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Research Review

Excessive Muscle Strain in Concentric Contractions A recent study conducted by Liu et al. (2017) reaffirmed the risk of hamstring injury during the late swing phase, but challenged the existing wisdom by proposing that it was the early stance phase, not the late stance phase, that posed the greater risk of hamstring injury. The authors hypothesized that it was therefore the period of transition between swing and stance phases where hamstring injuries were most likely to occur. Liu et al. proposed that hamstring injuries during sprinting were most likely caused by large passive forces at the knee and hip joints, which result in the lengthening of the hamstring muscles. These forces were found to be highest during the transition between the swing and stance phases, where highforce eccentric and concentric contractions occur. In response to this paper, Yu, Liu & Garrett (2017) published a rebuttal stating that there was no theoretical or experimental evidence that supported the view that concentric contraction could contribute to hamstring injury.

Something Different? An editorial in the Journal of Sport and Health Science written by Walter Herzog (2017) challenged both proposed mechanisms and raised an important consideration regarding eccentric vs. concentric contractions. Herzog posited that it may not be the stretching of the muscle tendon unit that is important when considering hamstring injuries, but the elongation of the muscle fibres and fascicles. Previous

Maybe we have it all wrong...Maybe hamstring injuries have nothing to do with eccentric loading...Maybe hamstring injuries are not mechanically mediated, but by small changes in hamstring activation when attempting to come from behind or when tightening up in a losing position Walter Herzog

research has demonstrated that when muscle force increases, fascicle lengths tend to decrease (de Brito Fontana & Herzog, 2016). During the late swing phase of sprinting there may be both eccentric contraction (the stretching of

BELOW Phases of the running gait including the phases of hamstring eccentric contraction highlighted according to Yu et al. (2008)

the muscle tendon unit) and concentric contraction (shortening of the muscle fibres and fascicles). Herzog also proposed an alternative hypothesis, suggesting that hamstring injuries may primarily be caused by changes in hamstring activation caused by psychological factors during competition. He gives the example of sprinters rarely pulling up with an injury while leading a race - is this because of excessive strain when trying to catch up or from



the loss of focus when an athlete considers a race unwinnable?





Ultimately, it appears that at this stage there is not enough evidence to definitively understand the causes of hamstring injuries during maximum-speed running. However, we do have agreement that the

High Concentric Forces

High Eccentric Forces

late swing phase is a period of high injury risk and as coaches we are able to use this information to assist in the design of prevention strategies to best support our athletes.

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Athletics Coach - Issue 3, 2018