Abstract
The extent of skeletal muscle hypertrophy in response to resistance training is highly variable in humans. The main objective of this study was to explain the nature of this variability. More specifically, we focused on the myogenic stem cell population, the satellite cell (SC) as a potential mediator of hypertrophy. Twenty-three males (aged 18–35 yrs) participated in 16 wk of progressive, whole body resistance training, resulting in changes of 7.9±1.6% (range of −1.9–24.7%) and 21.0±4.0% (range of −7.0 to 51.7%) in quadriceps volume and myofibre cross-sectional area (CSA), respectively. The SC response to a single bout of resistance exercise (80% 1RM), analyzed via immunofluorescent staining resulted in an expansion of type II fibre associated SC 72 h following exercise (pre: 11.3±0.9; 72 h: 14.8±1.4 SC/type II fibre; p<0.05). Training resulted in an expansion of the SC pool associated with type I (pre: 10.7±1.1; post: 12.1±1.2 SC/type I fibre; p<0.05) and type II fibres (pre: 11.3±0.9; post: 13.0±1.2 SC/type II fibre; p<0.05). Analysis of individual SC responses revealed a correlation between the relative change in type I associated SC 24 to 72 hours following an acute bout of resistance exercise and the percentage increase in quadriceps lean tissue mass assessed by MRI (r2 = 0.566, p = 0.012) and the relative change in type II associated SC following 16 weeks of resistance training and the percentage increase in quadriceps lean tissue mass assessed by MRI (r2 = 0.493, p = 0.027). Our results suggest that the SC response to resistance exercise is related to the extent of muscular hypertrophy induced by training.
Highlights
There is a high degree of inter-individual variation in skeletal muscle hypertrophy following resistance exercise training despite exposure to exercise of the same relative intensity [1,2]
Skeletal muscle hypertrophy and nuclear content Sixteen weeks of whole body resistance training resulted in skeletal muscle hypertrophy as assessed by dual-energy X-ray absorptiometry (DEXA), Magnetic resonance imaging (MRI) and immunofluorescence (Fig 1)
The fibre type-specific satellite cells (SC) response was quantified via immunofluorescence of muscle cross sections (Fig 3A). 72 hours following an acute bout of resistance exercise the number of SC associated with type II muscle fibres increased (Fig 3B)
Summary
There is a high degree of inter-individual variation in skeletal muscle hypertrophy following resistance exercise training despite exposure to exercise of the same relative intensity [1,2]. Various findings from studies of resistance exercise training-induced hypertrophy in humans highlight the association of SC with muscle hypertrophy [2,14]. Based on results from rodent models others propose that SC are dispensable in mediating muscle hypertrophy [15,16]. That because hypertrophy can occur in SC-depleted rodent models does not necessarily render their contribution unimportant in contributing to hypertrophy in humans under physiological conditions. Models of selective SC depletion that demonstrate hypertrophy, under conditions of extreme overload, merely establish the capacity of redundant mechanisms to compensate and result in hypertrophy
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