Sports Sci. 2016;Epub ahead of print:1-9. doi:10.1080/02640414.2016.1266018.
2. Van Hooren B, Bosch F. Preventing hamstring injuries – Part 2: There is possibly an isometric action of the hamstrings in high-speed running and it does matter. Sport Performance & Science Reports. 2018.
3. Gillis GB, Flynn JP, McGuigan P, Biewener AA. Patterns of strain and activation in the thigh muscles of goats across gaits during level locomotion. J Exp Biol. 2005;208(Pt 24):4599-611. doi:10.1242/jeb.01940.
4. Van Hooren B, Bosch F. Is there really an eccentric action of the hamstrings during the swing phase of high speed running? Part II: Implications for exercise. J Sports Sci. 2016;Epub ahead of print. doi:10.1080/02640414.2016.1266019.