Kazushige Sasaki

Acceleration and force reveal different mechanisms of electromechanical delay.

INTRODUCTION Electromechanical delay (EMD) represents a series of complex processes of converting an electrical stimulus to a mechanical response. To quantify the contribution of electrochemical and mechanical processes of EMD in the human biceps brachii muscle over a wide range of elbow joint angles, we determined the onset of muscle contraction and the beginning of force development by recording acceleration of skin surface over the muscle and elbow flexion force, respectively. METHODS Ten healthy male volunteers underwent two experimental sessions, in which submaximal paired-pulse stimuli were applied percutaneously to the resting biceps brachii muscle at 10 different elbow joint angles from 40° to 130° (0° represents full extension). RESULTS The electrical stimulation induced repeatable contractions, in which the test-retest reliability of time parameters was sufficiently high (intraclass correlation coefficient=0.84-0.88). The time for electrochemical process ranged between 3.1±0.8 and 3.6±0.9 ms and was independent of elbow joint angle (P=0.64). The time for mechanical process and the total duration of EMD, however, were significantly greater at elbow flexion positions than at 40°, the most extended position in this study (P<0.05). Regression analysis revealed that at elbow flexion positions, the time for mechanical process increased significantly with decreasing the muscle-tendon length of the biceps brachii calculated from a musculoskeletal model (R=0.54, P<0.001). CONCLUSIONS 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.

Vitamin C administration attenuates overload‐induced skeletal muscle hypertrophy in rats

This study aimed to investigate the effects of vitamin C administration on skeletal muscle hypertrophy induced by mechanical overload in rats.

Length-force characteristics of in vivo human muscle reflected by supersonic shear imaging.

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.

Endocrine responses to upper- and lower-limb resistance exercises with blood flow restriction

To compare endocrine responses to low-intensity resistance exercise with blood flow restriction (BFR) for upperlimb (UL) and lower-limb (LL) muscles, we measured blood lactate, plasma noradrenaline, and serum growth hormone (GH), testosterone, cortisol and insulin-like growth factor-I (IGF-I) before and after the UL (biceps curl and triceps press down) and LL (leg extension and leg curl) exercises with BFR in nine men (26.3 +/- 3.1 yr). The load of 30% of one-repetition maximum (1RM) was used in all the exercises, in which the first set of 30 repetitions was followed by the second and third sets to failure. In each exercise program, the proximal portions of their upper arms (UL) or thighs (LL) were compressed bilaterally by elastic belts. Both the UL and LL caused significant increases in lactate, noradrenaline, GH, testosterone, cortisol, and IGF-I concentrations when compared to the pre-exercise values. A significant difference between the UL and LL was observed only in the area under the curve (AUC) of serum GH concentration, indicating that the LL induced greater GH response than did the UL. The greater GH secretion following the LL may be more advantageous for muscle hypertrophy induced by a long-term training period.

Shortening velocity of human triceps surae muscle measured with the slack test in vivo

Unloaded shortening velocity (V0) of human triceps surae muscle was measured in vivo by applying the ‘slack test’, originally developed for determining V0 of single muscle fibres, to voluntary contractions at varied activation levels (ALs). V0 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, V0 tended to increase with AL (R2= 0.089; P= 0.035) up to 60%MVC (8.6 ± 2.6 rad s−1). This value of V0 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 V0 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, V0 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.

Activation of fast-twitch fibers assessed with twitch potentiation.

Introduction: The augmentation of twitch response following brief muscle activation, called twitch potentiation, has been shown to be much more pronounced in fast‐twitch than in slow‐twitch fibers. We thus explored the possibility of twitch potentiation as a noninvasive measure of fast‐twitch fiber activation, by studying its dependence on the intensity of preceding contraction. Methods: Twitch contraction of plantar flexor muscles was evoked with supramaximal stimulation of the posterior tibial nerve, before and immediately after 6‐s voluntary contractions at intensities of 10–100% of maximal voluntary contraction (MVC). Results: Except for low‐intensity contractions (< 30%MVC), voluntary contraction induced twitch potentiation, the magnitude of which increased with increasing contraction intensity (P < 0.001). The shortened contractile process was associated with the potentiation. Conclusions: 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. Muscle Nerve 46: 218–227, 2012

Unloaded shortening velocity of voluntarily and electrically activated human dorsiflexor muscles in vivo.

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.

Comments on point:counterpoint: skeletal muscle mechanical efficiency does/does not increase with age.

TO THE EDITOR: There is little doubt that the cost of locomotion is impaired with age as it is evident that centenarians, due to their exceptional longevity, present unique adaptations in their skeletal muscle efficiency to compensate for their extremely low VO2max. However, the underlying question of whether skeletal muscle efficiency is altered with age is unsettled (4, 6). In fact, both opponents present compelling evidence in support of their opinion, and the reason for this disagreement is likely that both authors are looking at different muscles. Indeed, there is accumulating evidence that age-related alterations in skeletal muscle efficiency vary among muscle group. For instance, a selective atrophy, independent of the fiber type, has been documented in skeletal muscles with age (5). It is therefore likely that a similar phenomenon occurs for muscle energetics properties. The suggestion that ATP cost of contraction is improved with age (6) is based on examinations of the tibialis anterior. Interestingly, this muscle also exhibits preserved features with age in terms of mitochondrial efficiency (1) and oxidative capacity (3). In contrast, reduced oxidative capacity (3), in conjunction with impaired mitochondrial and contractile efficiency (2), has been documented in the quadriceps, which would, at least partially, explain the increased cost of locomotion with age as this muscle group is a major contributor to force production during these activities. The reasons for the heterogeneous effect of aging on skeletal muscle efficiency remain unclear, but likely stem from differences in fiber type and chronic load associated with locomotion.