Eccentric vs. concentric muscle contraction: That is the question

Fascicle Behavior and Muscle Activity of The Biceps Femoris Long Head during Running at Increasing Speeds

Hamstring strain injuries (HSIs) are prevalent in sports involving high-speed running and most of the HSIs are biceps femoris long head (BFlh) injuries. The primary cause for HSIs during high-speed running remains controversial due to the lack of in vivo measurement of the BFlh muscle behavior during running. Therefore, the purpose of this study was to quantify the muscle-tendon unit (MTU) and fascicle behavior of BFlh during running. Seven college male sprinters (22.14 ± 1.8 years; 177.7 ± 2.5 cm; 70.57 ± 5.1 kg; personal bests in 100m: 11.1 ± 0.2 s) were tested on a motorized treadmill instrumented with two force plate for running at 4, 5, 6m/s. The ground reaction force (GRF), 3D lower limb kinematics, EMG, and ultrasound images of biceps femoris long head (BFlh) in the middle region were recorded simultaneously. BFlh fascicles undergo little length change (about 1 cm) in the late swing phase during running at three submaximal speeds. BFlh fascicle lengthening accounted for about 30% of MTU length change during the late swing phase. BFlh was most active during the late swing and early stance phases, ranging from 83%MVC at a running speed of 4 m/s to 116%MVC at 6 m/s. Muscle fascicles in the middle region of BFlh undergo relatively little lengthening relative to the MTU in the late swing phase during running in comparison to results from simulation studies. These results suggest that there is a decoupling between the fascicle in the middle region and MTU length changes during the late swing phase of running.

Intra- and Inter-Muscular Variations in Hamstring Architecture and Mechanics and Their Implications for Injury: A Narrative Review

Understanding the architecture, anatomy, and biomechanics of the hamstrings may assist in explaining the mechanisms that affect and improve their function. The aim of this review is to specifically examine intra- and inter-muscular variations in architecture and mechanical properties of the hamstrings. Of the hamstrings, the long head of the biceps femoris shows the shortest and more pennated fibers. The semimembranosus has a similar muscle architecture with a long head of the biceps femoris but it has a different proximal attachment as well as a different moment arm compared with the long head of the biceps femoris. For the same joint motion, the semitendinosus displays less relative strain than the other hamstrings probably owing to a greater length, longer fascicles and, possibly, a longer tendon. Intra-muscular variations in architecture are documented but their implications are currently unclear. Proximally, the long head of the biceps femoris has shorter and more pennated fibers coupled with a narrower aponeurosis than distally, while the semitendinosus is the only muscle that entails a tendinous inscription. In conclusion, some of the identified intra- and inter-muscular variations in architecture may help explain why some muscles sustain injuries more than others. In the same line, exercises designed for the hamstrings may not provide the same stimulus for all components of this muscle group. Future research could examine whether intervention strategies that target specific muscles or specific areas of the hamstrings may offer additional benefits for injury prevention or rehabilitation of their function.

Stretch Could Reduce Hamstring Injury Risk During Sprinting by Right Shifting the Length-Torque Curve

Ruan, M, Li, L, Chen, C, and Wu, X. Stretch could reduce hamstring injury risk during sprinting by right shifting the length-torque curve. J Strength Cond Res 32(8): 2190-2198, 2018-It was hypothesized that static stretch would shift the length-torque curve to the right, which may reduce the risk of muscle strain injuries. The purpose of this study was to evaluate the acute effects of static stretching of hamstring (SSH) on the risk of hamstring injury during sprinting indicated by the shift of the length-torque relationship. Twelve female college athletes (age: 20.8 ± 0.7 years; height: 1.61 ± 0.05 m; body mass: 54.25 ± 4.22 kg) participated in this study. Subjects performed overground sprinting under 2 conditions: after warm-up with 4 × 30 seconds SSH or after warm-up without SSH. Three-dimensional kinematic and kinetic data and electromyography of biceps femoris long head (BFlh), rectus femoris, and vastus medialis were collected during testing. The maximum length of BFlh during late swing phase increased after SSH with large effect size and close to statistically significant (p = 0.05, d = 1.22), but the knee flexion torque at the peak length did not change significantly. Static stretching of hamstring significantly reduced peak values of both horizontal (d = 1.46) and vertical (d = 1.79) ground reaction forces, and BFlh's activation level during the preactivation (late swing) phase (p = 0.05, d = 2.16). The results indicated that the length of BFlh-knee torque relationship and the length of BFlh-hip torque relationships during the late swing phase and initial stance phase were shifted to the right after SSH, which may reduce risk of hamstring strain injuries. We suggest that preactivity static stretching should not be simply removed and participators should give priority to stretch muscles that are vulnerable to strain injuries.