Substantial skeletal muscle loss occurs during only 5 days of disuse

<jats:title>Abstract</jats:title><jats:sec><jats:title>Aim</jats:title><jats:p>The impact of disuse on the loss of skeletal muscle mass and strength has been well documented. Given that most studies have investigated muscle atrophy after more than 2 weeks of disuse, few data are available on the impact of shorter periods of disuse. We assessed the impact of 5 and 14 days of disuse on skeletal muscle mass, strength and associated intramuscular molecular signalling responses.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>Twenty‐four healthy, young (23 ± 1 year) males were subjected to either 5 (<jats:italic>n </jats:italic>= 12) or 14 (<jats:italic>n </jats:italic>= 12) days of one‐legged knee immobilization using a full leg cast. Before and immediately after the immobilization period, <jats:italic>quadriceps</jats:italic> muscle cross‐sectional area (<jats:styled-content style="fixed-case">CSA</jats:styled-content>), leg lean mass and muscle strength were assessed, and biopsies were collected from the <jats:italic>vastus lateralis</jats:italic>.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p><jats:italic>Quadriceps</jats:italic> muscle <jats:styled-content style="fixed-case">CSA</jats:styled-content> declined from baseline by 3.5 ± 0.5 (<jats:italic>P </jats:italic>< 0.0001) and 8.4 ± 2.8% (<jats:italic>P </jats:italic>< 0.001), leg lean mass was reduced by 1.4 ± 0.7 (<jats:italic>P </jats:italic>= 0.07) and 3.1 ± 0.7% (<jats:italic>P </jats:italic>< 0.01) and strength was decreased by 9.0 ± 2.3 (<jats:italic>P </jats:italic>< 0.0001) and 22.9 ± 2.6% (<jats:italic>P </jats:italic>< 0.001) following 5 and 14 days of immobilization respectively. Muscle myostatin <jats:styled-content style="fixed-case">mRNA</jats:styled-content> expression doubled following immobilization (<jats:italic>P </jats:italic>< 0.05) in both groups, while the myostatin precursor isoform protein content decreased after 14 days only (<jats:italic>P </jats:italic>< 0.05). Muscle MAFBx <jats:styled-content style="fixed-case">mRNA</jats:styled-content> expression increased from baseline by a similar magnitude following either 5 or 14 days of disuse, whereas MuRF1 <jats:styled-content style="fixed-case">mRNA</jats:styled-content> expression had increased significantly only after 5 days.</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>We conclude that even short periods of muscle disuse can cause substantial loss of skeletal muscle mass and strength and are accompanied by an early catabolic molecular signalling response.</jats:p></jats:sec>