Bilateral erector spinae and biceps femoris co-activation following unilateral hamstring injury in young sprinters

This study compared coactivation and strength of the erector spinae (ES) and biceps femoris long head (BFLH) during isometric knee flexion and trunk extension in sprinters with a recent BFLH injury. Twenty-four sprinters with grade 2 BFLH injury and 21 controls performed prone trunk extension and knee flexion maximal voluntary contractions (MVCs) at 90° and 0° knee flexion. Torque and surface electromyography (EMG) of the ES and BFLH were recorded bilaterally and normalized to MVCs during 30° trunk extension and 0° knee flexion, respectively. Previously injured athletes showed a greater co-contraction index (CCI) during knee flexion at 0° and 90° than controls (p < 0.001), with higher ES activation (32.52 ± 17.08 to 47.72 ± 17.83% MVC) than the uninjured group (21.56 ± 14.52 to 44.68 ± 18.02% MVC; p < 0.05). No group differences were found in normalized BFLH EMG, CCI during trunk extension, or absolute knee flexion or trunk extension strength (p > 0.05). Previously injured sprinters showed larger between-limb knee flexion strength asymmetries (>10%, p < 0.01), which were absent in controls. This paraspinal overactivation and increased trunk–hamstring co-contraction after BFLH injury suggest that lumbopelvic control exercises may be beneficial for preventing BFLH re-injury.