Effect of the rest interval duration between contractions on muscle fatigue

Background: We aimed to investigate the effect of rest interval, between successive contractions, on muscular fatigue. Methods: Eighteen subjects performed elbow flexion and extension (30 repetitions) on an isokinetic dynamometer with 80 degrees of range of motion. The flexion velocity was 120 degrees/s, while for elbow extension we used 5 different velocities (30, 75, 120, 240, 360 degrees/s), producing 5 different rest intervals (2.89, 1.28, 0.85, 0.57 and 0.54 s). Results: We observed that when the rest interval was 2.89 s there was a reduction in fatigue. On the other hand, when the rest interval was 0.54 s the fatigue was increased. Conclusions: When the resting time was lower (0.54 s) the decline of work in the flexor muscle group was higher compared with different rest interval duration.

Metabolic and functional effects of beta-hydroxy-beta-methylbutyrate (HMB) supplementation in skeletal muscle

Beta-hydroxy-beta-methylbutyrate (HMB) is a metabolite derived from leucine. The anti-catabolic effect of HMB is well documented but its effect upon skeletal muscle strength and fatigue is still uncertain. In the present study, male Wistar rats were supplemented with HMB (320 mg/kg per day) for 4 weeks. Placebo group received saline solution only. Muscle strength (twitch and tetanic force) and resistance to acute muscle fatigue of the gastrocnemius muscle were evaluated by direct electrical stimulation of the sciatic nerve. The content of ATP and glycogen in red and white portions of gastrocnemius muscle were also evaluated. The effect of HMB on citrate synthase (CS) activity was also investigated. Muscle tetanic force was increased by HMB supplementation. No change was observed in time to peak of contraction and relaxation time. Resistance to acute muscle fatigue during intense contractile activity was also improved after HMB supplementation. Glycogen content was increased in both white (by fivefold) and red (by fourfold) portions of gastrocnemius muscle. HMB supplementation also increased the ATP content in red (by twofold) and white (1.2-fold) portions of gastrocnemius muscle. CS activity was increased by twofold in red portion of gastrocnemius muscle. These results support the proposition that HMB supplementation have marked change in oxidative metabolism improving muscle strength generation and performance during intense contractions.

Effect of N-acetylcysteine on markers of skeletal muscle injury after fatiguing contractile activity

The effects of N-Acetylcysteine (NAC), an unspecific antioxidant, on fatiguing contractile activity-induced injury were investigated. Twenty-four male Wistar rats were randomly assigned to two groups. The placebo group (N=12) received one injection of phosphate buffer (PBS) 1 h prior to contractile activity induced by electrical stimulation. The NAC group (NAC; N=12) received electrical stimulation for the same time period and NAC (500 mg/kg, i.p.) dissolved in PBS 1 h prior to electrical stimulation. The contralateral hindlimb was used as a control, except in the analysis of plasma enzyme activities, when a control group (rats placebo group not electrically stimulated and not treated) was included. The following parameters were measured: tetanic force, muscle fatigue, plasma activities of creatine kinase (CK) and lactate dehydrogenase (LDH), changes in muscle vascular permeability using Evans blue dye (EBD), muscle content of reactive oxygen species (ROS) and thiobarbituric acid-reactive substances (TBARS) and myeloperoxidase (MPO) activity. Muscle fatigue was delayed and tetanic force was preserved in NAC-treated rats. NAC treatment decreased plasma CK and LDH activities. The content of muscle-derived ROS, TBARS, EBD and MPO activity in both gastrocnemius and soleus muscles were also decreased by NAC pre-treatment. Thus, NAC has a protective effect against injury induced by fatiguing contractile activity in skeletal muscle.

Muscle Atrophy and Functional Deficits of Knee Extensors and Flexors in People With Chronic Stroke

Background. Further clarification is needed with regard to the degree of atrophy in individual muscle groups and its possible relationship to joint torque deficit poststroke. Objective. The purpose of this study was to investigate quadriceps and hamstring muscle volume and strength deficits of the knee extensors and flexors in people with chronic hemiparesis compared with a control group. Design. This was a cross-sectional study. Methods. Thirteen individuals with hemiparesis due to chronic stroke (hemiparetic group) and 13 individuals who were healthy (control group) participated in this study. Motor function, quadriceps and hamstring muscle volume, and maximal concentric and eccentric contractions of the knee extensors and flexors were assessed. Results. Only the quadriceps muscle of the paretic limb showed reduced muscle volume (24%) compared with the contralateral (nonparetic) limb. There were no differences in muscle volume between the hemiparetic and control groups. The peak torque of the paretic-limb knee extensors and flexors was reduced in both contraction modes and velocities compared with the nonparetic limb (36%-67%) and with the control group (49%-75%). The nonparetic limb also showed decreased extensor and flexor peak torque compared with the control group (17%-23%). Power showed similar deficits in strength (12%-78%). There were significant correlations between motor function and strength deficits (.54-.67). Limitations. Magnetic resonance imaging coil length did not allow measurement of the proximal region of the thigh. Conclusions. There were different responses between quadriceps and hamstring muscle volumes in the paretic limb that had quadriceps muscle atrophy only. However, both paretic and nonparetic limbs showed knee extensor and flexor torque and power reduction.