Effects of Nordic Hamstring Exercise on Knee Joint Muscle Strength in Adolescent Soccer Players. A Pilot Study.
Authors: Studnicki Rafał Jacek , Taraszkiewicz Marcin , Tomaszewicz Urszula , Koelmer Anna 
Published: July 2024

Introduction:The high interest in sports among young people has contributed to the growing importance of the phenomenon of sports specialization in recent years. It refers to players who train a particular sport for more than 80h per month. This is particularly noticeable in the group of young soccer players, in whom such a high training volume, is an important factor predisposing to the development of injuries. An important risk factor is the common lower limb muscle imbalance in the soccer player population between the extensors and flexors of the knee joint, showing a maximum isometric force ratio as low as 1:2. To compensate for this imbalance, it is important that eccentric exercises have a positive effect by improving the strength of the knee joint flexor muscles, especially the well-studied Nordic hamstring exercise(NHE). However, many studies only evaluate the effect of modified NHE exercise training volume, without isolating the effects of performing a single exercise session. It is worth noting that, there are warm-up protocols that incorporate the aforementioned exercise and enjoy high utility and effectiveness for sports injury prevention. Objective: Given the emerging imbalance in the strength of the knee stabilizing muscles, the aim of the study was to determine whether performing a single NHE intervention would alter the isometric strength of the knee flexor muscles (HAM). Methods: A group of 56 players of a youth soccer team (age range 13-16), were recruited for the pilot study. In doing so, the players constituted a group of healthy, training soccer players at the central junior level. After meeting the inclusion criteria, 32 players were entered into the study. Participants were randomly assigned to a study group (A) and a control group (B). In the study group, the intervention consisted of a single NHE exercise in the form of two series of trunk slumps in a kneeling position, eight repetitions each. The control group performed one series of forward trunk bends, five repetitions each. Participants (group A and B) were tested on the ` platform before the experimental procedure and immediately after the intervention. Maximum isometric strength of the knee joint extensors (QUAD) and knee joint flexors (HAM) were measured. Results: Statistical analysis of the obtained results indicated that the maximum muscle strength of the HAM increased after the performed, single intervention NHE (p=0.04) relative to the control group. At the same time, no increase in the strength of the quadriceps muscles of the thigh (extensors of the knee joint) was shown after the intervention, relative to the control group. Conclusions:The results obtained indicate that a single performance of the NHE exercise has an effect on increasing the maximum isometric strength of the HAM muscles. This increase, although not statistically significant, correlated with no change in quadriceps muscle strength. The work is a pilot study and further research may benefit in maintaining the balance between the two muscle groups, namely HAM and QAD, which contributes to the prevention of joint-muscle injuries. At the same time, the study supports the previously stated thesis that a single performed Nordic Eccentric Exercise is an important element in the prevention of muscle injuries in the lower limb, thus confirming the effectiveness of its use in football warm-ups. Nonetheless, implementation of the exercise into long-term training is necessary to achieve significant effects of muscle balance compensation in the lower limb, as confirmed by numerous training protocols.