The Effect of Fatiguing Exercise on Biceps Femoris Fascicle Behaviors During Maximal Isokinetic Contractions

The present study examined the acute effects of fatiguing, non-damaging exercise on biceps femoris long head (BFlh) fascicle strain and rotation at long and short muscle lengths during concentric versus eccentric contractions. BFlh fascicle length (L_f), fascicle angle (FA), and muscle thickness (MT) were quantified in 18 (8 women) healthy adults without a history of right hamstring strain injury. BFlh images were assessed using two serially aligned ultrasound devices whilst maximal concentric and eccentric isokinetic knee flexions were performed at 180°/s before and after fatiguing exercise. Ultrasound videos were exported and edited to create a single, synchronized video, and three fascicles were analyzed through the range of motion 10°-80° (0° = full knee extension). Changes (Δ) in L_f, FA, and MT at long (60°-80° knee angle) and short (10°-30°) muscle lengths and across the full knee flexion range were measured and compared. Greater ΔL_f was observed at long muscle lengths during eccentric contractions (p < 0.001) and at short muscle lengths during concentric contractions (p < 0.001). Average BFlh L_f was significantly longer (p < 0.001) after fatiguing exercise in both contraction modes. ΔFA was greater (p < 0.05) at long muscle length and during eccentric contractions, and ΔFA over the full ROM was greater in eccentric contractions (p < 0.001). No significant effect of fatigue was detected on ΔL_f, ΔFA, ΔMT, or muscle gear. The greater mean fascicle length after fatiguing exercise combined with greater ΔL_f and ΔFA at longer muscle lengths during eccentric contractions might provide a mechanism for BFlh strain injury during eccentric contractions in fatiguing sports.