The effect of adiposity on skeletal muscle size and strength in untrained women.

Background: It is not known how elevated adiposity in obese individuals or abnormally low levels of adiposity in underweight people affects the size and strength of skeletal muscle. Resistance training is known to induce gains in both muscle size and strength (Erskine et al., 2010), while prolonged muscle disuse has been shown to cause a reduction in these variables (de Boer et al., 2007). Moreover, simulated hypergravity for a 3 week period has been shown to increase muscle strength (Bosco et al., 1984), while low-load, high volume resistance exercise has been shown to enhance muscle protein synthesis more than conventional high-intensity resistance exercise (Burd et al., 2010). Therefore, we hypothesised that chronic over-loading of skeletal muscle in obese individuals would lead to greater lower-limb lean mass and higher absolute strength values compared to under-weight and normal-weight individuals. Method: Fifty untrained healthy women aged 19-49 yrs (mean ± SD: 25.1 ± 8.7 yrs) were recruited to take part in this study. Body fat and lean mass were assessed using dual-emission X-ray absorptiometry. Plantar flexion maximum voluntary isometric contraction (MVC) torque was assessed in the dominant limb using an isokinetic dynamometer at different ankle joint angles (-5 to +10 deg plantar flexion; 0 deg = neutral ankle position). MVC joint torque was corrected for voluntary muscle activation level (assessed using the interpolated twitch technique) and antagonist muscle co-activation (assessed via surface EMG). Results: Body mass index was 26.0 ± 7.0 kg/m2 (range: 17.0 - 51.9 kg/m2. Body fat percentage was 35.4 ± 8.8% (range: 19.6 - 51.9%). There was a positive relationship between lower-limb lean mass and MVC plantar flexion torque (r2 = 0.594; p<0.005). Moreover, total body fat mass was positively correlated with lower-limb lean mass (r2 = 0.500; p<0.005) and MVC plantar flexion torque at optimum joint angle (r2 = 0.332; p<0.005). A superior positive correlation though existed between total body mass and optimum MVC plantar flexion torque (r2 = 0.458; p<0.005). Conclusion: Our findings confirm that total body fat mass is associated with lower-limb fat-free mass and maximum strength in untrained women. This suggests that the antigravity muscles of the lower-limb may possibly adapt to chronically low and high levels of adiposity in a similar manner to chronic unloading or overloading of the muscles, as seen following prolonged periods of bed-rest or resistance training. [ABSTRACT FROM AUTHOR]