Muscle power is one of the most important parameters in almost every athletic action, and expresses the ability of the human muscle to produce great amounts of force with the greatest possible speed. Thus, muscle power is critical for high performance in athletic actions such as jumping, throwing, change of direction and sprinting. For enhancing their muscle power, athletes comprise several resistance training programs as part of their training. Core exercises as well as Olympic lifting has been used in muscle power training. The loads that are applied regarding the accomplishment of the most favorable power production are varying. Training load of 0% 1RM has been reported to favor power production at the countermovement squat jump, while loads of 56% 1RM and 80% 1RM, favored the power production at squat and clean, respectively. In the recent years, accentuated eccentric training has been proposed as a new training method for the enhancement of muscle power. This method emphasizes in the eccentric component of the muscle contraction, and there is evidence supporting the greater production of muscle force after accentuated eccentric training compared with the typical resistance exercise training method. Taking the above into consideration, muscle power training comprises of eccentric muscle actions, and the magnitude of the eccentric component depends on the emphasis that is given on the concentric or eccentric action, respectively, of the muscles during the exercises. However, eccentric muscle action, especially when unaccustomed, can lead to exercise-induced muscle damage (EIMD). Although concentric and isometric exercise may also lead to muscle injury, the amount of damage after eccentric muscle contractions is greater. EIMD, amongst others, is accompanied by increased levels of creatine kinase (CK) into the circulation, increased delayed onset of muscle soreness (DOMS), reduction of force production, reduction of agility and speed. Despite the fact that muscle power training comprises eccentric muscle actions and consequently can lead to muscle injury and muscle performance reduction during the following days, the recovery kinetics after acute muscle power training protocols have not been adequately studied. However, information regarding the recovery of the muscles after a power training protocol is critical for the correct design of a training microcycle, and the reduction of injury risk.

The aim of the present study is to investigate the muscle injury provoked after acute muscle power training using three different power training exercise protocols. Additionally, the effect of these protocols on muscle performance and neuromuscular fatigue indices will be examined.

According to a preliminary power analysis, a number of 8 - 10 participants is needed for significant differences to be observed at the variables that will be examined (α = 0.90). Thus, 10 participants will be included at the present study.

The study will be performed in a randomized, cross over, repeated measures design. During their 1st - 4th visit, all participants will sign an informed consent (1st visit) after they will be informed about all the benefits and risks of the study and they will fill and sign a medical history form. Participants will be instructed by a dietitian how to record a 7-days diet recall to ensure that they do not consume in greater extent nutrients that may affect EIMD and fatigue (e.g. antioxidants, amino acids, etc.) and to ensure that the energy intake during the trials will be the same. Subsequently, participants will have to be familiarized with the exercises that will be used during the three power training protocols, as well as with the measurements that will be used for the evaluation of performance indices.

During the 5th, 6th, 7th and 8th visit, baseline assessments will be performed. Fasting blood samples will be collected in order to estimate muscle damage concentration markers. Assessment of body mass and body height, body composition, and aerobic capacity (VO2max), will be performed. Squat jump and countermovement jump will be performed on a force platform to assess jump height, ground reaction force, peak and mean power, vertical stiffness and peak rate of force development; at the same time, peak and mean normalized EMG during the concentric phase of the squat jump, and during eccentric and concentric phases of the counter movement jump, for the vastus lateralis, biceps femoris, gastrocnemius, and gluteus maximum muscles will be assessed. The peak concentric, eccentric and isometric isokinetic torque of the knee flexors and knee extensors, in both limbs will be evaluated on an isokinetic dynamometer at 60°/sec. Maximal voluntary isometric contraction (MVIC) of the knee extensors at 65° in both limbs, as well as the fatigue rate during MVIC through the percent drop of peak torque between the first and the last three seconds of a 10-sec MVIC.

During their 9th visit, participants will be randomly assigned into one of the four different conditions of the study design: a) Core exercises protocol, b) Structural exercises protocol, c) Accentuated eccentric load exercises protocol, d) Control Condition. Prior to each experimental protocol, assessment of DOMS in the knee flexors and knee extensors of both limbs, as well as blood lactate assessment will be performed. Field activity will be continuously recorded during the sprint training protocols using global positioning system (GPS) technology. Heart rate will be continuously recorded during the sprint training protocols using heart rate monitors. Additionally, DOMS of knee flexors and knee extensors, peak concentric, eccentric and isometric isokinetic torque, squat and countermovement jump height, as well as ground reaction force, peak and mean power, vertical stiffness and peak rate of force development during squat and countermovement jump, alongside with peak and mean normalized EMG during the concentric phase of the squat jump, and during eccentric and concentric phases of the counter movement jump, for the vastus lateralis, biceps femoris, gastrocnemius, and gluteus maximum muscles will be assessed immediately after, 24h, 48h and 72h after the end of the trial. MVIC of the knee extensors of both limbs, as well as the fatigue rate during MVIC will also be assessed at 1h, 2h and 3h, as well as 24h, 48h, and 72h (10th, 11th and 12th visit) after the end of the trial. Blood lactate will also be assessed at 4 min, while creatine kinase at 24h, 48h, and 72h after the end of the trial. The exact same procedure (13rd - 16th visit, 17th - 20th visit, 22nd - 24th visit) will be repeated for the remaining three conditions. A 7-day wash out period will be mediated between trials.