Effects Of 4 Weeks Of Nordic Hamstring Exercise On The Strength And Structure Of Hamstring In People With Hamstring Strain Injury

Objective: The purpose of this study was to determine the effect of strength training on hamstring and determine the duration of protective effect of 4-week training on hamstring by measuring the strength and muscle structure parameters of hamstring at different time periods in the training of Nordic hamstring exercise(NHE).This study provided a biomechanical basis for the influence of NHE on hamstring,laid a theoretical foundation for the prevention and rehabilitation of hamstring strain injury and clinical research on other muscle strains in the future,and provides guidance for sports training and injury prevention in mass sports.Methods: 11 track and field male athletes with unilateral hamstring strain injury occurred during sprint were selected in this study.NHE was used to carry out intervention training for 4 weeks.The muscle strength of bilateral hamstring was measured before,after and two weeks after training by the Nordic hamstring strength tester.The muscle thickness,pennation angle and muscle fascicle length of the biceps femoris long head on both sides were measured before,after and two weeks after training by colour doppler ultrasound instrument(B-mode).Isokinetic dynamometry was used to measure the muscle strength of hamstring at 60°/s and 180°/s before and after training.Two-factor measurement variance analysis was used to determine the effect of injury and training on the muscle strength of the hamstring in the NHE and isokinetic strength tests and muscle parameters of biceps femoris long head..Results: There was an interaction between injury factors and training intervention factors of the muscle fascicle length of the biceps femoris long head(p=0.044).But there is no interaction between the muscle strength of NHE,the peak torque and the angle of peak torque of hamstring in isokinetic strength test,and the pennation angle and muscle thickness of the biceps femoris long head.The muscle fascicle length of the injured side was significantly longer than that of the non-injured side before and after intervention(+10.4%,p=0.027,d=0.92cm;+9.8%,p=0.009,d=0.94cm).After the intervention training,the muscle fascicle length of the injured side and the non-injured side increased significantly(+8.0%,p=0.012,d=0.78cm;+8.6%,p=0.046,d=0.75cm),and the muscle fascicle length of the non-injured side measured two weeks after training was significantly longer than the pre-training state(+10.8%,p=0.011,d=0.95cm).After the intervention training,the average peak force and the maximum peak force of hamstring in NHE were significantly increased(+10.4%,p=0.006,d=0.47N/kg;+8.0%,p=0.007,d=0.39N/kg)and still significantly higher than the pre-training state in the two weeks after intervention(+9.3%,p=0.022,d=0.42N/kg;+8.3%,p=0.011,d=0.40N/kg).After the intervention training,the muscle thickness of hamstring increased significantly(+3.5%,p=0.032,d=0.08cm),and the pennation angle decreased significantly(-5.1%,p=0.019,d=-0.72°).The pennation angle measured two weeks after training was significantly smaller than the pre-intervention state(-7.3%,p=0.008,d=-1.04°).The concentric peak torque of injured side at 180°/s was significantly lower than that of the non-injured side(-2.4%,p=0.036,d=-0.05N~*m/kg).After the intervention training,the eccentric peak torque of hamstring at 180°/s significantly increased(+11.6%,p=0.019,d=0.29N~*m/kg)and the angle of the eccentric peak torque significantly decreased(-18.7%,p=0.008).Conclusion:(1)There was no significant difference in the strength performance of the injured and non-injured hamstring in NHE and isokinetic strength tests.But the muscle fascicle length of the biceps femoris long head of the injured side is longer than that of the non-injured side.(2)4 weeks of NHE intervention training can significantly increase the strength of hamstring in NHE,and can lead to architecture alternations in the biceps femoris long head.At the same time it can improve the maximum eccentric force of hamstring at a relatively fast speed and reduce the angle of knee joint when generating the maximum force.(3)At two weeks after the intervention,the muscle strength of hamstring in NHE can still maintain a high level,while the pennation angle and the muscle fascicle length on the non-injured side of the biceps femoris long head were able to maintain the training effect to two weeks after the training.