Effects of essential amino acid supplementation on musculotendinous recovery following eccentric plantar flexor exercise: a randomized controlled trial

Background Exercise-induced muscle damage (EIMD) resulting from eccentric contractions leads to transient impairments in muscle function. Essential amino acids (EAAs) stimulate muscle protein synthesis and may support recovery following damaging exercise. However, limited research has examined the effects of EAAs on muscle function following eccentric plantar flexor exercise. The purpose of this study was to examine the effects of EAA supplementation on indirect markers of muscle and musculotendinous recovery following EIMD.Methods Thirty-six recreationally active males (age: 21.2 ± 2.7 years) were randomly assigned to an EAA group (10 g of EAAs), placebo (10 g of maltodextrin), or control (no supplementation). Supplements were consumed 30 minutes before and immediately after an eccentric plantar flexor protocol (4 × 50 repetitions followed by one set to failure) and during the 72 hours recovery period. Indirect markers of muscle damage were assessed pre-exercise, immediately post-exercise, and 24, 48, and 72 hours post-exercise, consisting of perceived soreness (NPRS), pain pressure threshold (PPT), calf muscle thickness and Achilles tendon thickness (ultrasound), and calf circumference.Results The eccentric protocol elicited responses consistent with EIMD, including increased soreness (p < 0.001), increased calf muscle thickness and circumference (p < 0.001), and reduced PPT (p = 0.018). EAA supplementation attenuated soreness at 24 hours compared with placebo. Calf muscle thickness increased following exercise in both groups; however, swelling returned to baseline by 72 hours in the EAA group but remained elevated in placebo (Condition × Time, p < 0.001). Achilles tendon thickness decreased immediately post-exercise (p = 0.005) but was not influenced by supplementation.Conclusions EAA supplementation modestly reduced soreness and was associated with faster recovery of muscle swelling but did not influence tendon morphology within 72 hours.