Assessing the Recovery of Muscle Fatigue Using Shear Wave Elastography

Variation in muscle stiffness, reflected by changes in shear modulus (G) measured via shear wave elastography (SWE), is linked to muscle fatigue and athletic performance. Although fatigue recovery varies across populations due to sex and training background, quantitative evidence remains limited. This study aimed to assess changes in passive muscle G during fatigue and recovery and to evaluate the applicability of SWE for monitoring muscle fatigue.

Thirty-five athletes and 16 non-athletes participated. Muscle fatigue was induced using unilateral eccentric dumbbell elbow flexion at 90% one-repetition maximum (three sets of 10 repetitions plus a final set to exhaustion). G was measured using SWE at baseline, immediately post-exercise, and at 24 and 48 h. Linear mixed-effects models examined the effects of group and time on G, with arm dominance included as a controlled factor.

A significant main effect of time on G was observed (P < 0.001), with no main effects of group or arm dominance and no significant interactions (all P > 0.05). At baseline, non-athlete females showed lower G than athletes in the dominant arm and lower G than male athletes in the non-dominant arm (P < 0.05, effect size [ES] = 1.05-1.39). Immediately, G increased in male athletes, non-athlete males, and non-athlete females (P < 0.05, ES = 0.43-1.34), with non-athlete males showing higher G in the dominant arm than non-athlete females (P < 0.05, ES = 0.95). At 24-hour follow-up, non-athlete females exhibited higher dominant-arm G and lower non-dominant-arm G than non-athlete males and male athletes (P < 0.05, ES = 1.16-1.57). By 48 h, G returned to baseline in male athletes and non-athlete males, while non-athlete females showed a significant decrease in dominant-arm G compared to 24 h measurement (P < 0.05, ES = 1.06). An inter-limb difference occurred only in non-athlete females at 24 h (P < 0.05, ES = 1.14).

Passive muscle modulus G reflects muscle stiffness and can be non-invasively monitored using a portable SWE device. Our findings suggest that passive G is a valid indicator for tracking muscle fatigue and recovery across populations.