Speed is one of the most important physical capacities for many sports, especially those that include speed and power as a major element, and plays a major role on performance. Thus, running speed improvement consists one of the most basic aims of a sprinter's and a power athlete's training program. One of the most commonly used strategies to improve the initial acceleration phase, is resisted sprint training. Evidence suggests that resisted sprint training is more effective in improving acceleration compared to sprint training without additional load. Sprinting is performed through the stretch-shortening cycle, where the pre-activated muscle is first stretched (eccentric action) and then followed by the shortening (concentric) action. Thus, sprint training highly includes the component of eccentric contraction. 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. Nevertheless, despite the fact that sprint 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 sprint training have not been adequately studied. However, such information is critical for both coaches and athletes to effectively design the training microcycles and incorporate the training components, as well as to reduce injury risk.

The aim of the present study is to examine the recovery kinetics of EIMD indices, muscle performance and neuromuscular fatigue, after different sprint training protocols.

According to a preliminary power analysis (a probability error of 0.05, and a statistical power of 80%), a sample size of 8 - 10 subjects per group was considered appropriate in order to detect statistically meaningful changes between groups.

The study will be performed in a randomized, cross over, repeated measures design. During the first 1st and 2nd visit, all participants will sign an informed consent form after they will be informed about all the benefits and risks of the study and they will fill in and sign a medical history questionnaire. Fasting blood samples will be collected in order to estimate muscle damage concentration markers. Participants will be instructed by a dietitian how to record a 7-days diet recalls to ensure that they do not consume to 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. Assessment of body mass and body height, body composition, and aerobic capacity (VO2max), will be performed. Running speed of 10 m, 20 m and 30 m sprint will be measured on a track and field stadium. 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 extensors, in both limbs will be evaluated on an isokinetic dynamometer at 60°/sec. Maximal voluntary isometric contraction (MVIC) of the knee extensors at 65o 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 the 3rd visit, participants will be randomly assigned into, and perform one of the four different conditions of the study design: a) unresisted sprint training, b) resisted sprint training with a load of 10% of body weight (BW), c) resisted sprint training with a load of 20% of BW d) control condition. Prior to each experimental protocol, assessment of DOMS in the knee flexors (KF) and extensors (KE) of both limbs, as well as blood lactate assessment will be performed. Additionally, DOMS of KF and KE, running speed at 10 m, 20 m and 30 m sprint, 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 electromyography (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 tibialis anterior 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 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 above procedures will be repeated by the participants during the remaining three experimental trials (7th - 10th, 11th - 13th, and 14th - 16th visits).