Changes in muscle size and MHC composition in response to resistance exercise

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The main finding of the below study was that light load (LL)

resistance training (15.5%RM) was sufficient to induce a small but

significant muscle hypertrophy in healthy young men. However, LL

resistance training was inferior to heavy load (70%1RM) training in

evoking adaptive changes in muscle size and contractile strength and

was insufficient to induce changes in MHC composition.

Changes in muscle size and MHC composition in response to resistance

exercise with heavy and light loading intensity

J Appl Physiol 105: 1454-1461, 2008. First published September 11,

2008;

L. Holm,1 S. Reitelseder,1 T. G. Pedersen,1 S. Doessing,1 S. G.

sen,1 A. Flyvbjerg,2 J. L. Andersen,1 P. Aagaard,3 and M. Kjaer1

Muscle mass accretion is accomplished by heavy-load resistance

training. The effect of light-load resistance exercise has been far

more sparsely investigated with regard to potential effect on muscle

size and contractile strength.

We applied a resistance exercise protocol in which the same

individual trained one leg at 70% of one-repetition maximum (1RM)

(heavy load, HL) while training the other leg at 15.5% 1RM (light

load, LL). Eleven sedentary men (age 25 ± 1 yr) trained for 12 wk at

three times/week. Before and after the intervention muscle

hypertrophy was determined by magnetic resonance imaging, muscle

biopsies were obtained bilaterally from vastus lateralis for

determination of myosin heavy chain (MHC) composition, and maximal

muscle strength was assessed by 1RM testing and in an isokinetic

dynamometer at 60°/s. Quadriceps muscle cross-sectional area

increased (P < 0.05) 8 ± 1% and 3 ± 1% in HL and LL legs,

respectively, with a greater gain in HL than LL (P < 0.05). Likewise,

1RM strength increased (P < 0.001) in both legs (HL: 36 ± 5%, LL: 19

± 2%), albeit more so with HL (P < 0.01). Isokinetic 60°/s muscle

strength improved by 13 ± 5% (P < 0.05) in HL but remained unchanged

in LL (4 ± 5%, not significant). Finally, MHC IIX protein expression

was decreased with HL but not LL, despite identical total workload in

HL and LL.

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Carruthers

Wakefield, UK