An in vivo investigation of residual force enhancement in healthy hamstring muscles

Background: Despite the prevalence of hamstring strain injuries and significant research interest, there is debate whether eccentric or isometric muscular contractions are most effective at modifying hamstring strain injury risk and rehabilitating injury. Post-stretch isometric (PS-ISO) contractions, incorporate both eccentric and isometric stimuli and have been shown to enhance isometric torque output, (termed residual force enhancement, rFE). This combination of contraction modes and subsequent effects may be beneficial in hamstring strain injury prevention. It is uncertain whether the hamstrings can generate rFE during PS-ISO contractions in vivo under relevant conditions and whether physical contractile element lengthening would occur in the eccentric phase. It is also unclear whether rFE would endure throughout multiple consecutive PS-ISO contractions as per a resistance training situation.
Research Aim: The aim of this thesis was to determine whether the knee flexors can reliably generate rFE during highly controlled PS-ISO contractions under a range of conditions relevant to hamstring strain injury prevention and rehabilitation.
Methods: Based on a broader literature review, a series of four studies were conducted targeting the thesis aim. A systematic review of rFE in vivo human muscles using voluntary PS-ISO contractions provided methodology for the three experimental studies. Experimental study one investigated the effects of PS-ISO contractions in the knee flexors using maximal contractions at long musculotendinous unit lengths. Experimental study two investigated the effects of PS-ISO contractions at both maximal and submaximal intensities and over long and short joint rotations. Ultrasonography provided direct muscle and tendon length change analysis of the biceps femoris long-head muscle during PS-ISO contractions. Finally, based
on study one and two, we investigated the effects of PS-ISO contractions under training conditions, a series of sets of consecutive submaximal PS-ISO repetitions.
Major Conclusions: The systematic review revealed that rFE of varying magnitude is observable in several muscle/joint arrangements in vivo. The experimental studies provide evidence that rFE was consistently generated in the knee flexors using single and multiple PS-ISO contractions incorporating stretch of the contractile element at maximal and submaximal contraction intensity, without increased muscle activation. We theorise that passive structures within the musculotendinous unit likely contributed additional force. We conclude that PS-ISO are likely to be beneficial in resistance training programs where it is desirable to provide a controlled eccentric stimulus, coupled with elevated isometric torque at lower activation levels. Specifically, there are implications for the potential application of PS-ISO contractions in hamstring strain injury prevention.