Quadriceps Femoris Muscle Fatigue in Obese Adolescents and Adults

Abstract

Very little is known on skeletal muscle function in obese populations. Even if obese individuals have significantly lower levels of physical activity than their lean counterparts, relative muscle strength and power of the lower limb muscles do not differ between obese and non-obese persons therefore indicating that these neuromuscular features are not affected by obesity. To our knowledge, the effect of obesity on muscle fatigue, which represents an important link to normal activities of daily living, has never been addressed to date. PURPOSE: To compare quadriceps femoris muscle fatigue and the origin of this phenomenon between obese individuals, both adolescents and adults, and their non-obese counterparts. METHODS: Four groups of male subjects were considered (N = 10 each): obese adolescents (age: 15.6 yr; BMI: 34.0 kg/m2), lean adolescents (age: 14.9 yr; BMI: 19.4 kg/m2), obese adults (age: 25.3 yr; BMI: 41.3 kg/m2) and lean adults (age: 27.0 yr; BMI: 22.6 kg/m2). Quadriceps muscle fatigue was quantified as the percent torque decrease during a voluntary (50 isokinetic contractions at 180°/s) and an electrostimulation protocol (5 min at 40 Hz, 1 s on-2 s off, initial level 10% of isometric MVC torque). Isometric (60° knee flexion) and isokinetic (180°/s) MVC torque were also measured. RESULTS: Obese individuals were significantly stronger than their lean counterparts, both in isometric (266.7 vs. 229.4 Nm; P <0.05) and in isokinetic conditions (170.1 vs. 140.2 Nm; P <0.01). Voluntary torque loss was significantly larger in obese than in lean adults (−63.5% vs. −50.6%; P <0.05), while no difference was observed between obese and lean adolescents (−52.6% vs. −52.2%). Electrically evoked torque loss was comparable in the four subject groups (−25.8% in the obese groups and −26.0% in the lean groups). CONCLUSIONS: When activating the quadriceps femoris muscle voluntarily, obese adults were less fatigue-resistant than their lean counterparts, but also than their younger counterparts. In the absence of a difference between groups in muscle fatigue induced by electrical stimulation, i.e., partly bypassing the central nervous system, this impairment in the obese adult population was considered as central rather than peripheral in origin. Impaired quadriceps muscle activation, but also motivation, skill and coordination mainly account for the present findings, and probably contribute to the reduced motor performance of obese adults, especially during more complex and physiologically demanding motor tasks.