Norihiro Shima

Changes in blood volume and oxygenation level in a working muscle during a crank cycle.

PURPOSE This study examined circulatory and metabolic changes in a working muscle during a crank cycle in a pedaling exercise with near-infrared spectroscopy (NIRS). METHODS NIRS measurements sampled under stable metabolic and cadence conditions during incremental pedaling exercise were reordered according to the crank angles whose signals were obtained in eight male subjects. RESULTS The reordered changes in muscle blood volume during a crank cycle demonstrated a pattern change that corresponded to changes in pedal force and electrical muscle activity for pedal thrust. The top and bottom peaks for muscle blood volume change at work intensities of 180 W and 220 W always preceded (88 +/- 32 and 92 +/- 23 ms, respectively) those for muscle oxygenation changes. Significant differences in the level of NIRS parameters (muscle blood volume and oxygenation level) among work intensities were noted with a common shape in curve changes related to pedal force. In addition, a temporary increase in muscle blood volume following a pedal thrust was detected at work intensities higher than moderate. This temporary increase in muscle blood volume might reflect muscle blood flow restriction caused by pedal thrusts. CONCLUSION The results suggest that circulatory and metabolic conditions of a working muscle can be easily affected during pedaling exercise by work intensity. The present method, reordering of NIRS parameters against crank angle, serves as a useful measure in providing additional findings of circulatory dynamics and metabolic changes in a working muscle during pedaling exercise.

Effect of intermittent hypoxia on cardiovascular adaptations and response to progressive hypoxia in humans.

The aim of the present study was to elucidate (1) the cardiovascular adaptations and response to hypoxic stimuli during short-term intermittent hypoxia and (2) whether the change in cardiovascular response to hypoxia is correlated to the change in hypoxic ventilatory chemosensitivity. Fourteen subjects were decompressed in a chamber to 432 torr, simulating an altitude of 4500 m, over a period of 30 min and were maintained at that pressure for 1 h daily for 7 days. Ventilatory (DeltaV(I)/DeltaSa(O2); Sa(O2) is arterial oxygen saturation), systolic and diastolic blood pressure (DeltaSBP/DeltaSa(O2) and DeltaDBP/DeltaSa(O2)), and heart rate (DeltaHR/DeltaSa(O2)) responses to progressive isocapnic hypoxia were measured before and after intermittent hypoxia. Resting ventilation, SBP, DBP, and HR did not change after intermittent hypoxia. DeltaSBP/DeltaSa(O2) and DeltaDBP/DeltaSa(O2) increased significantly after intermittent hypoxia accompanied by an enhanced DeltaV(I)/DeltaSa(C2), but there was no change in DeltaHR/DeltaSa(C2). There were significant correlations between the change in DeltaV(I)/DeltaSa(O2) and both the changes in DeltaSBP/DeltaSa(O2) and DeltaDBP/DeltaSa(O2) following intermittent hypoxic exposure. These results suggest that short-term intermittent hypoxia leads to the enhanced arterial BP response to hypoxic stimuli in humans, and that the enhanced peripheral chemosensitivity to hypoxia after intermittent hypoxia may play an important role in the increased arterial BP response.

Mechanomyographic and electromyographic responses to stimulated and voluntary contractions in the dorsiflexors of young and old men

The effect of age on mechanomyography (MMG) has not been examined for electrically evoked contractions. Similar to torque, we expected that postactivation potentiation of the MMG would differ between young and old subjects. Additionally, under voluntary conditions, we compared normalized MMG and electromyographic (EMG) signals in relation to torque, and expected that MMG, unlike EMG, would be affected by age. In 10 young and 10 old men, electrical stimulation was delivered before and after a 10‐s maximal voluntary contraction (MVC) to assess potentiation of contractile (twitch torque; Pt), electrical (M‐wave amplitude), and mechanical (MMG amplitude) properties of the dorsiflexors. Subsequently, subjects performed voluntary contractions at 20%, 40%, 60%, 80%, and 100% MVC for calculation of normalized MMG–torque and EMG–torque relationships. Following the MVC, Pt and evoked MMG were larger than at rest in both groups, but M‐wave amplitude was unchanged. Twitch potentiation was greater in young than old, whereas evoked MMG was unaffected by age. Under voluntary conditions, values for MMG and EMG were similar between groups, except for greater MMG at MVC in young men. The shape of MMG and EMG relationships to torque was similar only in young men. Using the aging model, our results indicate that potentiation of the mechanical components (MMG) differ from those of twitch torque. Furthermore, the comparison of normalized voluntary MMG with age provides additional support for the concept of age‐related motor unit remodeling. Muscle Nerve, 2006

In vivo behaviour of human muscle architecture and mechanomyographic response using the interpolated twitch technique

This study investigated the origin of curvilinear change in the superimposed mechanomyogram (MMG) amplitude of the human medial gastrocnemius muscle (MG) with increasing contraction intensity. The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening were measured using ultrasonic images of electrical stimulation during isometric plantar flexions at levels 20%, 40%, 60%, 80%, and 100% of the maximal voluntary contraction (MVC). The superimposed twitch amplitude, the superimposed MMG amplitude and the extent of fascicle shortening decreased with increasing contraction intensity. The superimposed MMG amplitude and the extent of fascicle shortening showed a curvilinear decrease, while the superimposed twitch amplitude showed a linear decrease at levels up to 80% of the MVC. There was a linear relationship between the superimposed MMG amplitude and the extent of fascicle shortening at different contraction intensities. These results indicate that the superimposed MMG amplitude reflects changes in the extent of fascicle shortening at different contraction intensities better than the superimposed twitch amplitude. Our study suggests that the origin of the curvilinear decrease of superimposed MMG amplitude is associated with a curvilinear decrease of the extent of fascicle shortening with increasing contraction intensity in the human MG.

Changes in intra-abdominal pressure and spontaneous breath volume by magnitude of lifting effort: highly trained athletes versus healthy men

Intra-abdominal pressure (IAP) is closely related to breathing behavior during lifting. Abdominal muscles contribute to both IAP development and respiratory function. The purpose of this study was to examine whether spontaneous breath volume and IAP altered with increased isometric lifting effort, and to compare the effect of different abdominal muscle strengths on these parameters. Maximal IAP during the Valsalva maneuver (maxIAP) and maximal isometric trunk flexor strength were measured in 10 highly trained judo athletes (trained) and 11 healthy men (controls). They performed isometric lifting with 0 (rest), 30, 45, 60, 75, 90, and 100% of maximal lifting effort (MLE). Natural inspiratory and expiratory volumes were calculated from air-flow data immediately before and after the start of lifting. IAP, measured using an intra-rectal pressure transducer during lifting, was normalized by maxIAP (%maxIAP). Trained athletes had higher maxIAP and stronger trunk flexor muscles than controls. A significant main effect of lifting effort was found on %maxIAP and respiratory volume. An interaction (lifting effort by group) was found only for %maxIAP. No significant group main effect or interaction was found for respiratory volume. Inspiratory volume increased significantly from tidal volume to above 60 and 45% of MLE in trained athletes and controls, respectively. Expiratory volume decreased significantly from tidal volume at above 30% of MLE in both the groups. These results suggest that spontaneous breath volume and IAP development are coupled with increased lifting effort, and strong abdominal muscles can modify IAP development and inspiratory behavior during lifting.

Changes in force and tendinous tissue elongation during the early phase of tetanic summation in in vivo human tibialis anterior muscle

The purpose of this study was to determine the changes that occur in tendinous tissue properties during the early phase of tetanic summation in the in vivo human tibialis anterior muscle (TA). The torque response and tendinous tissue elongation following single stimuli, two-pulse trains, and three-pulse trains were recorded in the TA during isometric contractions. The elongation, compliance, and lengthening velocity of tendinous tissue were determined by real-time ultrasonography. The contribution of the response to the second stimulation (C2) was obtained by subtracting the response to the single stimulation (C1) from the response of doublet. The third contribution (C3) was obtained by subtracting the response to the doublet from that of the triplet. C2 (7.8+/-0.5 Nm) and C3 (7.3+/-0.6 Nm) had torque responses significantly higher than C1 (3.6+/-0.7 Nm). In contrast, the elongations of tendinous tissue for C2 (2.8+/-0.4mm) and C3 (1.7+/-0.2mm) were significantly lower than for C1 (4.9+/-0.3mm), indicating that the summation pattern of tendinous tissue elongation is different from the summation pattern of torque response. In addition, this showed considerable difference both between C1 (0.12+/-0.01 mm/N; 83+/-4.6mm/s) and C2 (0.03+/-0.005 mm/N; 50+/-6.3mm/s) and between C1 and C3 (0.02+/-0.002 mm/N; 39+/-6.4mm/s) in the compliance and lengthening velocity of tendinous tissue. These results suggest that changes in tendinous tissue properties between first and second contraction are related to different summation patterns of force and tendinous tissue elongation during early phase of tetanic summation.

Muscle Activation Characteristics of the Front Leg During Baseball Swings with Timing Correction for Sudden Velocity Decrease

This study aimed to clarify the activation characteristics of the vastus lateralis muscle in the front leg during timing correction for a sudden decrease in the velocity of a target during baseball swings. Eleven male collegiate baseball players performed coincident timing tasks that comprised constant velocity of 8 m/s (unchanged) and a sudden decrease in velocity from 8 to 4 m/s (decreased velocity). Electromyography (EMG) revealed that the muscle activation was typically monophasic when responding unchanged conditions. The type of muscle activation during swings in response to decreased velocity condition was both monophasic and biphasic. When biphasic activation appeared in response to decreased velocity, the impact time and the time to peak EMG amplitude were significantly prolonged and the timing error was significantly smaller than that of monophasic activation. However, the EMG onset from the target start was consistent both monophasic and biphasic activation in response to conditions of decreased velocity. In addition, batters with small timing errors in response to decreased velocity were more likely to generate biphasic EMG activation. These findings indicated that timing correction for a sudden decrease in the velocity of an oncoming target is achieved by modifying the muscle activation characteristics of the vastus lateralis muscle of front leg from monophasic to biphasic to delay reaching peak muscle activation and thus prolong impact time. Therefore, the present findings suggests that the extent of timing errors in response to decreased velocity is influenced by the ability to correct muscle activation after its initiation rather than by delaying the initiation timing of muscle activation during baseball swings.

The effect of summation of contraction on acceleration signals in human skeletal muscle.

The purpose of this study was to determine the effects of summation of contraction on acceleration signals in human skeletal muscle. The torque parameters of dorsiflexion and acceleration signals in the tibialis anterior muscle were measured during evoked isometric contractions. In an examination of two-pulse trains with different inter-pulse intervals, the torque and accelerometer responses to inter-pulse intervals of 10-100 ms were recorded. In an investigation of the effects of different numbers of stimuli, the torque and accelerometer responses to 1-8 pulses with a constant inter-pulse interval of 10 ms were recorded. The present study found that there was a difference in acceleration amplitude between the single-pulse and two-pulse trains with an inter-pulse interval of 10 ms but not two-pulse trains with an inter-pulse interval of 20 ms or more. In the investigation of different numbers of stimuli, we found a similar MMG amplitude across 2-8 pulses. Moreover, we observed that the maximal time to the peak acceleration signal was approximately 27 ms. In a comparison of torque parameters with acceleration signals, the present study clearly shows that acceleration amplitude is poorly correlated to changes in force parameters when the inter-pulse interval or the number of stimuli are increased. These results suggest that the absence of associated changes in acceleration peak is due to the long interval for the subsequent pulses relative to the time at which acceleration peak is achieved ( approximately 27 ms). These findings will provide useful information concerning the method for assessing summation of contraction with an accelerometer.

Utility of noninvasive brachial-ankle pulse wave velocity measurement in people with spinal cord injury

The purpose of the present study was to examine the utility of non-invasive brachial-ankle pulse wave velocity measurement in people with spinal cord injury based on the comparison of the data of the compliance of common femoral artery measured by ultrasonography. Five physically active persons with spinal cord injury volunteered for this study. Subjects were in a supine position on the bed, and estimate brachial-ankle pulse wave velocity and the compliance coefficient of common femoral artery. The brachial-ankle pulse wave velocity measurement is related to was significantly (p<;0.05) related to the compliance coefficient of common femoral artery. The brachial-ankle pulse wave velocity measurement may be helpful for assessment of arterial stiffness in people with spinal cord injury.

Ventilatory and circulatory responses at the onset of rapid changes in posture.

The transition from rest to light or moderate intensity exercise is typically accompanied by an abrupt step-like increment in ventilation at the first exercise breath. This phase I response is observed during not only voluntary exercise and passive movement, but also during electrically induced muscle contraction. Many investigators have pursued the mechanisms of this phase I response; however, it is still a matter of dispute as to its nature. Since changes in ventilation are so rapid, a number of neural mechanisms, such as reflexogenic influences from the cerebral cortex and hypothalamic motor regions, afferent stimulus through the group III and IV fibers, and central and/or peripheral circulatory influences, are postulated to explain the phase I response.1 Regarding the last factor, Huszczuk et al.2 suggested that ventilation is controlled by a mechanism which acts rapidly, and which relies on the integrity of baroreception and/or vasoregulation; baroreception appears to be more important. Although it has been reported that hemodynamic responses are considerably affected by a sudden change in posture, it is likely that respiratory response is also modified by postural changes in man. To our knowledge, however, the initial ventilatory responses to rapid change in posture have not been studied in detail. The purpose of this study was to clarify the ventilatory and circulatory responses at the onset of sudden tilts from supine to upright, and vice versa.