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 (Lf), 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 Lf, 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 ΔLf 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 Lf 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 ΔLf, ΔFA, ΔMT, or muscle gear. The greater mean fascicle length after fatiguing exercise combined with greater ΔLf and ΔFA at longer muscle lengths during eccentric contractions might provide a mechanism for BFlh strain injury during eccentric contractions in fatiguing sports.