The evidence indicates that the hamstrings do not behave isometrically during the swing phase of the sprint cycle: a narrative review

In opposition to the majority of research, it has recently been suggested that the hamstrings behave isometrically during the swing phase of sprinting. Despite little evidence, this theory postulates that hamstring muscle slack enables passive hamstring lengthening during the initial- and mid-swing phases of sprinting. It has also been theorised that the hamstrings then contract isometrically in the late-swing phase whilst the hamstring tendons stretch and store elastic energy. There are a number of deficiencies with this theory. Evidence from electromyogram studies demonstrate that the hamstrings are active during the entirety of the sprint cycle, indicating that hamstring muscle slack does not exist whilst sprinting and that hamstring lengthening occurs eccentrically. Additionally, there are substantial mechanical benefits to eccentric muscle behaviour that do not present under isometric conditions, such as force enhancement, force augmentation per unit of energy cost, and higher contraction velocities through sarcomere recoil. All of which are expected to contribute to hamstring muscle functioning whilst sprinting. Kinematically, joint velocity and acceleration profiles further refute the idea that the hamstring is functioning isometrically when sprinting. Kinetically, hamstring forces exceed isometric capacity during the late-swing phase of sprinting, dictating that eccentric contributions are required in this phase. Collectively, the evidence indicates that passive and isometric behaviour of the hamstrings in the swing phase of sprinting is unlikely, with the hamstrings acting eccentrically instead. Suggestions that isometric exercises should be prescribed on the basis that they mimic isometric hamstring muscle function whilst sprinting, are unreasonable and should be dismissed.