Biceps femoris long head sarcomere and fascicle length adaptations after three weeks of eccentric exercise training

PurposeEccentric exercise is widely used to increase muscle fascicle lengths and thus decrease the risk of muscle strain injuries. However, the mechanisms behind this protection are still unknown. The aim of this study was to determine whether Biceps femoris long head (BFlh) fascicle length increases in response to three weeks of eccentric exercise training are the result of addition of in-series sarcomeres within muscle fibres. MethodsTen recreationally active participants (age: 27 {+/-} 3 years, mass: 70 {+/-} 14 kg, height: 174 {+/-} 9 cm) completed three weeks of Nordic hamstring exercise (NHE) training. We collected in vivo sarcomere and muscle fascicle images of the BFlh in two regions (central and distal), utilising microendoscopy and 3D ultrasonography. These images allowed us to estimate sarcomere length, sarcomere number and fascicle lengths before and after the training intervention. ResultsEccentric knee flexion strength increased after the training (15%, P < 0.001, {eta}p2= 0.75). Further, we found a significant increase in fascicle (21%, P < 0.001, {eta}p2 = 0.81) and sarcomere (17%, P < 0.001, {eta}p2 = 0.9) lengths in the distal but not in the central portion of the muscle. The estimated number of in series sarcomeres did not change in either region. ConclusionFascicle length adaptations appear to be heterogeneous in the BFlf in response to three weeks of NHE training. An increase in sarcomere length, rather than the addition of sarcomeres in series, appears to be underlying this adaptation. The mechanism driving regional increases in fascicle and sarcomere length remain unknown, but we speculate it may be driven by regional changes in the passive tension of muscle or connective tissue adaptations.