It is known that eccentric exercise induces muscle microtrauma. Also, eccentric type of exercise are associated with inflammatory response. The leukocytes and the immune cells during the phagocytosis process alter the redox status causing secondary muscle damage to the muscle tissue and also increase oxidative stress concentration. Despite of the existence of studies that examine the muscle damage indicators, inflammatory response, performance and neuromuscular fatigue and oxidative stress concentrations after eccentric exercise, however no studies are detected in order to compare the differences among three eccentric protocols. The eccentric protocols will be differentiated in training volume and specifically in the eccentric repetitions in isokinetic dynamometer. Specifically, a randomized, fourth-trial, cross-over, repeated measures design will be applied. Healthy male adults (age 18-30 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 exercise protocols. Also the participants will not be smokers and will not consume alcohol and nutritional supplements.

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 (20-gauge) in order to measure muscle damage markers (CK), inflammation markers (WBC) and oxidative stress markers (TBARS, PC, GSH, ΤΑC, CAT, UA). 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 5 days diet recalls estimating the energy intake during the trials will be the same. The knee range of motion (KJRM) will be determined by the use of a goniometer, 24 hours after (second phase). The counter movement jump (CMJ) will be evaluated 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 medialis (VM) and vastus lateralis 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 65° in both limbs. 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.

96 hours after, the participants are going to perform one of the three eccentric protocols randomly, on an isokinetic dynamometer. The eccentric protocol will be performed on a different limb for each trial. The ECC 75 trial will include 75 eccentric maximum repetitions (5 sets, 15 reps/set and recovery period: 30sec.) at 60°/sec. The ECC 150 trial will include 150 eccentric maximum repetitions (10 sets, 15 reps/set and recovery period: 30sec.) at 60°/sec. The ECC 300 trial will include 300 eccentric maximum repetitions (20 sets, 15 reps/set and recovery period: 30sec.) at 60°/sec. The DOMS indicator in the knee flexors (KF) and extensors (KE) of both limbs and the KJRM will be evaluated immediately after each protocol. Also, the (CMJ) will be evaluated 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 medialis (VM) and vastus lateralis muscles. Electromyography data will be collected wirelessly at 2.000 Hz using a Myon MA-320 EMG system. In addition, the peak eccentric and concentric isokinetic torque of the knee flexors and extensors, in both limbs will be evaluated on an isokinetic dynamometer at 60o/sec. Also, the isometric peak torque of the knee extensors will be evaluated at 65o in both limbs. All the above markers will be evaluated 24, 48 and 192 hours post eccentric protocol, however at these time points, fasting blood samples (20ml) will be collected in order to estimate hematological and biochemical indicators. Α 2 - week washout period will be adapted among trials. After, the participants will perform the process until they complete the trials.