In a constant effort to find ways to make a quicker recovery between demanding workouts and football matches, this study is the first to investigate the benefits of protein supplementation, and compares two types of proteins, an animal-derived (whey) and a plant-derived (soy) protein, after an exercise-induced muscle injury caused by a speed endurance training protocol. Soy protein could be a cheaper and more environment-friendly alternative for athletes involved in high-velocity strength training.

A randomized, three-trial (placebo vs. whey protein isolate vs. soy protein isolate consumption), cross-over, double-blind design.

1. Baseline performance testing: Initially, volunteers will participate in a performance testing procedure at University facilities. These measurements include:

   A) measurement of descriptor variables: a) body composition (DXA), b) VO2max, c) Yo-Yo intermittent endurance level 2 (Yo-Yo IE2), d) Yo-Yo intermittent recovery test level 2 (Yo-Yo IR2), e) technical skill level, f) daily dietary intake profile (over a 7-day period), g) gabitual physical activity level (over a 7-day period) and h) resting mtabolic rate (RMR).

   B) Measurement of dependent variables: a) isokinetic strength (concentric, eccentric) and maximal voluntary isometric contraction (MVIC) of knee extensors and flexors of both lower limbs, b) counter movement jump height (CMJ), c) repeated sprint ability (RSA), d) speed (10 and 30 m), e) delayed onset of muscle soreness (DOMS), f) blood sampling for measurement of lactate concentration, creatine kinase activity (CK) and inflammatory markers [i.e.Glutathione (GSH), total antioxidant capacity (TAC) and protein carbonyls (PC)].

2. A 1-week adaptive period: based on a dietary analysis, participants will be given a dietary plan [taking into account the resting metabolic rate (RMR) and total daily physical activity related energy expenditure], providing a standard protein intake of 0.8-1 g protein/kg/day over the 1-week adaptive period. This protein intake is accepted as the average and population-safe protein intake during periods of very low physical activity and/or exercise levels. RMR, daily dietary intake and physical activity will be measured before the adaptive period. Familiarization with the training protocol will take place during this phase.

3. Participants will randomly participate in three trials including: whey protein (WP), soy protein (SP) and placebo (PL) supplementation:

   • A 7-day pre-loading phase: A 7-day pre-loading phase will be applied prior to each trial, during which volunteers will receive the respective supplement (placebo, whey protein or soy protein). In WP and SP trials, participants will consume daily the appropriate amount of protein to reach a total protein intake of 1.5 g protein/kg body weight (BW).
   • On the 7th day (end of pre-loading phase), participants will repeat performance testing (assessment of all dependent variables according to baseline testing) and blood sampling.
   • On the 8th day, participants will perform the speed-endurance production training (SEPT) session 1 (1st trial). During training, participants internal (heart rate) and external load (total distance, speed zones, acceleration and decelerations, impacts) will be continuously monitored using heart rate monitors and global positioning system (GPS) instrumentation.
   • Immediately after the training protocol: Determination of blood lactate concentration (3-4 minutes post-training) and assessment of MVIC (1 hour, 2 hours and 3 hours post-training).
   • 1-Day post-training (9th day; 24 hours post-training): Blood sampling (for the determination of CK, GSH, TAC and PC) and measurement of dependent variables (i.e. isokinetic strength, MVIC, Speed, RSA, CMJ and DOMS).
   • 2-Days post-training (10th day; 48 hours post-training): Blood sampling (for the determination of CK, GSH, TAC and PC) and assessment of DOMS. Thereafter, the SEPT session 2 will take place during which the particpants' internal and external load will be continuously monitored (as described in SEPT session 1).

4. Wash-out period: 2-week without any activity or supplementation.

5. 2nd trial: repeat of stages 3 and 4.

6. 3rd trial: repeat of stage 3.

Inclusion Criteria:

• Playing at a competitive level (top three divisions) for ≥3 years
• Free of any recent history of illnesses, musculoskeletal problems and metabolic diseases
• No use of supplements and medications (for ≤6 months prior to the study)
• Non-smokers

Exclusion Criteria:

• A known milk intolerance or allergy
• A recent febrile illness
• History of muscle lesion
• Lower limb trauma
• Metabolic diseases.