Effects of knee flexor submaximal isometric contraction until exhaustion on semitendinosus and biceps femoris long head shear modulus in healthy individuals

Abstract

This study examined whether a knee flexor isometric contraction at 20% of maximal voluntary isometric contraction until exhaustion would alter the biceps femoris long head (BFlh) and semitendinosus (ST) active stiffness, assessed using ultrasound-based shear wave elastography. Twelve healthy individuals participated in 2 sessions separated by 7 days. Time to exhaustion was similar in both sessions (day 1: 443.8 ± 192.5 s; day 2: 474.6 ± 131.7 s; p = 0.323). At the start of the fatigue task, the ST showed greater active stiffness than the BFlh (p < 0.001), with no differences between days (p = 0.08). The ST active stiffness then decreased from 40% of the task time to exhaustion (− 2.2 to − 13.3%, p = 0.027) until the end of the task (− 16.1 to − 22.9%, p = 0.012), while no significant changes were noted in the BFlh (p = 0.771). Immediately after the fatigue task, a decrease in active stiffness was observed in the ST (− 11.8 to − 17.8%, p < 0.001), but not in the BFlh (p = 0.551). Results were consistent between the 2 testing sessions (p = 0.07–0.959). The present results indicate that fatigue alters the hamstring active stiffness pattern.

Fatigue-induced changes in hamstrings' active muscle stiffness: Effect of contraction type and implications for strain injuries

“…These changes are opposite to those seen in ECC, despite similar reductions in MVT, suggesting that fatigue affects hamstrings load bearing differently, depending on contraction type. The greater relative load borne by BFlh during ISO is consistent with previous shear wave elastography studies 21,22 . In those studies, BFlh active stiffness remained constant during a sustained isometric contraction at 20%MVC, while that of ST was reduced, thereby increasing the BFlh/ST ratio towards the end of the task 21,22 .…”

“…In contrast, two recent SWE studies reported a decrease in ST shear elastic modulus (i.e. index of tissue stiffness, directly related to muscle force), but no change in BFlh during a low-intensity sustained isometric contraction in healthy individuals 21 and previously injured elite footballers 22 , suggesting greater fatigue in ST. These ndings support the concept of selective fatigue within the hamstrings; however, they point to different muscle being affected.…”

“…Previous studies suggested the existence of selective fatigue within the hamstrings group, however, their ndings were con icting potentially due to the different fatiguing tasks and contraction types examined [20][21][22] . Indeed, our results from direct measurement of local active muscle stiffness support this notion.…”

Regional Differences in Biceps Femoris Long Head Stiffness during Isometric Knee Flexion

“…The present results provide important methodological insights regarding the use of SWE to examine the localized muscle stiffness during contraction. Previous studies have only assessed stiffness at a single muscle region [ 8 , 9 , 10 , 23 ], assuming a homogeneous distribution along the muscle’s length. Our results show that such an assumption is flawed and that regionalized differences should be taken into consideration, particularly during low intensity contraction.…”

“…Through the use of ultrasound-based shear wave elastography (SWE), several studies have recently investigated the stiffness of different muscles in vivo during contraction (i.e., active stiffness) through estimates of localized shear modulus [ 8 , 9 , 10 ]. Although the validity of these measurements has not been fully explored, the aforementioned studies have only assessed the muscle stiffness at a single region, assuming stiffness homogeneity throughout the muscle length.…”