No abstract

Performance in high-intensity exercise is dependent on the ability to activate motor units. The main aim of this study was to test the hypothesis that adult men and women (age 19-27 years) are able to maintain higher levels of voluntary activation (VA) in knee extensor muscles than boys and girls (age 12-14 years). The volunteers (n = 7 in each group) performed three 5-s maximal voluntary contractions (MVCs) and a continuous 2-min MVC. The VA and fatigue of the muscles was assessed by applying 250-ms 100-HZ test tetani (TT100HZ). During brief MVCs girls showed lower VA than women, but the difference between boys and men was not significant. During the 2-min MVC, VA in boys and girls was more depressed than in adults. The end-exercise values of the relative TT100HZ torque correlated with the average VA during the exercise. Thus, the results of the study support the hypothesis that children are more susceptible to central fatigue than adults. This should be taken into account when evaluating results of fitness tests that require high levels of motor unit activation.

Sarcopenia is the loss of muscle size and function during ageing. The aim of this study was to test whether serum concentrations of myostatin and interacting proteins (GASP-1, FLRG, and follistatin) differed between young and elderly sarcopenic men. Isometric knee extensor maximal voluntary contraction and quadriceps cross-sectional area (magnetic resonance imaging measurement) were significantly higher in young (22 ± 2 years; 266 ± 54 N/m; 8,686 ± 1,154 mm(2)) than in mildly sarcopenic (69 ± 3 years; 183 ± 17 N/m; 6,621±718 mm(2)) and severely sarcopenic men (76 ± 6 years; 127 ± 23 N/m; 5,846 ± 591 mm(2)), respectively (p ≤ .01 for all comparisons). There was a trend (p = .06) toward higher FLRG in young (20 ± 8 ng/mL) than in mildly (15 ± 6 ng/mL) and severely sarcopenic men (17 ± 8 ng/mL). Myostatin, follistatin, GASP-1, tumor necrosis factor α, and interleukin-6 did not differ significantly. Insulin-like growth factor-1 and free testosterone were both significantly lower in sarcopenic men (p < .001). This suggests that altered serum concentrations of myostatin and myostatin-interacting proteins are not contributing to sarcopenia with the possible exception of FLRG.

Regular exercise can reduce the risk of CVD (cardiovascular disease). Although moderate-intensity exercise can attenuate postprandial TAG (triacylglycerol), high-intensity intermittent exercise might be a more effective method to improve health. We compared the effects of high-intensity intermittent exercise and 30 min of brisk walking on postprandial TAG, soluble adhesion molecules and markers of oxidative stress. Nine men each completed three 2-day trials. On day 1, subjects rested (control), walked briskly for 30 min (walking) or performed 5×30 s maximal sprints (high-intensity). On day 2, subjects consumed a high-fat meal for breakfast and 3 h later for lunch. Blood samples were taken at various times and analysed for TAG, glucose, insulin, ICAM-1 (intracellular adhesion molecule-1), VCAM-1 (vascular adhesion molecule-1), TBARS (thiobarbituric acid- reactive substances), protein carbonyls and β-hydroxybutyrate. On day 2 of the high-intensity trial, there was a lower (P<0.05) incremental TAG AUC (area under the curve; 6.42±2.24 mmol/l per 7 h) compared with the control trial (9.68±4.77 mmol/l per 7 h) with no differences during day 2 of the walking trial (8.98±2.84 mmol/l per 7 h). A trend (P=0.056) for a reduced total TAG AUC was also seen during the high-intensity trial (14.13±2.83 mmol/l per 7 h) compared with control (17.18±3.92 mmol/l per 7 h), walking showed no difference (16.33±3.51 mmol/l per 7 h). On day 2 of the high-intensity trial plasma TBARS and protein carbonyls were also reduced (P<0.05) when compared with the control and walking trials. In conclusion, high-intensity intermittent exercise attenuates postprandial TAG and markers of oxidative stress after the consumption of a high-fat meal.

The aims of this study were to investigate if low-frequency fatigue (LFF) dependent on the duration of repeated muscle contractions and to compare LFF in voluntary and electrically induced exercise. Male subjects performed three 9-min periods of repeated isometric knee extensions at 40% maximal voluntary contraction with contraction plus relaxation periods of 30 plus 60 s, 15 plus 30 s and 5 plus 10 s in protocols 1, 2 and 3, respectively. The same exercise protocols were repeated using feedback-controlled electrical stimulation at 40% maximal tetanic torque. Before and 15 min after each exercise period, knee extension torque at 1, 7, 10, 15, 20, 50 and 100 Hz was assessed. During voluntary exercise, electromyogram root mean square (EMGrms) of the vastus lateralis muscle was evaluated. The 20-Hz torque:100-Hz torque (20:100 Hz torque) ratio was reduced more after electrically induced than after voluntary exercise (P < 0.05). During electrically induced exercise, the decrease in 20:100 Hz torque ratio was gradually (P < 0.05) reduced as the individual contractions shortened. During voluntary exercise, the decrease in 20:100 Hz torque ratio and the increase in EMGrms were greater in protocol 1 (P < 0.01) than in protocols 2 and 3, which did not differ from each other. In conclusion, our results showed that LFF is dependent on the duration of individual muscle contractions during repetitive isometric exercise and that the electrically induced exercise produced a more pronounced LFF compared to voluntary exercise of submaximal intensity. It is suggested that compensatory recruitment of faster-contracting motor units is an additional factor affecting the severity of LFF during voluntary exercise.

Purpose: To investigate MRI biomarkers of muscle atrophy during cast immobilization of the lower leg. Materials and Methods:Eighteen patients (8 male, 10 female), who had one lower leg immobilized in a cast, underwent 3.0 Tesla (T) MR imaging 5, 8, 15, 29, and 43 days after casting. Measurements were made on both lower legs of total muscle volume. Cross-sectional area (CSA), fractional water content, and T 2 were measured in tibialis anterior (TA), gastrocnemius medialis (GM) and lateralis (GL) and soleus (SOL). Fiber pennation angle was measured in GM.Results: Total muscle volume decreased by 17% (P < 0.001) over the 6 weeks of immobilization. The greatest loss in CSA (mean[SD]) was seen in GM (À23.3(8.7)%), followed by SOL (À19.0(9.8)%), GL (À17.1(6.5)%), and TA (À10.7(5.9)%). Significant reductions of CSA were also detectable in the contra-lateral leg. T 2 increased in all muscles: TA 27.0(2.5) ms to 29.6(2.8) ms (P < 0.001), GM 34.6(2.9) ms to 39.8(5.4) ms (P < 0.001) and SOL 34.4 (2.9) ms to 44.9(5.9) ms (P < 0.001). Small reductions were found in fractional water content. Pennation angle decreased in the cast leg (P < 0.001).Conclusion: Quantitative MR imaging can detect and monitor progressive biochemical and biophysical changes in muscle during immobilization.

Phosphocreatine (PCr) and intracellular pH detected by 31P NMR in the gastrocnemius and soleus muscles were evaluated in order to compare the anaerobic ATP costs of voluntary and electrically induced exercise. Continuous isometric contraction at 40 % of maximum force and repeated isometric contractions at approximately 75 % of maximum force (contraction plus relaxation period of 0.5 s plus 2 s) were studied. Anaerobic ATP turnover in soleus and gastrocnemius muscles was slower during continuous voluntary contraction than during continuous electrically induced contraction (0.36 ± 0.04 versus 0.63 ± 0.05 mmol (kg wet wt)−1 s−1, P < 0.05, in soleus; 0.19 ± 0.03 versus 1.04 ± 0.04 mmol (kg wet wt)−1 s−1, P < 0.001, in gastrocnemius). There was no significant difference in anaerobic ATP turnover between voluntary and electrically induced exercise when repeated brief contractions were performed (0.22 ± 0.05 and 0.30 ± 0.04 mmol (kg wet wt)−1 s−1, respectively, for the soleus muscle and 0.57 ± 0.03 and 0.66 ± 0.07 mmol (kg wet wt)−1 s−1, respectively, for the gastrocnemius muscle). During continuous voluntary contraction, in contrast to continuous stimulated contraction, anaerobic ATP turnover was slower (P < 0.05) in the gastrocnemius than in the soleus muscle, which also showed a higher electromyogram amplitude (41.1 ± 1.1 % of maximum) than the medial gastrocnemius muscle (21.4 ± 3.6 % of maximum, P < 0.001). Anaerobic ATP turnover was faster (P < 0.05) in the gastrocnemius than in the soleus muscle during brief voluntary and brief electrically induced contractions. The results show that the anaerobic ATP costs were higher for electrically induced exercise than for voluntary exercise when continuous submaximal contraction was performed but not when brief high‐intensity contractions were performed. The gastrocnemius muscle contributes to total force production relatively less than the soleus muscle during continuous voluntary plantar flexion at 40 % of the maximum voluntary contraction.

The influence of age on performance and muscle activity was studied during computer mouse tasks designed to induce high demands on motor control. Eight young (mean age 25 years) and nine elderly (mean age 63 years) women participated. When the speed was self-determined, the elderly subjects performed 13%-18% slower than did the young. When speed was predefined, the error rate was higher in the elderly subjects than in the young ones (medium precision 7.8% compared to 2.5%, high precision 16.5% compared to 7.9%, respectively). The highest error rate was found for double-clicking (32.9% compared to 13.5%, respectively). The reduced performance in the elderly subjects was hypothesised to be a combined effect of deteriorated proprioception, increased motor unit size, and changes in the central nervous system. Electrical activity (EMG) was recorded from the forearm, shoulder and neck muscles. Higher levels of EMG activity were found in the elderly compared to the young. A likely explanation is that the impaired motor control necessitated an increased muscle activity. The highest levels of EMG activity and lack of EMG gaps were found for the forearm extensor muscles, especially the extensor digitorum muscle (mean EMG activity 10.4% compared to 8.1% of maximal electrical activity, EMGmax) whereas lower EMG activity levels were found for the shoulder region (e.g. right trapezius muscle mean EMG 2.8% compared to 1.1% EMGmax, respectively). The latter was possibly due to a relieving effect of the forearm support. Differences in muscle activity among the tasks were found, however they were minor for the shoulder and neck muscles. Consideration of the demands on motor control when designing user interfaces is recommended, to the benefit of both the young and the elderly.