Taku Wakahara

Nonuniform Muscle Hypertrophy: Its Relation to Muscle Activation in Training Session

PURPOSE Muscle hypertrophy in response to resistance training has been reported to occur nonuniformly along the length of the muscle. The purpose of the present study was to examine whether the regional difference in muscle hypertrophy induced by a training intervention corresponds to the regional difference in muscle activation in the training session. METHODS Twelve young men participated in a training intervention program for the elbow extensors with a multijoint resistance exercise for 12 wk (3 d · wk(-1)). Before and after the intervention, cross-sectional areas of the triceps brachii along its length were measured with magnetic resonance images. A series of transverse relaxation time (T2)-weighted magnetic resonance images was recorded before and immediately after the first session of training intervention. The T2 was calculated for each pixel within the triceps brachii. In the images recorded after the session, the number of pixels with a T2 greater than the threshold (mean + 1 SD of T2 before the session) was expressed as the ratio to the whole number of pixels within the muscle and used as an index of muscle activation (percent activated area). RESULTS The percent activated area of the triceps brachii in the first session was significantly higher in the middle regions than that in the most proximal region. Similarly, the relative change in cross-sectional area induced by the training intervention was also significantly greater in the middle regions than the most proximal region. CONCLUSION The results suggest that nonuniform muscle hypertrophy after training intervention is due to the region-specific muscle activation during the training session.

Relationships Between Muscle Strength and Indices of Muscle Cross-sectional Area Determined During Maximal Voluntary Contraction in Middle-aged and Elderly Individuals

Akagi, R, Takai, Y, Kato, E, Fukuda, M, Wakahara, T, Ohta, M, Kanehisa, H, Kawakami, Y, and Fukunaga, T. Relationships between muscle strength and indices of muscle cross-sectional area determined during maximal voluntary contraction in middle-aged and elderly individuals. J Strength Cond Res 23(4): 1258-1262, 2009-The present study examined how muscle cross-sectional area (CSA) indices determined at rest and during maximal voluntary contraction (MVC) are related to muscle strength in middle-aged and elderly individuals (22 men and 36 women, 51-77 years). The muscle thickness (MT) of elbow flexors and circumference (C) at the level 60% distal to the upper arm was measured by ultrasonography and a measuring tape, respectively, both at rest and during isometric MVC of elbow flexion. The muscle strength (F) of elbow flexors was calculated by dividing the torque developed during MVC by the forearm length of each subject. The product of MT and C (MT×C) and the square of MT (MT2) were defined as the muscle CSA indices. The F was significantly correlated with MT×C during MVC (r = 0.905, p ≤ 0.001) and at rest (r = 0.778, p ≤ 0.001), with the former relationship significantly stronger than the latter (p ≤ 0.001). Similarly, F was significantly correlated with MT2 both during MVC (r = 0.896, p ≤ 0.001) and at rest (r = 0.780, p ≤ 0.001), and there was also a significant difference between the correlation coefficients (p ≤ 0.01). These findings show that, in middle-aged and elderly individuals, muscle strength is more closely related to muscle CSA indices during MVC than at rest. It is concluded that the present muscle CSA indices taken during MVC enable easy and practical evaluation of the muscle strength per size of elbow flexors in middle-aged and elderly individuals.

In vivo measurement of human rectus femoris architecture by ultrasonography: Validity and applicability

The architectural feature of the rectus femoris (RF) has been scarcely investigated despite its substantial contribution to knee extension torque and large plasticity in the muscularity. This study aimed to examine the reproducibility and validity of ultrasound measurements of RF architecture and interrelationships among the architectural parameters. After confirming the measurement accuracy of an examiner (measurement errors <1%), in vivo and cadaveric measurements of fascicle lengths and pennation angles of RF were performed. Day‐to‐day reproducibility of measurements was investigated in vivo including muscle thickness measurement. Validity of measurements was investigated by comparing the values between direct and ultrasound measurements for the cadaver. The intraclass correlation coefficients were 0·984, 0·960 and 0·932, and root mean square difference between measured values on 2 days was 0·8 mm, 3·1 mm and 1·4° for muscle thickness, fascicle length and pennation angle, respectively. The validity of measurements was similar or better than those of previous reports on other muscles. We also confirmed a positive correlation between the muscle thickness and the pennation angle as has been shown in other muscles. The current results warrant the use of B‐mode ultrasonography for examining the architectural characteristics of RF in vivo.

Validity of ultrasound muscle thickness measurements for predicting leg skeletal muscle mass in healthy Japanese middle-aged and older individuals

BackgroundThe skeletal muscle mass of the lower limb plays a role in its mobility during daily life. From the perspective of physical resources, leg muscle mass dominantly decreases after the end of the fifth decade. Therefore, an accurate estimate of the muscle mass is important for the middle-aged and older population. The present study aimed to clarify the validity of ultrasound muscle thickness (MT) measurements for predicting leg skeletal muscle mass (SM) in the healthy Japanese middle-aged and older population.FindingsMTs at four sites of the lower limb and the bone-free lean tissue mass (LTM) of the right leg were determined using brightness-mode ultrasonography and dual-energy X-ray absorptiometry (DXA), respectively, in 44 women and 33 men, 52- to 78-years old. LTM was used as a representative variable of leg skeletal muscle mass. In the model-development group (30 women and 22 men), regression analysis produced an equation with R2 and standard error of the estimate (SEE) of 0.958 and 0.3 kg, respectively: LTM (kg) = 0.01464 × (MTSUM×L) (cm2) - 2.767, where MTSUM is the sum of the product of MTs at four sites, and L is length of segment where MT is determined. The estimated LTM (7.0 ± 1.7 kg) did not significantly differ from the measured LTM (7.0 ± 1.7 kg), without a significant systematic error on a Bland-Altman plot. The application of this equation for the cross-validation group (14 women and 11 men) did not yield a significant difference between the measured (7.2 ± 1.6 kg) or estimated (7.2 ± 1.6 kg) LTM and systematic error.ConclusionThe developed prediction equation may be useful for estimating the lean tissue mass of the lower extremity for the healthy Japanese middle-aged and older population.

Effects of muscle cooling on the stiffness of the human gastrocnemius muscle in vivo

Background/Aims: The effects of muscle cooling on the stiffness of the human gastrocnemius muscle (GAS) were examined in vivo. Methods: The knee joint was passively extended from 90 to 0° (0° = full knee extended position) with a constant ankle angle of 10° dorsiflexed position (0° = the sole of the foot is approximately perpendicular to the anterior margin of the shaft of the tibia) in a control condition (room temperature of 18–23°C) and a cooling condition (muscle temperature decreased by 5.8 ± 1.7°C after cooling using a cold water bath at a temperature of 5–8°C for 60 min). The change in passive Achilles tendon force, muscle fascicle length of GAS and muscle temperature were measured (n = 6) during the motion. Results and Conclusion: GAS stiffness was significantly greater in the cooling condition (20 ± 8 N/mm) than the control condition (18 ± 8 N/mm). There was no cooling effect on the muscle slack length, beyond which passive muscle force arises. The maximum passive Achilles tendon force significantly increased by 19 ± 20% after cooling. These results suggested that cooling increased the passive muscle force due to the increase in the muscle stiffness rather than the shift of the muscle slack length.

Applicability of ultrasound muscle thickness measurements for predicting fat-free mass in elderly population

Objective: This study aimed to examine the applicability of ultrasound muscle thickness (MT) measurements for predicting whole body fat-free mass (FFM) in elderly individuals. Design and setting: Crosssectional study of 77 healthy elderly individuals. Methods: MTs at nine sites of the body and FFM were determined using B-mode ultrasound and dual-energy x-ray absorptiometry (DXA), respectively, in 44 women and 33 men aged 52 to 78 yrs. Stepwise multiple regression analysis produced two equations for predicting DXA-based FFM with sex (dummy: woman = 0 and man = 1) and either MTs at the anterior and posterior of thigh and lower leg (Eq1) or the product of MT and limb length (MT×LL) at thigh anterior and posterior, lower leg posterior, and upper arm anterior (Eq2) as independent variables. Results: The R2 and SEE for each of the two equations were 0.929 and 2.5 kg for Eq1 and 0.955 and 2.0 kg for Eq2. The estimated FFM from each of Eq1 (44.4 ± 8.9 kg) and Eq2 (44.4 ± 9.0 kg) did not significantly differ from that of the DXA-based FFM (44.4 ± 9.2 kg), without systematic error. However, the absolute value of the difference between the DXA-based and estimated FFM was significantly greater with Eq1 (2.0 ± 1.5 kg) than with Eq2 (1.5 ± 1.3 kg). Conclusion: The current results indicate that ultrasound MT measurement is useful to predict FFM in the elderly, and its accuracy is improved by using the product of MT and limb length as an independent variable.

Task-Dependent Inhomogeneous Muscle Activities within the Bi-Articular Human Rectus Femoris Muscle

The motor nerve of the bi-articular rectus femoris muscle is generally split from the femoral nerve trunk into two sub-branches just before it reaches the distal and proximal regions of the muscle. In this study, we examined whether the regional difference in muscle activities exists within the human rectus femoris muscle during maximal voluntary isometric contractions of knee extension and hip flexion. Surface electromyographic signals were recorded from the distal, middle, and proximal regions. In addition, twitch responses were evoked by stimulating the femoral nerve with supramaximal intensity. The root mean square value of electromyographic amplitude during each voluntary task was normalized to the maximal compound muscle action potential amplitude (M-wave) for each region. The electromyographic amplitudes were significantly smaller during hip flexion than during knee extension task for all regions. There was no significant difference in the normalized electromyographic amplitude during knee extension among regions within the rectus femoris muscle, whereas those were significantly smaller in the distal than in the middle and proximal regions during hip flexion task. These results indicate that the bi-articular rectus femoris muscle is differentially controlled along the longitudinal direction and that in particular the distal region of the muscle cannot be fully activated during hip flexion.

Non‐uniform muscle oxygenation despite uniform neuromuscular activity within the vastus lateralis during fatiguing heavy resistance exercise

Previous studies have reported for the vastus lateralis (VL) that the extent of muscle hypertrophy in response to resistance training is greater in the distal than in the middle region, despite uniform muscle fibre composition within VL along its length. In the present study, to investigate mechanism(s) for such non‐uniform muscle hypertrophy, we simultaneously measured neuromuscular activity and muscle oxygenation state at the middle and distal regions of VL during fatiguing heavy resistance exercise. Twelve males performed unilateral knee extension exercise which consisted of 4 sets of 8 repetitions at intensity of 80% of the individual one repetition maximum. During the resistance exercise, neuromuscular activities and muscle oxygenation status at the middle and distal regions (50% and 70% of the thigh length, respectively) of VL were measured by using electromyography and near‐infrared spectroscopy, respectively. Neuromuscular activities were similar between the distal and middle regions of VL, whereas muscle tissue oxygenation saturation was significantly lower at the distal than at the middle region of VL. These results suggest a possibility that the regional difference in muscle oxygenation but not in neuromuscular activity during fatiguing heavy resistance exercise is responsible for the regional difference in hypertrophy within a muscle.

Influence of muscle anatomical cross-sectional area on the moment arm length of the triceps brachii muscle at the elbow joint

The purpose of this study was to test the hypothesis that the musculotendon moment arm length is affected by the muscle anatomical cross-sectional area. The moment arm length of the triceps brachii (TB) muscle at 30°, 50°, 70°, 90°, 110° elbow flexion positions was measured in sagittal magnetic resonance images (MRI) of 18 subjects as the perpendicular distance between the center of the pulley of the humerus to the line through the center of the TB tendon. The moment arm increased as the elbow flexion angle decreased, from 1.74±0.13 cm at 110° to 2.39±0.14 cm at 30°. The maximal anatomical cross-sectional area of the TB muscle was significantly correlated with the moment arms at all joint positions (r=0.545-0.803, p<0.05). Furthermore, the circumference of the upper arm was also significantly correlated with the moment arms at all joint positions, except for 70° (r=0.504-0.702, p<0.05). These results indicate that the moment arm length of the TB muscle is affected by the muscle anatomical cross-sectional area.

Variability of limb muscle size in young men

The purpose of this study was to determine the interindividual variability of the upper and lower limb muscle size in young men. Subjects were 655 Japanese men aged 18–39 years. They were sedentary and mildly to highly active individuals, including college athletes of various sports. Muscle thicknesses at each of the anterior and posterior portions of the upper arm, thigh, and lower leg were measured using B‐mode ultrasonography. Interindividual variability of muscle thickness was evaluated by coefficients of variation (CVs). The CVs of muscle thicknesses were found to be in the order of upper arm posterior (17.7%), thigh anterior (14.8%), thigh posterior (12.6%), upper arm anterior (12.2%), lower leg anterior (9.8%), and lower leg posterior (9.4%). The CVs were significantly different between each pair of measurement sites except for those of upper arm anterior‐thigh posterior and lower leg anterior‐posterior. These differences remain significant even when the muscle thicknesses were normalized to the segment length. The observed differences in the size variability can be interpreted as muscle‐related differences in hypertrophic responsiveness to resistance training. The muscle‐dependent size variability may be related to the differences in the fiber‐type composition and/or muscle usage in daily life among examined muscle groups. Am. J. Hum. Biol. 2010.