Changes in muscle strength, muscle fibre size and myofibrillar gene expression after immobilization and retraining in humans

1. Changes in muscle strength, vastus lateralis fibre characteristics and m yosin heavy-chain (MyoHC) gene expression were examined in 48 men and women following 3 weeks of knee immobilization and after 12 weeks of retraining with 1866 eccentric, concentric or mixed contractions. 2. Immobilization reduced eccentric, concentric and isometric strength by 4 7%. After 2 weeks of spontaneous recovery there still was an average streng th deficit of 11%. With eccentric and mixed compared with concentric retrai ning the rate of strength recovery was faster and the eccentric and isometr ic strength gains greater. 3. Immobilization reduced type I, IIa and IIx muscle fibre areas by 13, 10 and 10%, respectively and after 2 weeks of spontaneous recovery from immobi lization these fibres were 5% smaller than at baseline. Hypertrophy of type I, IIa and IIx fibres relative to baseline was 10, 16 and 16% after eccent ric and 11, 9 and 10% after mixed training (all P < 0.05), exceeding the 4, 5 and 5% gains after concentric training. Type IIa and IIx fibre enlargeme nts were greatest after eccentric training. 4. Total RNA/wet muscle weight and type I, IIa and IIx MyoHC mRNA levels di d not change differently after immobilization and retraining. Immobilizatio n downregulated the expression of type I MyoHC mRNA to 0.72-fold of baselin e and exercise training upregulated it to 0.95 of baseline. No changes occu rred in type IIa MyoHC mRNA. Immobilization and exercise training upregulat ed type IIx MyoHC mRNA 2.9-fold and 1.2-fold, respectively. For the immobil ization segment, type I, IIa and IIx fibre area and type I, IIa and IIx Myo HC mRNA correlated (r = 0.66, r = 0.07 and r = -0.71, respectively). 5. The present data underscore the role muscle lengthening plays in human n euromuscular function and adaptation.