Speed endurance maintenance training is utilized in order to improve the repeated sprint and high speed running ability of soccer players during a soccer match. Also, speed endurance maintenance training includes soccer drills which stimulate high speed and powerful actions such as changes of directions, accelerations and decelerations. However these actions are associated with eccentric component and it is known that eccentric exercises induces muscle damage microtrauma. Thus, the aim of this study will be to investigate the recovery kinetics on performance, neuromuscular fatigue and muscle microtrauma that will occur after two speed-endurance maintenance training protocols in elite male soccer players. The speed endurance maintenance training (SEMT) sessions will be differentiated in training volume and specifically in the work to rest period. Specifically, a randomized three-trial, cross-over, repeated measures design will be applied. Elite male soccer players (aged 18-35 years) will participate in the present study. Also, it is considered necessary that the participants will not suffer from any musculoskeletal injuries that will limit their ability to perform the SEMT sessions. Also the participants will not be smokers and will not consume alcohol and ergogenic nutritional supplements before and during the study.

In the first phase all participants will sign an informed consent form after they will be informed about all benefits and risks of this study and they will sign a recent historical of musculoskeletal injury or illness form. Subsequently, fasting blood samples will be collected by venipuncture using a disposable needle (10-gauge) in order to estimate muscle damage concentration markers (CK), blood lactate and inflammation markers (WBC). After, delayed onset muscle soreness (DOMS) in the knee flexors (KF) and extensors (KE) of both limbs, body weight (BW), height and body composition (DXA method) will be measured in the lab. Completing the first phase, participants will be instructed by a dietitian how to record a 7 days diet recalls estimating that the energy intake during the trials will be the same. The counter movement jump (CMJ) will be measured 24 hour after (second phase) on a force platform using two force platforms at 1000 Hz, with each foot in parallel on the two platforms providing a separate yet time-synchronized measurement of the data for each leg. During the CMJ will be measured the jump height (cm), the ground reaction force (N), the peak and mean power (W/kg), the vertical stiffness (Kvert, N/m/kg) and the peak rate of force development (RFD, N/s), while at the same time will be evaluated the change in peak and mean normalized EMG during the eccentric and concentric phases of the counter movement jump, for the vastus lateralis (VL), biceps femoris (BF), gastrocnemius (GAS), and gluteus maximus (GM) muscles. Electromyography data will be collected wirelessly at 2.000 Hz using a Myon MA-320 EMG system. The peak eccentric and concentric isokinetic torque of the knee flexors and extensors, in both limbs will be evaluated on an isokinetic dynamometer at 60°/sec. Also, the isometric peak torque of the knee extensors will be evaluated at 65o in both limbs. Also the fatigue rate during maximal voluntary isometric contraction will be estimated through the percent drop of peak torque between the first and the last three seconds of a 10-secong maximal isometric contraction. Also, the speed of 10m and 30m will be measured using light cells Chronojump system. The repeated sprint ability (RSA) (5x30m) will be measure using light cells Chronojump with 25 seconds rest in-between.

The soccer agility drill with a ball will be measured using light cells Chronojump in order to evaluate the soccer technical skills, 24 hour after. Finally, Open-circuit spirometry will be utilized for assessment of maximal oxygen consumption (VO2max) using an automated online pulmonary gas exchange system via breath-by-breath analysis during a graded exercise testing on a treadmill. The ability to recover after repeated intense exercise will be evaluated through the YO-YO Intermittent Recovery test level 2 and YO-YO Intermittent Endurance test level 2. 96 hours after, the participants are going to perform one of the two speed endurance maintenance training session randomly in the field. The SEMT(1:1) session will include 1 set, 6 repetitions per set and the work to rest ratio will be 1:1. The SEMT(1:3) session will iclude 1 set, 6 repetitions per set and the work to rest ratio will be 1:3. During the sessions will be assessed the change in Heart rate using a heart rate monitor and the field activity will be recorded using a global positiong system (GPS). The blood lactate will be evaluated immediately after each session. The DOMS indicator in the knee flexors (KF) and extensors (KE) of both limbs will be evaluated immediately after, 1h, 2h, 3h, 24h, 48h, and 72 hours after each session. The isometric peak torque of the knee extensors will be evaluated at 65° in limbs, post and at 1h, 2h and 3h, hours after each session. Also the fatigue rate during maximal voluntary isometric contraction will be estimated through the percent drop of peak torque between the first and the last three seconds of a 10-secong maximal isometric contraction.

Blood samples will be collected at 24h, 48h, and 72 hours after sessions. The counter movement jump (CMJ) will be measured immediately after the session, at 1h, 2h, 3h, at 24h, 48h and 72 hours after session on a force platform using two force platforms at 1000 Hz, with each foot in parallel on the two platforms providing a separate yet time-synchronized measurement of the data for each leg. During the CMJ will be measured the jump height (cm), the ground reaction force (N), the peak and mean power (W/kg), the vertical stiffness (Kvert, N/m/kg) and the peak rate of force development (RFD, N/s), while at the same time will be evaluated the change in peak and mean normalized EMG during the eccentric and concentric phases of the counter movement jump, for the vastus lateralis (VL), biceps femoris (BF), gastrocnemius (GAS), and gluteus maximus (GM) muscles. Electromyography data will be collected wirelessly at 2.000 Hz using a Myon MA-320 EMG system. Also, the speed of 10m and 30m will be measured using light cells Chronojump system immediately after, at 24h, 48h and 72 hours after session. Finally the RSA test will be measured at 24h, 48h and 72 hours after sessions. Α 2 - week washout period will be adapted among trials. After, the participants will perform the process until they complete the trials.