Biceps Femoris Long Head Muscle Fascicles Actively Lengthen During the Nordic Hamstring Exercise.

Brent J Raiteri,Daniel Hahn,Ronja Beller

doi:10.3389/fspor.2021.669813

Abstract

Current debate exists around whether a presumed eccentric exercise, the Nordic hamstring exercise (NHE), actually causes active hamstring muscle lengthening. This is because of the decoupling that can occur between the muscle fascicle and muscle-tendon unit (MTU) length changes in relatively compliant human lower-limb MTUs, which results in MTU lengthening not necessarily causing muscle fascicle lengthening. This missing knowledge complicates the interpretation of why the NHE is effective at reducing running-related hamstring muscle injury risk in athletes previously unfamiliar with performing this exercise. The purpose of the study was therefore to investigate if the most-commonly injured hamstring muscle, the biceps femoris long head (BF), exhibits active muscle lengthening (i.e. an eccentric muscle action) during the NHE up until peak force in Nordic novices. External reaction force at the ankle, knee flexion angle, and BF and semitendinosus muscle activities were recorded from the left leg of 14 participants during the NHE. Simultaneously, BF muscle architecture was imaged using B-mode ultrasound imaging, and muscle architecture changes were tracked using two different tracking algorithms. From ~85 to 100% of peak NHE force, both tracking algorithms detected that BF muscle fascicles (n = 10) significantly lengthened (p < 0.01) and had a mean positive lengthening velocity (p ≤ 0.02), while knee extension velocity remained positive (17°·s−1) over knee flexion angles from 53 to 37° and a duration of 1.6 s. Despite some individual cases of brief isometric fascicle behavior and brief fascicle shortening during BF MTU lengthening, the predominant muscle action was eccentric under a relatively high muscle activity level (59% of maximum). Eccentric hamstring muscle action therefore does occur during the NHE in relatively strong (429 N) Nordic novices, which might contribute to the increase in resting BF muscle fascicle length and reduction in running-related injury risk, which have previously been reported following NHE training. Whether an eccentric BF muscle action occurs in individuals accustomed to the NHE remains to be tested.

Highlights

Despite extensive research efforts, hamstring injuries remain a large burden for high-speed running athletes (Lysholm and Wiklander, 1987; Opar et al, 2014; Ekstrand et al, 2016)
biceps femoris long head (BF) fascicles significantly lengthened and had a mean positive velocity while mean BF muscle activity was 59% of its maximum during the Nordic hamstring exercise (NHE). These findings provide the first direct experimental support that the NHE causes eccentric action of at least one hamstring muscle when performed by Nordic novices, which might contribute to the increase in resting BF muscle fascicle length (Bourne et al, 2017; Alonso-Fernandez et al, 2018; Presland et al, 2018; Pollard et al, 2019) and reduction in running-related injury risk (Al Attar et al, 2017; van Dyk et al, 2019) that have previously been reported following NHE training
Two different ultrasound tracking algorithms detected net BF muscle fascicle lengthening and a positive mean fascicle velocity from ∼85 to 100% of peak force during the NHE. This fascicle behavior was observed while knee extension velocity was positive over knee flexion angles from 53 to 37◦

Summary

Introduction

Despite extensive research efforts, hamstring injuries remain a large burden for high-speed running athletes (Lysholm and Wiklander, 1987; Opar et al, 2014; Ekstrand et al, 2016). The NHE induces a repeated-bout effect (Brockett et al, 2001), and training the exercise for 5–10 weeks increases muscle fascicle lengths of the BF (Bourne et al, 2017; Presland et al, 2018). Despite this indirect evidence, there is no direct experimental support that the NHE causes eccentric hamstring muscle action, and assuming muscle fascicle length changes from muscle-tendon unit (MTU) length changes is complicated because of the decoupling that can occur due to MTU compliance (Griffiths, 1991). Obtaining muscle fascicle length changes from the hamstrings (especially the BF MTU, which is relatively compliant) (Zajac, 1989) during the NHE is needed to help justify the interpretation of why this exercise is effective in reducing running-related hamstring muscle injury risk

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