Yasuhide Yoshitake

MUSCLE SHEAR MODULUS MEASURED WITH ULTRASOUND SHEAR-WAVE ELASTOGRAPHY ACROSS A WIDE RANGE OF CONTRACTION INTENSITY

Introduction: In this study we examine the repeatability of measuring muscle shear modulus using ultrasound shear‐wave elastography between trials and between days, and the association between shear modulus and contraction intensity over a wide range of intensities. Methods: Shear modulus of the biceps brachii was determined using ultrasound shear‐wave elastography during static elbow flexion (up to 60% of maximal contraction) in healthy young adults. Results: The correspondence of shear modulus was confirmed in phantoms between the manufacturer‐calibrated values and the shear‐wave elastography values. The intraclass correlation coefficient of muscle shear modulus was high: 0.978 between trials and 0.948 between days. Shear modulus increased linearly with elbow flexion torque across contraction intensity, and its slope was associated negatively with muscle strength. Conclusions: Muscle shear modulus measured with ultrasound shear‐wave elastography may be useful for inferring muscle stiffness across a wide range of contraction intensity. In addition, it has high repeatability between trials and between days. Muscle Nerve 50: 103–113, 2014

Validity of measurement of shear modulus by ultrasound shear wave elastography in human pennate muscle.

Ultrasound shear wave elastography is becoming a valuable tool for measuring mechanical properties of individual muscles. Since ultrasound shear wave elastography measures shear modulus along the principal axis of the probe (i.e., along the transverse axis of the imaging plane), the measured shear modulus most accurately represents the mechanical property of the muscle along the fascicle direction when the probe’s principal axis is parallel to the fascicle direction in the plane of the ultrasound image. However, it is unclear how the measured shear modulus is affected by the probe angle relative to the fascicle direction in the same plane. The purpose of the present study was therefore to examine whether the angle between the principal axis of the probe and the fascicle direction in the same plane affects the measured shear modulus. Shear modulus in seven specially-designed tissue-mimicking phantoms, and in eleven human in-vivo biceps brachii and medial gastrocnemius were determined by using ultrasound shear wave elastography. The probe was positioned parallel or 20° obliquely to the fascicle across the B-mode images. The reproducibility of shear modulus measurements was high for both parallel and oblique conditions. Although there was a significant effect of the probe angle relative to the fascicle on the shear modulus in human experiment, the magnitude was negligibly small. These findings indicate that the ultrasound shear wave elastography is a valid tool for evaluating the mechanical property of pennate muscles along the fascicle direction.

Muscle Deoxygenation during Sustained and Intermittent Isometric Exercise in Hypoxia

PURPOSE It is reported that the rate of locomotor muscle fatigue development during intermittent isometric exercise in hypoxia is accelerated compared with normoxia. In contrast, when sustained isometric contractions are used, some studies do not show any effect of hypoxia on fatigue development. Increased intramuscular pressure during sustained isometric exercise causes substantial and sustained ischemia, even in normoxia. Therefore, we hypothesized that the difference in muscle deoxygenation between normoxia and hypoxia would be small during sustained exercise compared with intermittent exercise and that this may contribute to the inconsistent findings. METHODS Subjects performed sustained and intermittent isometric, unilateral, and submaximal knee-extension exercises (60% maximal voluntary contraction to exhaustion) while breathing normoxic (inspired O2 fraction = 0.21) or hypoxic gas mixtures (inspired O2 fraction = 0.10-0.12). Muscle oxygenation (deoxyhemoglobin/myoglobin and tissue oxygenation index) using near-infrared spectroscopy and surface EMG were measured from the left vastus lateralis. RESULTS During intermittent isometric exercise in hypoxia, increases in deoxyhemoglobin/myoglobin and reductions of tissue oxygenation index were larger (P < 0.05) than those in normoxia. The rate of rise in integrated EMG during intermittent exercise was accelerated (P < 0.05) in hypoxia. In contrast, there were no significant differences in changes in near-infrared spectroscopy variables and integrated EMG during sustained isometric exercise between normoxia and hypoxia. CONCLUSIONS These results suggest that muscle deoxygenation is exaggerated during intermittent isometric exercise in hypoxia compared with normoxia, whereas during sustained isometric exercise, the extent of muscle deoxygenation is the same between normoxia and hypoxia. The different extent of muscle deoxygenation during sustained and intermittent isometric exercise in normoxia and hypoxia could affect muscle fatigability, which results from the varied rate of accumulation of metabolites.

Alterations in synergistic muscle activation impact fluctuations in net force.

Net muscle force recorded during voluntary contractions with multiple agonist muscles is the summated result of individual muscle forces. The purpose of this article is to synthesize recent findings from several studies on the modulations of muscle activity and force fluctuations during steady voluntary contractions with multiple agonist muscles in humans. During a sustained low-force contraction with the knee extensor muscles, fluctuations in knee extension force changed concurrently with the involuntary alternate muscle activity between the rectus femoris muscle and the vasti muscles. After prolonged bed rest, the amount of change in the fluctuations in net force was associated with the amount of change in the EMG distribution among individual muscles within the knee extensor group and the ankle extensor group. As an adaptation to bed rest, an increase in the fluctuations in plantarflexion force accompanied an increase in the relative contribution of the medial gastrocnemius muscle to net force. In contrast, an increased contribution of the medial gastrocnemius by an acute increase in muscle length reduced force fluctuations. The discrepancy in the changes in force fluctuations due to an increased contribution of the medial gastrocnemius between two interventions (bed rest and muscle length change) is associated with increased low-frequency power in the EMG of the medial gastrocnemius after bed rest. In summary, the fluctuations in net force were influenced by the alteration in muscle activity among agonist muscles in terms of relative contributions to net force and frequency characteristics.

Association between Knee Extensor Strength and EMG Activities during Squat Movement

PURPOSE The present study aimed to clarify how the force-generating capability of quadriceps femoris (QF) is associated to its surface EMG activity during a body mass-based squat movement. METHODS Isometric knee extension torque (KET) during maximal voluntary contraction and EMG activities of the rectus femoris and vastus lateralis muscles during a body mass-based squat movement were determined in 53 men and 48 women age 19-90 yr, including 18 frail elderly persons who used the long-term care insurance system. The rectified EMG signals during the squat movement were averaged and normalized as the relative value (%EMG(max)) to that during maximal voluntary contraction. The %EMG(max) values for rectus femoris and vastus lateralis were averaged and used as an index representing the level of muscular activities of QF during the squat movement (QF %EMG(max)). RESULTS QF %EMG(max) was nonlinearly related to KET relative to body mass (KET/BM). Linear piecewise continuous regression analysis showed that there was a breakpoint of 1.9 N·m·kg(-1) in the relationship between the two variables. In individuals with KET/BM less than 1.9 N·m·kg(-1), QF %EMG(max) rapidly increased as KET/BM decreased. CONCLUSIONS The current results indicate that the activity level of QF during a body mass-based squat movement is influenced by its force generation capability. For individuals with a KET/BM less than 1.9 N·m·kg(-1), body mass-based squat movement is considered to be a fairly high-intensity exercise. The breakpoint of 1.9 N·m·kg(-1) may be assumed to be a threshold level of knee extensor strength, which should be maintained for performing the activities of daily living without great difficulty.

Low-frequency component of rectified EMG is temporally correlated with force and instantaneous rate of force fluctuations during steady contractions.

Introduction: The usefulness of surface EMG for assessing motor output variability during steady contraction is unclear. The purpose of this study was to examine the temporal correlation in signal characteristics between low‐frequency components of full‐wave rectified EMG (with or without high‐pass filtering before rectification) and force fluctuations or the instantaneous rate of force fluctuations (dF/dt) during steady contractions. Methods: Fourteen young adults produced steady force with the first dorsal interosseus muscle at various intensities. Cross‐correlation functions were calculated between the following signals: <5 Hz: force, dF/dt, and rectified EMG with or without high‐pass filtering at 300 Hz before rectification. Results: Rectified EMG correlated better with dF/dt compared with force itself, and high‐pass filtering of interference EMG before rectification improved the temporal correlation. Conclusions: The low‐frequency component of rectified EMG following high‐pass filtering may provide temporal information on the instantaneous rate of force fluctuations during steady contractions. Muscle Nerve 47: [?show

Laser-detected lateral muscle displacement is correlated with force fluctuations during voluntary contractions in humans

1534v]–[?show

Effect of short-term maximal voluntary co-contraction training on neuromuscular function.

1535v], 2013

Body mass-based exercise in middle-aged and older women.

Fluctuations in muscle force during steady voluntary contractions result from the summation of twitch forces produced by asynchronous activation of multiple motor units. We hypothesized that oscillatory lateral muscle displacement, measured with a non-contact high-resolution laser displacement sensor, is correlated with force fluctuations during steady, voluntary contractions with a human muscle. Eight healthy young adults (20-33 yrs) performed steady isometric contractions with the first dorsal interosseus muscle. Contraction intensity ranged from 2.5% to 60% of the maximal voluntary contraction force. Oscillatory lateral displacement of the muscle surface was measured with a high-resolution laser displacement sensor (0.5 microm resolution), concurrently with abduction force of the index finger. In the time-domain analysis, there was a significant positive peak in the cross-correlation function between lateral muscle displacement and force fluctuations. In addition, the amplitude increased linearly with contraction intensity in both signals. In the frequency-domain analysis, frequency content was similar in both signals, and there was significant coherence between signals for the major frequency range of the signals (<5 Hz). In conclusion, laser-detected lateral displacement of a hand muscle is correlated with force fluctuations across a wide range of contraction intensity during steady voluntary contractions in humans.

Subthreshold electrical stimulation reduces motor unit discharge variability and decreases the force fluctuations of plantar flexion

The present study aimed to examine the effect of short-term training utilizing voluntary co-contraction with maximal efforts. 23 healthy young men (training group: TG, n = 13; control group: CG, n = 10) participated in this study. TG conducted a 4-week training program (3 days/week), which consisted of 4 s simultaneous maximal voluntary contractions of elbow flexors and extensors at 90° of the elbow joint, followed by 4 s muscle relaxation (10 repetitions/set, 5 sets/day). Before and after the intervention, maximal voluntary isometric and isokinetic torques and the muscle thicknesses of the elbow flexors and extensors were determined. The electromyograms (EMGs) of the 2 muscle groups during isometric maximal voluntary contraction (MVC) were also recorded. After the intervention, CG did not show any significant changes in all measured variables. In TG, significant increases were found in the agonist EMG activities during MVC, and maximal isometric and isokinetic torques of the elbow flexors and extensors, without significant changes in the muscle thicknesses and involuntary coactivation levels during MVC. The current results indicate that the training mode with maximal voluntary co-contraction is effective for improving the force-generating capabilities of the exercising muscles, without any increases in the level of involuntary coactivation during MVC.