The purpose of this study was to investigate whether the voluntary neural drive and the excitability of the reflex arc could be modulated by training, even in old age. To this aim, the effects of a 16-wk strengthening program on plantar flexor voluntary activation (VA) and on the maximum Hoffman reflex (H(max))-to-maximum M wave (M(max)) ratio were investigated in 14 elderly men (65-80 yr). After training, isometric maximum voluntary contraction (MVC) increased by 18% (P < 0.05) and weight-lifting ability by 24% (P < 0.001). Twitch contraction time decreased by 8% (P < 0.01), but no changes in half relaxation time and in peak twitch torque were observed. The VA, assessed by twitch interpolation, increased from 95 to 98% (P < 0.05). Pretraining VA, also evaluated from the expected MVC for total twitch occlusion, was 7% higher (P < 0.01) than MVC. This discrepancy persisted after training. The interpolated twitch torque-voluntary torque relationship was fitted by a nonlinear model and was found to deviate from linearity for torque levels >65% MVC. Compared with younger men (24-35 yr), the H(max)- to M(max) ratio and nerve conduction velocity (H index) of the older group were significantly lower (42%, P < 0.05; and 29%, P < 0.001, respectively) and were not modulated by training. In conclusion, older men seem to preserve a high VA of plantar flexors. However, the impaired functionality of the reflex pathway with aging and the lack of modulation with exercise suggest that the decrease in the H(max)- to M(max) ratio and H index may be related to degenerative phenomena.

The present study aimed to compare quadriceps femoris muscle strength and fatigue between obese (grade II and III) and nonobese adults. Ten obese (mean age: 25 years; mean BMI: 41 kg/m(2)) and ten lean (mean age: 27 years; mean BMI: 23 kg/m(2)) men were tested. Quadriceps muscle fatigue was quantified as the (percent) torque loss during a voluntary isokinetic (50 maximal contractions at 180 degrees /s) and an electrostimulated (40 Hz) isometric protocol (5 min, 10% of the maximal torque). Maximal voluntary isometric and isokinetic torque and power were also measured. Voluntary torque loss was significantly higher (P < 0.05) in obese (-63.5%) than in lean subjects (-50.6%). Stimulated torque decreased significantly (P < 0.05) but equally in the two subject groups. Obese subjects displayed higher absolute (+20%; P < 0.01) but lower relative (i.e., normalized to body mass) (-32%; P < 0.001) muscle torque and power than their lean counterparts. Obese individuals demonstrated lower fatigue resistance during voluntary but not during stimulated knee extensions compared to their nonobese counterparts. Peripheral mechanisms of muscle fatigue -- at least those associated to the present stimulated test -- were not influenced by obesity. The observed quadriceps muscle function impairments (voluntary fatigue and relative strength) probably contribute to the reduced functional capacity of obese subjects during daily living activities.

NA.Comparison between voluntary and stimulated contractions of the quadriceps femoris for growth hormone response and muscle damage. J Appl Physiol 104: 75-81, 2008. First published November 1, 2007 doi:10.1152/japplphysiol.00335.2007.-This study aimed to compare voluntary and stimulated exercise for changes in muscle strength, growth hormone (GH), blood lactate, and markers of muscle damage. Nine healthy men had two leg press exercise bouts separated by 2 wk. In the first bout, the quadriceps muscles were stimulated by biphasic rectangular pulses (75 Hz, duration 400 s, on-off ratio 6.25-20 s) with current amplitude being consistently increased throughout 40 contractions at maximal tolerable level. In the second bout, 40 voluntary isometric contractions were performed at the same leg press force output as the first bout. Maximal voluntary isometric strength was measured before and after the bouts, and serum GH and blood lactate concentrations were measured before, during, and after exercise. Serum creatine kinase (CK) activity and muscle soreness were assessed before, immediately after, and 24, 48, and 72 h after exercise. Maximal voluntary strength decreased significantly (P Ͻ 0.05) after both bouts, but the magnitude of the decrease was significantly (P Ͻ 0.05) greater for the stimulated contractions (Ϫ22%) compared with the voluntary contractions (Ϫ9%). Increases in serum GH and lactate concentrations were significantly (P Ͻ 0.05) larger after the stimulation compared with the voluntary exercise. Increases in serum CK activity and muscle soreness were also significantly (P Ͻ 0.05) greater for the stimulation than voluntary exercise. It was concluded that a single bout of electrical stimulation exercise resulted in greater GH response and muscle damage than voluntary exercise. neuromuscular electrical stimulation; isometric strength; blood lactate; creatine kinase; muscle soreness IN RECENT YEARS, THE ACUTE effects of neuromuscular electrical stimulation (NMES) on neuromuscular and metabolic responses have received attention (16,28,33,37,39). It has been reported in several studies that electrically evoked contractions result in greater strength loss and greater increases in oxygen consumption and blood lactate compared with voluntary contractions at the same intensity (16,28,33,37,39). It has been speculated that the specific recruitment pattern of motor units during NMES is mainly attributed to the phenomena (16,28,37,39). Indeed, it is documented that the recruitment of motor units during stimulated contractions is different from voluntary contraction such that fast-twitch fibers could be activated at relatively low force levels (i.e., random/nonselective motor unit recruitment) (10,17,20,30).It is known that acute voluntary resistance exercise increases growth hormone (GH) secretion (for review see 22); however, limited information is available for acute GH responses to NMES. To the best of our knowledge, only two studies have reported GH responses to NMES. Greisen et al. (11) showed a significant increa...

BACKGROUND:Motor capabilities are reduced in obese (OB) individuals, and this impairment may result also from quantitative variation of muscle mass due to alterations in body composition. OBJECTIVE: This study aims to evaluate the differences in body mass (BM) and composition, as well as in muscle strength (ST) and power output (Ẇ ) between OB and NW males and females, and to test the hypothesis that variations in body composition affect muscle performance in OB subjects. DESIGN AND METHODS: Body composition (determined by BIA with a two-compartment model), upper and lower limb maximum ST (evaluated with isotonic machines) and lower limb maximum anaerobic Ẇ (measured with a jumping test) were studied in a group of 95 extremely OB subjects (OB: 28 males, 67 females; mean age7s.d.: 29.377.0 y; BMI: 41.274.4 kg/m 2 ) and in a control group of 18 NW voluntary subjects (NW: eight males, 10 females; age: 30.375.3 y; BMI: 22.672.1 kg/m 2 ). RESULTS: OB male and female subjects differed significantly with increases in BM being attained by a similar contribution of fat mass (FM) and fat-free mass (FFM) in male subjects, but mainly contributed by FM in female subjects. Compared with NW, both OB men and women had a greater amount of FFM (Po0.001) and, since a general linear correlation was found between ST and FFM (ST (N) ¼ 64.4 FFM (kg)À190.0, R 2 ¼ 0.612, Po0.001), they developed higher values of ST (Po0.05) than their respective NW counterparts. For the same reason, both OB and NW male subjects had higher ST (Po0.001) than their female counterparts. Correction for FFM eliminated all gender-and obesity-related ST differences. On the contrary, in spite of their higher absolute muscle strength, both OB men and women could develop absolute Ẇ similar to that of NW subjects, and were notably less powerful per unit BM than NW subjects (Po0.001), women being most affected among the OB. CONCLUSIONS: Obesity-related variation in body composition differs considerably by gender, and is responsible for differences in muscle performance: the higher muscle strength observed in OB subjects (both men and women) and in male subjects (both OB and NW) is accounted for by a greater amount of FFM. Nonetheless, biomechanical limitations appear to impair muscle power development during jumping in OB individuals.