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.

In the last decade there has been a growing interest in infrared thermography in the field of sports medicine in order to elucidate the mechanisms of thermoregulation. The aim of this study was to describe bilateral variations in skin temperature of the anterior thigh and patellar tendon in healthy athletes, to provide a model of baseline tendon and muscle thermoregulation in healthy sprinters following a unilateral isokinetic fatigue protocol. Fifteen healthy national-level sprinters underwent unilateral isokinetic force testing and electrostimulation in which body temperature was measured before, during, and after the protocol using an infrared thermographic camera. ANOVA detected a significant difference in the time x side interaction for patellar temperature changes (p≤0.001) and a significant difference in the time x side interaction for quadriceps temperature changes (p≤0.001). The thermal challenge produces homogeneous changes evident in quadriceps areas, but not homogeneous in tendon areas. these data show that metabolic and blood flow changes may depend on the physical and mechanical properties of each tissue.

Isokinetic testing of the knee joint is the “gold standard” in strength assessment in sport especially in soccer, where it is part of pre-season screening to establish norms in case of injuries. There are many devices on the market that make it difficult to use of normative data. Isokinetics literature in females’ soccer players is scarce in comparison to males. Furthermore, evaluation protocols strongly differ from each other, using different ranges of speeds, movement patterns and output data. Thus, it is difficult for the researcher and the clinician to choose a correct protocol for isokinetic evaluation in female soccer players. The indexes of isokinetic performance that are widely accepted are the hamstring/quadriceps (H/Q) ratio and dominant/non-dominant (D/ND) ratio as reference. In this review are highlighted some common features of isokinetic testing in female soccer players of different levels of qualification. From this study, it emerges that, for a reliable test, 3 reps at the speed of 60°/sec in knee flexion/extension is preferable due to the high reliability and that an optimal H/Q ratio in healthy female soccer players of different level of qualification and age is 50%, while the interlimb differences (D/ND) should be below 10%.