Background The majority of hamstring strain injuries (HSIs) occur in the late swing phase of high-speed running when hamstrings work eccentrically. The proximal region of the biceps femoris (BF) seems to be the most common site of injury. Elongation of the fascicles in eccentric contractions is higher in the proximal region compared to the distal region of BF, but the neural background to this phenomenon is not clear.
Objective To examine the region-specificity of BF muscle activity at different running speeds.
Design Observational study. Running speeds were randomized.
Setting Laboratory, amateur level.
Participants Football players without acute injury/illness, and without HSI history were recruited. Data collection is ongoing; six players have been measured and analysed so far.
Assessment After warm-up, players ran on a treadmill at 12, 15, 18 km/h, and maximum speed. Electromyographic activity (EMG) along the BF and joint angular displacements were recorded with a 16-channel high-density EMG array and 3-D motion analysis, respectively. EMG data were normalized to maximal voluntary isometric contraction.
Main Outcome Measures Normalized root-mean-square activity was calculated in the proximal, middle and distal muscle regions, and BF elongation was calculated using musculoskeletal modelling.
Results Higher BF elongation and activity were observed in all regions with increasing speed. Activity in the proximal region was lower compared to the distal region at 15, 18 km/h and maximum speed, by 12, 18, and 35%, respectively.
Conclusions Lower relative activity in the proximal region and greater proximal-distal differences in activity with increasing speed may indicate why HSIs occur at higher speeds, and mainly in the proximal region. Lower proximal activity may be associated with the higher elongation in this region during eccentric contractions that has been reported previously. Future studies are needed to examine whether players with higher within-muscle heterogeneity in muscle recruitment are at higher risk for HSI.