Unloaded Shortening Velocity of Voluntarily and Electrically Activated Human Dorsiflexor Muscles In Vivo

We have previously shown that unloaded shortening velocity (V0) of human plantar flexors can be determined in vivo, by applying the “slack test” to submaximal voluntary contractions (J Physiol 567:1047–1056, 2005). In the present study, to investigate the effect of motor unit recruitment pattern on V0 of human muscle, we modified the slack test and applied this method to both voluntary and electrically elicited contractions of dorsiflexors. A series of quick releases (i.e., rapid ankle joint rotation driven by an electrical dynamometer) was applied to voluntarily activated dorsiflexor muscles at three different contraction intensities (15, 50, and 85% of maximal voluntary contraction; MVC). The quick-release trials were also performed on electrically activated dorsiflexor muscles, in which three stimulus conditions were used: submaximal (equal to 15%MVC) 50-Hz stimulation, supramaximal 50-Hz stimulation, and supramaximal 20-Hz stimulation. Modification of the slack test in vivo resulted in good reproducibility of V0, with an intraclass correlation coefficient of 0.87 (95% confidence interval: 0.68–0.95). Regression analysis showed that V0 of voluntarily activated dorsiflexor muscles significantly increased with increasing contraction intensity (R2 = 0.52, P<0.001). By contrast, V0 of electrically activated dorsiflexor muscles remained unchanged (R2<0.001, P = 0.98) among three different stimulus conditions showing a large variation of tetanic torque. These results suggest that the recruitment pattern of motor units, which is quite different between voluntary and electrically elicited contractions, plays an important role in determining shortening velocity of human skeletal muscle in vivo

THREE-DIMENSIONAL ACCELERATION DURING RUNNING IN TRAINED ATHLETES AND UNTRAINED SUBJECTS

The present study aimed to evaluate the three-dimensional (30) accelerations during running in trained athletes compared to untrained subjects. Ten endurance-trained male athletes and 10 untrained male subjects wore a 3D accelerometer at the L3 level and ran on a 400-m track at three different speeds. The acceleration amplitude was calculated for each direction from the acceleration time series for 10 strides. Additionally, the positive and negative peak values of vertical and anterior-posterior accelerations were calculated. Analysis of covariance tests showed a significant difference between athletes and untrained subjects in the vertical acceleration amplitude. Significant interaction effects were found for upward acceleration and forward-backward acceleration ratio. These results suggest that athletes have characteristic patterns in the vertical acceleration and forward-backward acceleration ratio

Higher Training Frequency Is Important for Gaining Muscular Strength Under Volume-Matched Training

Background: This study investigated the effect of volume-matched strength training programs with different frequency and subsequent detraining on muscle size and strength.Methods: During a training period of 11 weeks, untrained subjects (age: 22.3 ± 0.9 years, height: 173.1 ± 4.8 cm and body mass: 66.8 ± 8.4 kg) performed knee-extension exercise at 67% of their estimated one-repetition maximum either one session per week (T1 group: 6 sets of 12 repetitions per session; n = 10) or three sessions per week (T3 group: 2 sets of 12 repetitions per session; n = 10). Rating of perceived exertion (RPE) and muscle stiffness were measured as an index of muscle fatigue and muscle damage, respectively. The magnitude of muscle hypertrophy was assessed with thigh circumference and the quadriceps muscle thickness. The changes in muscle strength were measured with isometric maximum voluntary contraction torque (MVC).Results: During the training period, RPE was significantly higher in the T1 than in the T3 (p &lt; 0.001). After 11 weeks of training, both groups exhibited significant improvements in thigh circumference, muscle thickness, and MVC compared with baseline values. However, there was a significant group difference in MVC improvement at week 11 (T1: 43.5 ± 15.5%, T3: 65.2 ± 23.2%, p &lt; 0.05). After 6 weeks of detraining, both groups showed the significant decreases in thigh circumference and muscle thickness from those at the end of training period, while no significant effect of detraining was observed in MVC.Conclusion: These results suggest that three training sessions per week with two sets are recommended for untrained subjects to improve muscle strength while minimizing fatigue compared to one session per week with six sets

TIME COURSE CHANGE OF IGF1/AKT/MTOR/P70S6K PATHWAY ACTIVATION IN RAT GASTROCNEMIUS MUSCLE DURING REPEATED BOUTS OF ECCENTRIC EXERCISE

The purpose of this study was to examine whether insulin-like growth factor (IGF-1) and Akt/mTOR/p70S6K pathway activity is altered by chronic eccentric exercise in rat medial gastrocnemius muscle. Male Wistar rats (n = 24) were randomly assigned to 1 of the 2 groups: eccentric exercise (ECC) group or sham-operated control (CON) group. Rats in the ECC group were trained every second day for 10 days (5 sessions in total) or 20 days (10 sessions in total). After either 5 or 10 exercise sessions, muscle specimens were dissected and weighed. The mRNA expression of IGF-1 and its variant, mechano growth factor (MGF), was evaluated using real time reverse transcriptase-polymerase chain reaction (RT-PCR). Tissue concentrations of Akt (P), mTOR (P), and p70S6K (P) were measured by using western blot analysis. The medial gastrocnemius muscle mass of the ECC group did not show any significant difference after 5 exercise sessions, whereas the muscle mass increased significantly after 10 exercise sessions with a concomitant increase in the cross-sectional area of muscle fibers (p < 0.05). The expression of IGF-1 mRNA and the tissue concentrations of Akt (P) and p70S6K (P) after 10 exercise sessions was significantly higher than those of the age-matched controls and the rats that received 5 exercise sessions. The expression of MGF mRNA in both ECC5S and ECC10S were significantly higher than that in each period-matched control (p < 0.01). The tissue concentration of mTOR (P) after 10 sessions showed a significant increase when compared with period-matched controls (p < 0.01). These results suggest that activation of the IGF-1/Akt/mTOR/p70S6K signaling pathway becomes dominant in the later phase of chronic exercise, when significant muscular hypertrophy is observe

Relations between contraction intensity and unloaded shortening velocity (<i>V</i>₀) of human dorsiflexors during voluntary (A) and electrically-elicited (B) contractions.

<p>Shown are individual line plots from 6 subjects. Contraction intensity is expressed as a percentage of maximal voluntary contraction (MVC). <i>V</i>₀ of voluntarily activated muscle significantly increased with increasing contraction intensity (solid line: <i>R</i>² = 0.52, <i>n</i> = 18, <i>P</i><0.001), whereas that of electrically activated muscle did not (dashed line: <i>R</i>²<0.001, <i>n</i> = 18, <i>P</i> = 0.98).</p

Examples of linear regression lines determined with ordinary least-squares method (dashed line) and iterative reweighted least-squares algorithm (solid line).

<p>Representative data from <i>subjects</i> L (left panel) and I (right panel) are shown. The iterative reweighted least-squares regression proved to be robust against the outliers due to the fluctuations in torque baseline and the attenuated torque redevelopment.</p

Schematic diagram of motor-driven ankle dynamometer for the slack test.

<p>The torque produced by the ankle dorsiflexor as well as plantar flexor muscles was measured with a pair of strain gauges, whereas the ankle rotation was measured with a potentiometer. The torque of a gear motor was transmitted to a footplate via dual clutch system. A quick release was applied to isometrically contracting dorsiflexor muscles by moving a brake lever. A coil spring was attached to the edge of a rotor for increasing initial acceleration of quick-release movement. Two mechanical stops were used to preset the range of angular displacement. For further details, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0013043#pone.0013043-Sasaki1" target="_blank">[8]</a>.</p