Essential amino acids modestly reduce soreness and accelerate muscle swelling recovery after eccentric exercise

Thirty-six recreationally active men (average age 21 years) were randomly assigned to receive 10 g of EAAs, 10 g of placebo maltodextrin, or no supplement. All participants performed an intense eccentric plantar flexor protocol (4 sets of 50 repetitions followed by one set to failure), with supplements or no supplement given 30 minutes before exercise, immediately after, and during the 72-hour recovery window. The eccentric exercise successfully induced muscle damage, with increased soreness (p < 0.001), increased calf muscle thickness and circumference (p < 0.001), and reduced pain pressure threshold (p = 0.018). EAA supplementation reduced perceived soreness specifically at 24 hours post-exercise versus placebo. Calf muscle swelling (measured by ultrasound thickness) returned to baseline by 72 hours in the EAA group but remained elevated in the placebo group (p < 0.001). Achilles tendon thickness decreased immediately post-exercise but was not influenced by supplementation. The results suggest EAA supplementation may provide modest benefit for soreness perception and swelling resolution, though the effect was modest and did not extend to tendon recovery within three days.

The soreness reduction appeared specifically at 24 hours and did not necessarily persist through 72 hours, suggesting the benefit may be narrowly timed. Muscle thickness—a proxy for edema or swelling—showed the most meaningful difference: EAA-supplemented participants' calves returned to pre-exercise size by day 3, whereas placebo participants' remained swollen. Notably, neither supplementation nor placebo prevented the initial damage response (both groups showed the same immediate soreness, thickness increase, and pressure threshold reduction), and neither influenced Achilles tendon morphology, which is relevant because tendons may be more resistant to EAA-driven adaptation than muscle tissue. The study used 10 g of EAAs dosed three times around the exercise window, a higher frequency than typical daily supplementation; whether lower or single-dose schedules work as well is unknown. This is a small, short-term study (n=36, 72-hour window) in young active men, so findings may not generalize to older, sedentary, or female populations, or to recovery beyond three days.