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

Abstract

1. Changes in muscle strength, vastus lateralis fibre characteristics and myosin 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 47 %. After 2 weeks of spontaneous recovery there still was an average strength deficit of 11 %. With eccentric and mixed compared with concentric retraining the rate of strength recovery was faster and the eccentric and isometric 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 immobilization 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 eccentric 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 enlargements were greatest after eccentric training. 4. Total RNA/wet muscle weight and ty I, IIa and IIx MyoHC mRNA levels did not change differently after immobilization and retraining. Immobilization downregulated the expression of type I MyoHC mRNA to 0.72-fold of baseline and exercise training upregulated it to 0.95 of baseline. No changes occurred in type IIa MyoHC mRNA. Immobilization and exercise training upregulated type IIx MyoHC mRNA 2.9-fold and 1.2-fold, respectively. For the immobilization segment, type I, IIa and IIx fibre area and type I, IIa and IIx MyoHC 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 neuromuscular function and adaptation.

Figures

Figure 1. Study design

Twenty-four young male (M) and female (F) subjects were randomly assigned to a control group and three exercise groups following 3 weeks of left leg immobilization of 42 subjects. Tests 1, 2a and 3 included unilateral strength and surface electromyography assessment of both legs and muscle biopsy of the left vastus lateralis. Test 2b involved the same measurements as Test 2a but were administered in 6 subjects after 2 weeks of spontaneous recovery from immobilization. Exercising subjects performed a total of 1866 maximal quadriceps contractions over 12 weeks using 4–6 bouts of 6–12 repetitions per session.

Figure 2. Myosin heavy chain mRNA analysis

Radiograph of polyacrylamide gels from ribonuclease protection assay for type I and IIa (A) and IIx (B) MyoHC mRNAs before (C, control), after 3 weeks of leg immobilization (I) and after 12 weeks of eccentric exercise training (T). In each panel the data from one subject are shown. Vastus lateralis samples were taken 3 h after the cast was removed and 3 h after last contraction.

Figure 3. Immobilization and muscle strength

Changes in quadriceps muscle strength following 3 weeks of leg immobilization. For IM, non-IM and 2 wk rec. data are the averages of eccentric, concentric and isometric quadriceps strength, while the control data are the averages of eccentric, concentric and isometric quadriceps strength of both legs. IM, immobilized leg; Non-IM, non-immobilized leg of subjects whose other leg was immobilized; 2wk rec., muscle strength after 2 weeks of spontaneous recovery from 3 weeks of immobilization; and Control, non-immobilized and non-exercised subjects' strength data 15 weeks after an initial test. *P < 0.05 compared with all other groups and †P < 0.05 compared with non-IM and control. Vertical bars denote + or – 1 s.d.

Figure 4. Immobilization and strength recovery

Rate of muscle strength recovery from immobilization. * Recovery occurred at a significantly faster rate at about week 2 in eccentric strength with eccentric (•) or mixed (□) training compared with the recovery of concentric strength with mixed (▪) or concentric training (▵). In this latter group recovery occurred at about week 4. The horizontal line at 100 % indicates recovery to strength levels before immobilization. Standard deviations are omitted for clarity.

Figure 5. Strength training effects after immobilization

Percentage changes in eccentric (▪), isometric (□) and concentric () quadriceps muscle strength following 1866 eccentric, concentric or mixed contractions for the comparisons between Tests 1 and 3 (cf. Fig. 1). * Significantly more change than other changes within the same group (P < 0.05); † significantly more change than the changes in eccentric, isometric and concentric strength in the other two groups; ² significantly more change than isometric and eccentric strength in the concentric group; ³ significantly more change than concentric strength in eccentric group. Data for the control group are not shown for clarity. Vertical bars denote +1 s.d.

Figure 6. Changes in fibre type composition

Fibre type distribution in the vastus lateralis muscle before immobilization (▪, n = 36), after 3 weeks of immobilization (□, n = 36), after 2 weeks of spontaneous recovery from immobilization (, n = 6), after 12 weeks of knee extension exercise training (, n = 36) and after 15 weeks of spontaneous activity (, n = 6). * Significantly different compared with all other conditions (P < 0.05). Vertical bars denote + 1 s.d.

Figure 7. Myosin heavy chain gene expression

Changes in type I, IIa and IIx myosin heavy chain mRNA levels (in arbitrary units, a.u.) at baseline (▪), after 3 weeks of immobilization (□) and after 12 weeks of resistive exercise training of the human quadriceps muscle (). * Significantly different compared with baseline and exercise training (P < 0.05, n = 22). Vertical bars denote + 1 s.d.