The aim of the present study was to compare "absolute" and "relative" knee extension strength between obese and nonobese adolescents. Ten nonobese and 12 severely obese adolescent boys of similar chronological age, maturity status, and height were compared. Total body and regional soft tissue composition were determined using dual-energy X-ray absorptiometry (DXA). Knee extensors maximum voluntary contraction (MVC) torque was measured using an isometric dynamometer at a knee angle of 60° (0° is full extension). Absolute MVC torque was significantly higher in obese adolescents than in controls. However, although MVC torque expressed per unit of body mass was found to be significantly lower in obese adolescent boys, no significant difference in MVC torque was found between groups when normalized to fat-free mass. Conversely, when correcting for thigh lean mass and estimated thigh muscle mass, MVC torque was significantly higher in the obese group (17.9% and 22.2%, respectively; P <0.05). To conclude, our sample of obese adolescent boys had higher absolute and relative knee extension strength than our nonobese controls. However, further studies are required to ascertain whether or not relative strength, measured with more accurate in vivo methods such as magnetic resonance imaging, is higher in obese adolescents than in nonobese controls.

Despite the considerable efforts that have been made to characterize in vivo human skeletal muscle function in the last 20 years, there is still controversy about whether obesity affects muscle performance in people of different ages. We therefore reviewed the available literature to determine the impact of obesity on skeletal muscle strength and fatigue. Obese individuals have (i) higher absolute muscle strength, (ii) lower strength per unit body mass, (iii) a similar strength to total fat-free mass ratio and (iv) a similar/higher strength to muscle size ratio compared to their nonobese peers. These results suggest that obesity does not negatively affect the intrinsic muscle contractile properties. Moreover, the available evidence does not show differences in muscle fatigue between obese and nonobese individuals. Therefore, factors such as the handicapping effect of excess fat mass and/or impaired motor coordination may account for the poor physical performance of obese people of all ages.

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