Introduction: Skeletal muscle disuse atrophy (DA) through inactivity and immobilization is not fully understood, and as such remains a major goal of global muscle biology science efforts. Nonetheless, remarkably not all leg muscles atrophy at the same rate or total degree despite similar anatomical roles/ proximity (1). Moreover, temporal aspects of DA remain ambiguous, with rates of muscle loss over chronic periods (i.e., >7 days) often inferred between repeated measures spanning several days (2), which lack resolution. We thus investigated this temporality amongst atrophy resistant and atrophy susceptible muscles of the lower leg, i.e. the tibialis anterior (TA) and medial gastrocnemius(MG), respectively (3).
Methods: We recruited 10 healthy young men (22±1y, BMI 23±1kg.m2) who underwent 15 days unilateral leg immobilisation (ULI) using a knee and foot brace to immobilise both the upper and lower leg. To assess muscle mass, participants received sequential magnetic resonance imaging (MRI) scans at baseline and thereafter on alternate days to quantify the volume of individual muscles (focussing upon the TA and MG) throughout the 15 days of immobilisation. Functional assessments were also undertaken (i.e., knee extensor 1-RM, dorsiflexion/ plantarflexion MVC) before and after immobilisation. Data are mean±SEM, analysed by repeated measures 2-way ANOVA.
Results: After 7 days ULI, MG volume decreased from baseline values by 8.8±2.9% (p=0.02), and thereafter remained largely stable at this lower level throughout the remaining immobilisation period (i.e., day 9: -7.7±4.4% (p=0.02), day 16: -9.7±3.1%, (p=0.006)). TA muscle volume remained unchanged in both legs at all time-points as did the non-immobilised MG. In the immobilised leg, knee extensor 1-RM (-19±7%, p<0.001) and MVC (-31±5%, p=0.0003) each decreased from baseline, yet were unchanged in the non-immobilised leg. Following the 15 days of ULI, a similar degree of muscle strength loss was observed in measures of plantarflexion MVC (i.e., in MG) (-16±18%, p=0.04) and dorsiflexion MVC (i.e., in TA) (-25±9%, p=0.001).
Conclusion: Investigating atrophy susceptible vs. atrophy resistant muscles is a novel paradigm to study the mechanistic basis of DA. Moreover, muscle mass decreases in a non-linear manner; with early losses (i.e., 5-7 days) remaining consistent up to 15 days of ULI. Importantly, despite divergent responses in DA, the disparity between relative mass and functional losses of both muscles may be due to detrimental changes in neuromuscular aspects. We contend that targeting early atrophy is central to mitigating even long-term losses.