Minoru Shinohara

Oxygen availability and motor unit activity in humans

SummarySix men were studied to determine the interrelationships among blood supply, motor unit (MU) activity and lactate concentrations during intermittent isometric contractions of the hand grip muscles. The subjects performed repeated contractions at 20% of maximal voluntary contraction (MVC) for 2 s followed by 2-s rest for 4 min with either unhindered blood circulation or arterial occlusion given between the 1st and 2nd min. The simultaneously recorded intramuscular MU spikes and surface electromyogram (EMG) data indicated that mean MU spike amplitude, firing frequency and the parameters of surface EMG power spectra (mean power frequency and root mean square amplitude) remained constant during the experiment with unhindered circulation, providing no electrophysiological signs of muscle fatigue. Significant increases in mean MU spike amplitude and frequency were, however, evident during the contractions with arterial occlusion. Similar patterns of significant changes in the surface EMG spectra parameters and venous lactate concentration were also observed, while the integrated force-time curves remained constant. These data would suggest that the metabolic state of the active muscles may have played an important role in the regulation of MU recruitment and rate coding patterns during exercise.

Efficacy of tourniquet ischemia for strength training with low resistance.

Abstract To investigate the efficacy of ischemia in strength training with low mechanical stress, tourniquet ischemia was utilized in low-resistance training. Five untrained subjects conducted one-legged isometric knee extension training with one leg ischemic (I-leg) and the other non-ischemic (NI-leg). Repeated isometric contractions for 2 s with 3 s relaxation in between were continued for 3 min and conducted 3 days/week for 4 weeks as training. Training resistance was 40% of maximal voluntary contraction (MVC) of respective leg and tourniquet ischemia was applied during I-leg training. MVC in I-leg after 2 weeks (9% gain) and 4 weeks (26% gain) were significantly higher than pre-training value (p < 0.05). A significant increase in maximal rate of torque development in I-leg was observed after 4 weeks (p < 0.05). On the contrary, there was no significant changes in either of the parameters in NI-leg. As a consequence, the differences between legs for both parameters were significant after 2 and 4 weeks (p < 0.05 or p < 0.01). The substantial gain in strength and maximal rate of torque development in I-leg demonstrated the efficacy of tourniquet ischemia during low-resistance training of short duration, and suggested the importance of neuromuscular and/or metabolic activity, other than high mechanical stress, to the adapting responses to strength training.

Mechanomyography of the human quadriceps muscle during incremental cycle ergometry.

Abstract The mechanical activity of the human quadriceps muscle during maximal incremental cycle ergometry was investigated by mechanomyography (MMG). MMG and surface electromyography (EMG) recordings of vastus lateralis muscle activity were obtained from nine males. Cycle ergometry was performed at 60 rev/min and work load was incremented step wise by 20 W (3.2 Nm) every minute until volitional fatigue. The mean amplitudes of MMG (mMMG) and EMG (mEMG) during the contraction phase were calculated from the last six contractions in each load. The duration, load and work rate of exercise at exhaustion were 13.3 (1.6) min, 44.1 (5.5) Nm, 276.7 (34.7) W, respectively. A linear relationship between mMMG and load was evident in each subject (r = 0.868–0.995), while mEMG seemed to dissociate as the load became greater. In the grouped mean data, mMMG was linearly related to load whether aligned to the absolute (r = 0.995) or maximal (r = 0.995) load. Involvement of the noise component was further investigated by studying passive cycling by four subjects. Pedals were rotated passively for the first half of each stage (PAS) and the subject then pushed the pedals for the second half (ACT). In the lighter load region, the mMMG of ACT was as small as that of PAS. However, the change in the mMMG of PAS was very small compared with that of ACT. In conclusion, this study demonstrates a linear relationship between the mMMG of the quadriceps muscle and work load during maximal incremental cycle ergometry. The effect of movement noise was thought to be small and stable.

Non-uniform mechanical activity of quadriceps muscle during fatigue by repeated maximal voluntary contraction in humans

Abstract To determine the non-uniform surface mechanical activity of human quadriceps muscle during fatiguing activity, surface mechanomyogram (MMG), or muscle sound, and surface electromyogram (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles of seven subjects during unilateral isometric knee extension exercise. Time- and frequency-domain analyses of MMG and of EMG fatigued by 50 repeated maximal voluntary contractions (MVC) for 3 s, with 3-s relaxation in between, were compared among the muscles. The mean MVC force fell to 49.5 (SEM 2.0)% at the end of the repeated MVC. Integrated EMG decreased in a similar manner in each muscle head, but a marked non-uniformity was found for the decline in integrated MMG (iMMG). The fall in iMMG was most prominent for RF, followed by VM and VL. Moreover, the median frequency of MMG and the relative decrease in that of EMG in RF were significantly greater (P < 0.05) than those recorded for VL and VM. These results would suggest a divergence of mechanical activity within the quadriceps muscle during fatiguing activity by repeated MVC.

The ventilatory threshold gives maximal lactate steady state

SummaryThe purpose of this study was to examine whether the ventilatory threshold (Thv) would give the maximal lactate steady state ([1a]ss, max), which was defined as the highest work rate (W) attained by a subject without a progressive increase in blood lactate concentration [1a]b at constant intensity exercise. Firstly, 8 healthy men repeated ramp-work tests (20 W·min−1) on an electrically braked cycle ergometer on different days. During the tests, alveolar gas exchange was measured breath-by-breath, and theW atThv (WThv) was determined. The results of two-way ANOVA showed that the coefficient of variation of a singleWThv determination was 2.6%. Secondly, 13 men performed 30-min exercise atWThv (Thv trial) and at 4.9% aboveWThv (Thv + trial), which corresponded to the 95% confidence interval of the single determination. The [1a]b was measured at 15 and 30 min from the onset of exercise. The [1a]b at 15 min (3.15 mmol·1−1, SEM 0.14) and at 30 min (2.95 mmol·1−1, SEM 0.18) were not significantly different inThv trial. However, the [1a]b ofThv+ trial significantly increased (P<0.05) from 15 min (3.62 mmol·1−1, SEM 0.36) to 30 min (3.91 mmol·1−1, SEM 0.40). These results indicate thatThv gives the [1a]ss,max, at which one can perform sustained exercise without continuous [1a]b accumulation.

Mechanomyogram from the different heads of the quadriceps muscle during incremental knee extension

Abstract To investigate the time- and frequency-domain responses of mechanomyograms (MMGs) during the progressive fatigue induced by intermittent incremental contractions, a surface MMG was obtained from the three muscle heads of the quadriceps muscle in seven subjects while they performed isometric knee extensions lasting 7.6 min. Isometric intermittent incremental contractions started at 1% of the maximal voluntary contraction (MVC) for 3 s, with a 3-s relaxation period in between each contraction, and the contraction level was increased by 1% of MVC for every contraction (by 10% of MVC per min) up to exhaustion. Separate contractions with sufficient rest periods were also conducted to serve for the MMG characteristics without fatigue. The integrated MMG (iMMG) was linearly related to force in all of the muscles when fatigue was not involved. With regard to the incremental contractions, the relationship exhibited an ascending-descending shape, but the behavior was not the same for the individual muscle heads, especially for the rectus femoris muscle. A steep increase in the median frequency of MMG from around 60% of MVC corresponded to a decrease in iMMG. These results suggest that analysis of MMG in the time- and frequency-domain during an incremental protocol is a useful way of characterizing the motor unit recruitment strategy and fatigue properties of individual muscles.

Nonlinear time-course of lumbar muscle fatigue using recurrence quantifications

Abstract. Isometric skeletal muscle fatigue is usually assumed to be a linear process based upon the monotonic decrease in spectral frequency of the EMG. Since spectral analysis by fast Fourier transform (FFT) constitutes a linear transformation of the data, the present study was designed to reevaluate the time-course of muscle fatigue with a nonlinear tool, recurrence quantification analysis (RQA). Surface EMG recordings were obtained from the multifidus muscle of 17 human subjects during isometric posture-holding of the upper torso. The process of muscle fatigue was found to be linear for 59% of the subjects by FFT criteria, but nonlinear for 76% by RQA criteria. As a demonstrative control, both slow and fast transients occurring within a nonlinear mathematical process could be accurately depicted by RQA, but not by FFT. It is concluded that assessment of EMG patterns by nonlinear techniques can give insight into the time-course of fatiguing muscles attributed to the summation of several nonlinear and competing processes.

Exhausting handgrip exercise reduces the blood flow in the active calf muscle exercising at low intensity

The calf and forearm blood flows (Qcalf and Qforearm respectively), blood pressure, heart rate and oxygen uptake of six men and women were studied during combined leg and handgrip exercise to determine whether a reduction of exercise-induced hyperaemia would occur in the active leg when exhausting rhythmic handgrip exercise at 50% maximal voluntary contraction (MVC) was superimposed upon rhythmic plantar flexion lasting for 10 min at 10% MVC (P10) prior to this combined exercise. The Qcalf and Qforearm were measured by venous occlusion plethysmography during 5-s rests interposed during every minute of P10 exercise and immediately after combined exercise. The muscle sympathetic nerve activity (MSNA) changes were also recorded during leg exercise alone and combined exercise. During plantar flexion performed 60 times · min−1 with a load equal to 10% MVC (P10), Qcalf was maintained at a constant level, which was significantly higher than the resting value (P < 0.001). When rhythmic handgrip contraction at 50% MVC (H50) and P10 were performed simultaneously, the combined exercise was concluded due to forearm exhaustion after a mean of 51.2 (SEM 5.5) s. At exhaustion, Qcalf had decreased significantly from 20.6 (SEM 3.0) ml · 100 ml−1 · min−1 (10th min during P10 exercise) to 15.3 (SEM) ml · 100 ml−1 · min−1 (P = 0.001), whereas Qforearm had increased significantly (0.001 < P < 0.01) from 8.6 (SEM 1.9) ml · 100 ml−1 · min−1 (10th min of P10 exercise) to 26.2 (SEM 3.2) ml · 100 ml−1 · min−1. The mean blood pressure remained at an almost constant level during the 3rd to 10th min of P10 exercise and increased markedly when H50 was added. The calf vascular conductance during combined exercise decreased significantly (0.001 < P < 0.01) compared with that at the 10th min of P10 alone. Although the MSNA (expressed as burst rate) remained unchanged during P10 exercise for 10 min, it increased markedly when exhausting H50 and P10 exercise were performed simultaneously. These findings indicated that superimposition of exhausting handgrip exercise at 50% MVC caused a vasoconstriction in the exercising calf due to increased MSNA, which counteracted the vasodilatation in this active muscle.

New trends for practice in telecommunication applied to preventive and environmental medicine.

This paper presents survey results of connectivity to the Internet from preventive and environmental medicine-related departments in medical schools and other institutions in Japan and propose means to establish connectivity among them. Of 191 facilities surveyed, 134 (70%) responded by March 31, 1996. The data presented here are from 132 facilities. One hundred seventeen facilities (89%) answered that they were connected to the Internet. More than 80% of them got access to the Internet in the past two years. One hundred three facilities (78%) answered that e-mail was available. Despite the large percentage being connected, only 11 facilities (8%) had their own homepages. However, just 6 months later more than 25 facilities could be found by their own homepages. The Global Health Network (GHNet) has been developed in the USA based upon the concept that the best means to produce improved health is a better surveillance and information system applying the latest telecommunication technology to public health. The GHNet will offer an initial homepage for Preventive and Environmental Medicine related facilities in Japan to promote and establish sustainable connectivity among them.