Spatial neuromuscular activation of the M. Biceps femoris and the force-length-relationship in isometric contractions

Abstract-Text (inkl. Referenzen und Bildunterschriften)

Despite extensive research on hamstring injuries, their incidence and time-loss rates are rising and the biceps femoris long head (BFlh) is the most prevalent muscle, primarily injured at the proximal muscle-tendon junction1. The behavior of neuromuscular activation of the BFlh in different situations and its impact on BFlh injury risk is unclear. The study aims to describe and compare regional spatial muscle activation amplitude and distribution of the BFlh in isometric contractions at short, intermediate and long Muscle-Tendon-Unit (MTU) lengths. High-density EMG was used to gain spatiotemporal insight into muscle activation and its relation to the Force-Length-Relationship (FLR) of the BFlh.

Isometric knee flexion contractions were performed using an Isomed2000 and a 64-electrode HD-EMG matrix (positioned at 35% BFlh muscle length) three MTU lengths (0°, 45°, 90° hip flexion with 45° knee flexion). Activity maps were generated from Root Mean Square values from contractions in each MTU length at a specified target knee flexion torque (constant across conditions), both of which were normalized to a maximal-effort contraction in the 45° hip position.

Results showed a decrease in the required nRMS to reach 25% Isomed torque of MVC45 with increasing MTU length, suggesting that the proximal BFlh operates on the ascending limb of the FLR. Entropy (measure for heterogeneity / homogeneity of a signal) values indicated homogenous neuromuscular activation across all trials, and did not show meaningful differences between conditions.

In summary, in this setup the BFlh operates on the ascending limb of the FLR. Findings regarding spatial distribution remain inconclusive as Entropy values indicated homogeneous activation with minimal differences between conditions. Further investigation is warranted to enhance our understanding of spatial muscle activity in the BFlh and interpret these results in the context of injury mechanisms.

1 Ekstrand, J. et al. (2023). Hamstring injury rates in men's professional football: The UEFA Elite Club Injury Study, 2001/02-2021/22. Br J Sports Med.