The Effect of Eccentric Hamstring Strength Training on Muscle Function

Abstract

The high prevalence of hamstring strain injury in sports, coupled with detrimental performance and financial effects of hamstring injuries, stress the necessity to implement an intervention capable of minimizing hamstring injuries for athletes. Nordic Hamstring eccentric strength training has shown itself to be an effective method of preventing hamstring injury. Eccentric strength training has also been shown to alter muscle architecture, joint stiffness, improve strength, and enhance dynamic performance, specifically vertical jump height. While there is limited research investigating the adaptations of the hamstrings to Nordic Hamstring training, determining these adaptations would allow for a better understanding of how the body responds to this injury-preventing stimulus. The purpose of this study was to examine the effects of Nordic Hamstring eccentric strength training on hamstring muscle architecture, stiffness, strength, and dynamic performance. We hypothesized that Nordic Hamstring eccentric strength training will increase hamstring fascicle lengths and cross-sectional area, properties of muscle stiffness as measured by shear modulus and passive knee flexor torque, maximum torque and angle of max peak torque, and vertical jump height. A total of 17 recreationally active, adult participants between the age of 18 and 21 were randomly assigned to control or experimental groups. Control subjects (n=7) performed a warm-up and static stretching for 6 weeks while experimental subjects (n=10) performed a warm-up, static stretching, and progressive Nordic Hamstring strength training for 6 weeks. Pre- and post-intervention measurements included: muscle architecture and stiffness of the biceps femoris long head using ultrasound imaging, maximal isokinetic and isometric hamstring strength measured on a dynamometer, and vertical jump height performance. Muscle volume and physiological cross-sectional area (PSCA) were calculated from the ultrasound measurements. Within and between groups two-way repeated measures ANOVAs were used to determine significant interactions and main effects with an alpha level of p<0.05. The experimental group increased volume in both hamstring muscles (biceps femoris 11%, semitendinosus 20%), with a 12% increase in PCSA for the biceps femoris long head muscle but not the semitendinosus (p<0.05). There were no changes to fascicle length, pennation angle, or stiffness that were unique to the experimental group. The 6-week intervention did not produce any significant group by time interactions for concentric, isometric, eccentric strength measurements or peak passive torque measurements. Vertical jump height was also not affected by the intervention. Overall, the 6-week Nordic Hamstring training intervention was a good training method for muscle growth and this adaptation in architecture translated to changes in eccentric hamstring muscle strength, although these changes did not reach statistical significance largely due to the dichotomy of responses in our training group. A sub-group analysis of the experimental subjects revealed apparent "responders" and "non-responders" to the training stimulus. From a clinically-relevant standpoint, the Nordic Hamstring training was 70% effective at improving muscle function for participants in this study. Future exploration of the mechanics behind the Nordic Hamstring strength training exercise, shown to reduce hamstring injury, is a necessary step in understanding muscular adaptations to resistance exercise and enhancing training effectiveness on hamstring muscle function.