Author(s): WALKER, J., BROGDEN, C., MARTIN, J., JONES, A., ALT, T., NICHOLSON, G., Institution: LEEDS BECKETT UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 1007

INTRODUCTION:
Knee flexors are the most frequently injured muscle-tendon unit (MTU) in sporting populations. The Nordic Hamstring Exercise (NHE) is often used to reduce injury risk, as the knee flexors undergo large amounts of negative work (sometimes termed “eccentric load”), possibly leading to a favourable shift in knee flexor length-tension properties. NHE are characterised by metrics like the “break point angle”, implying that an athlete is unable to complete a full NHE, thus not exposing the MTU to load at longer lengths. Therefore, variations of NHE and training progression models have been proposed [1], theorising to train MTUs at longer lengths, albeit at lower force levels, to mitigate injury risk. This study aimed to understand biceps femoris long head (BFlh) neuromechanical activity during different NHE variations.
METHODS:
Sixteen recreationally active individuals took part in a protocol where participants completed movement from an established progression model [1] until failure. Movements included NHE variations that involved static and dynamic components with a range of lower-limb kinematic patterns (including hip and knee flexion angles, and shank inclination), all using the HamsHell NHE training device [1]. Proximal and distal muscle activity patterns were measured at 1,000 Hz using high-density electromyography (Delsys, Inc., USA). Joint kinematics were obtained at 100 Hz with a 14-camera optoelectronic system (Qualisys AB, Sweden) and were used to calculate BFlh MTU lengths [2].
RESULTS:
MTU kinematic patterns were significantly affected by exercise variation, where variations involving hip flexion increased peak length (p < 0.001), and variations with dynamic hip or knee angle changes increased MTU excursion (p < 0.001). Although there were no significant differences between regions for mean muscle activity (p ≥ 0.567), a region × exercise interaction (p = 0.005) showed that the proximal region of BFlh appears more responsive to NHE variations (p ≤ 0.028). These findings were corroborated by a time-series analysis of proximal and distal muscle activity across the exercise duration, displaying with extremely large effect sizes between regions (Cohen’s d > 0.9).
CONCLUSION:
These findings demonstrate that NHE variations involving hip flexion can be used to target longer MTU lengths, overcoming the limitation of the standard NHE version in lower-performing individuals. This insight offers a trade-off between MTU force and operating length, which is important when considering adaptations to length-tension properties and serial sarcomere number through NHE training modalities, with the aim of reducing knee flexor strain injury incidence. Given NHE variations preferentially cause responses in the proximal region of BFlh, these should be considered given the higher injury rates in this region, particularly in young athletic populations.
References:
[1] Alt, T et al. (2023) J Athletic Training;58:329-37
[2] Hawkins, D & Hull, ML (1990) J Biomech;23:487-94