No abstract

These results suggest that, in the human biceps brachii muscle, the prolongation of EMD at short muscle-tendon length is not attributed to the impairment of the electrochemical process of muscle contraction but to the increased slack within the muscle-tendon unit.

Recently, an ultrasound-based elastography technique has been used to measure stiffness (shear modulus) of an active human muscle along the axis of contraction. Using this technique, we explored 1) whether muscle shear modulus, like muscle force, is length dependent; and 2) whether the length dependence of muscle shear modulus is consistent between electrically elicited and voluntary contractions. From nine healthy participants, ankle joint torque and shear modulus of the tibialis anterior muscle were measured at five different ankle joint angles during tetanic contractions and during maximal voluntary contractions. Fascicle length, pennation angle, and tendon moment arm length of the tetanized tibialis anterior calculated from ultrasound images were used to reveal the length-dependent changes in muscle force and shear modulus. Over the range of joint angles examined, both force and shear modulus of the tetanized muscle increased with increasing fascicle length. Regression analysis of normalized data revealed a significant linear relationship between force and shear modulus (R(2) = 0.52, n = 45, P < 0.001). Although the length dependence of shear modulus was consistent, irrespective of contraction mode, the slope of length-shear modulus relationship was steeper during maximal voluntary contractions than during tetanic contractions. These results provide novel evidence that length-force relationship, one of the most fundamental characteristics of muscle, can be inferred from in vivo imaging of shear modulus in the tibialis anterior muscle. Furthermore, the estimation of length-force relationship may be applicable to voluntary contractions in which neural and mechanical interactions of multiple muscles are involved.

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 < 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 < 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.

Unloaded shortening velocity (V 0 ) of human triceps surae muscle was measured in vivo by applying the 'slack test', originally developed for determining V 0 of single muscle fibres, to voluntary contractions at varied activation levels (ALs). V 0 was measured from 10 subjects at five different ALs defined as a fraction (5, 10, 20, 40 and 60%) of the maximum voluntary contraction (MVC) torque. Although individual variability was apparent, V 0 tended to increase with AL (R 2 = 0.089; P = 0.035) up to 60%MVC (8.6 ± 2.6 rad s −1 ). This value of V 0 at 60%MVC was comparable to the maximum shortening velocity of plantar flexors reported in the previous studies. Electromyographic analysis showed that the activities of soleus, medial gastrocnemius and lateral gastrocnemius muscles increased with AL during isometric contraction and after the application of quick release in a similar manner. Also, it showed that the activity of an antagonist, tibialis anterior muscle, was negligible, even though a slight increase took place after the quick release of agonist. Correlation analysis showed that there were no significant correlations between V 0 and MVC torque normalized with respect to body mass, although the correlation coefficient was relatively high at low ALs. The results suggest that in human muscle, V 0 represents the unloaded velocity of the fastest muscle fibres recruited, and increases with AL possibly because of progressive recruitment of faster fibres. Individual variability may be explained, at least partially, by the difference in fibre-type composition. In the locomotory performance of organisms including humans, contractile force and velocity of skeletal muscles play primary roles. Whereas muscle force basically depends on the amount of overlap between actin and myosin filaments in the sarcomere (length-force relation), shortening velocity depends on the ratio between the maximum isometric force and the load applied to the muscle (force-velocity relation). When the load approaches zero, the velocity is ultimately independent of force-generating capacity and depends on the rate of cross-bridge cycling and the number of sarcomeres in series.Most of the information on contractile force and velocity in human muscles has been obtained by using isokinetic tests, in which joint torque is measured under constant angular velocity. Previous studies have shown the similarity between force-velocity relations obtained from isolated muscles and those from human muscles in vivo, i.e. muscle force (or joint torque) decreases progressively as shortening velocity increases. However, a recent study suggests that isokinetic tests can describe the proper force-velocity characteristics of human muscles only in a limited range of velocities (Desplantez & Goubel, 2002). In fact, when using a hyperbola (Hill, 1938) to extrapolate the isokinetic torque-velocity data to the velocity at zero torque (extrapolated maximum velocity; V max ), unrealistic values can be found for V max (Desplantez & Goubel, 2002). Therefore, a different appr...

These results are consistent with the concept of "hierarchical order of fiber activation", suggesting that the magnitude of twitch potentiation reflects the activation of fast-twitch fibers during a brief contraction.

We have previously shown that unloaded shortening velocity (V 0) 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 V 0 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 V 0, with an intraclass correlation coefficient of 0.87 (95% confidence interval: 0.68–0.95). Regression analysis showed that V 0 of voluntarily activated dorsiflexor muscles significantly increased with increasing contraction intensity (R 2 = 0.52, P<0.001). By contrast, V 0 of electrically activated dorsiflexor muscles remained unchanged (R 2<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.

Compression stockings are known to be effective in reducing peripheral oedema or leg swelling by increasing tissue pressure and venous blood flow. While previous studies on compression stockings have focused on its prolonged, preventive effect on leg swelling, the aim of this study was to investigate an acute effect of wearing compression stockings on lower leg swelling and muscle stiffness. Twenty healthy women aged 18-23 years participated in the experiment conducted in the evening, in which they wore below-knee graduated compression stockings and rested in a seated position for 30 min. Before and after the application of stockings, maximum calf circumference, volume, extracellular water resistance (R ) and muscle stiffness of the right lower leg were determined by tape measure, water displacement volumetry, segmental bioelectrical impedance spectroscopy and ultrasound shear-wave elastography, respectively. The maximum calf circumference and the reciprocal of R (an index of extracellular fluid volume) significantly decreased after the application of stockings, whereas the total lower leg volume and the stiffness (shear modulus) of the medial gastrocnemius muscle tended to decrease. These changes, except for that in the medial gastrocnemius muscle stiffness, were opposite to those from morning to evening studied in the subgroup of participants (n = 8). However, partial correlation analysis failed to detect significant associations among these changes. These results suggest that even for a short period of application, compression stockings have some positive effects against lower leg swelling.