Tetsuo Fukunaga

Sex differences in whole body skeletal muscle mass measured by magnetic resonance imaging and its distribution in young Japanese adults

Objectives: To determine sex differences in the distribution of regional and total skeletal muscle (SM) using contiguous whole body magnetic resonance imaging (MRI) data, and to examine the relations between fat free mass (FFM) and total and regional SM masses. Methods: A total of 20 Japanese college students (10 women and 10 men) volunteered for the study. FFM was measured by two compartment densitometry. Whole body MRI images were prepared using a 1.5 T scanner. Contiguous transverse images with 1.0 cm slice thickness were obtained from the first cervical vertebra to the ankle joints. All MRI scans were segmented into four components (SM, subcutaneous adipose tissue, bone, and residual tissues). In each slice, the SM tissue cross sectional areas (CSAs) were digitised, and the muscle tissue volume per slice was calculated by multiplying muscle CSA by slice thickness. SM volume units (litres) were converted into mass units (kg) by multiplying the volumes by the assumed constant density (1.041 mg/ml) for SM. Results: The SM distribution pattern (shape of curve) from the contiguous whole body slices was essentially similar for the two sexes, with two large peaks and three smaller peaks (arms excluded). However, the largest peak was observed at the upper portion of the thigh for women and at the level of the shoulder for men. Men had larger (p<0.01) total and regional SM mass than women. All regional SM masses correlated highly (r = 0.90–0.99, p<0.01) with total SM mass. A strong positive correlation was observed between FFM and total and regional SM masses in both sexes (women, r = 0.95; men, r = 0.90; all p<0.01). As FFM increased, there was a corresponding increase in SM/FFM ratio for all subjects (r = 0.86, p<0.01). Conclusions: Sex differences in total SM/FFM ratio and regional SM distributions are associated with the degree of absolute FFM accumulation in men and women.

Muscle volume compared to cross-sectional area is more appropriate for evaluating muscle strength in young and elderly individuals

OBJECTIVE the present study examined which of muscle volume (MV) and cross-sectional area (CSA) is appropriate for evaluating the relation with elbow flexor muscle strength in young and elderly individuals. METHODS the subjects were 52 young (20-34 year; 30 men and 22 women) and 51 elderly individuals (60-77 year, 19 men and 32 women). The MV and maximal anatomical CSA (ACSA) of elbow flexors were determined by magnetic resonance imaging. The torque developed during maximal voluntary contraction of isometric elbow joint flexion was converted to force by dividing it by the forearm length of each subject. RESULTS torque was significantly correlated with MV in young and elderly individuals (r = 0.564-0.926). Similarly, force was also significantly correlated with ACSA in each of them (r = 0.637-0.906). However, the y-intercepts of the regression lines for the ACSA-force relationship in young men and women were significantly higher than zero. There was no age effect on torque per MV, whereas force per ACSA was significantly higher in young adults than in elderly individuals. CONCLUSION for elbow flexors, MV compared to ACSA is appropriate for evaluating the size-strength relationship and the existence of age-related difference in muscle strength per size.

Activation of agonist and antagonist muscles at different joint angles during maximal isometric efforts

The purpose of this study was to investigate the influence of different angles of the knee joint on the activation level of an agonist (quadriceps femoris muscle) and antagonist (biceps femoris muscle) from electromyographic activities and activation levels (twitch interpolation). Isometric torque measurements were performed on 23 healthy subjects at 10° intervals between 40° and 110° of knee joint flexion. Superimposed twitches at maximal voluntary contraction were applied and the voluntary activation estimated. To quantify the antagonist muscle activity, we normalized its integrated EMG (iEMG) value at each joint angle with respect to its iEMG value at the same angle when acting as an agonist at maximal effort. The activation levels at the knee-flexed position (80–110°) were higher than that at the knee-extended position (40–70°). The co-activation levels at 90, 100, and 110° were significantly higher than that the other knee angle. These results suggest that the activation level of an agonist (quadriceps femoris) muscle and the co-activation level of an antagonist (biceps femoris) muscle were higher in longer muscles than in shorter muscles. It was also concluded that the risk of knee injuries could be reduced by applying these mutual relationships between activation levels of agonist and antagonist muscles.

Larger center of pressure minus center of gravity in the elderly induces larger body acceleration during quiet standing.

When an inverted pendulum approximates quiet standing, it is assumed that the distance between the center of pressure and the vertical projection of the center of mass on the ground (COP-COG) reflects the relationship between the controlling and controlled variables of the balance control mechanism, and that the center of mass acceleration (ACC) is proportional to COP-COG. As aging affects the control mechanism of balance during quiet standing, COP-COG must be influenced by aging and, as a result, ACC is influenced by aging as well. The purpose of this study was to test the hypotheses that aging results in an increased COP-COG amplitude and, as a consequence, that ACC becomes larger in the elderly than the young. Fifteen elderly and 11 young subjects stood quietly on a force platform with their eyes open or closed. We found that (1) the standard deviations of COP-COG and ACC were larger in the elderly than in the young, irrespective of the eye condition; (2) COP-COG is proportional to ACC in both age groups, i.e., the inverted pendulum assumption holds true for quiet standing. The results suggest that a change in the control strategy that is due to aging causes a larger COP-COG in the elderly and, as a consequence, that ACC becomes larger as well.

Effects of 20 days of bed rest on the viscoelastic properties of tendon structures in lower limb muscles

Objectives: The purpose of this study was to investigate the effects of 20 days’ bed rest on the viscoelastic properties of human tendon structures in knee extensor and plantar flexor muscles in vivo. Methods: Eight healthy men (age: 24±4 years, height: 172±9 m, body mass: 69±13 kg) carried out a 6° head-down bed rest for 20 days. Before and after bed rest, elongation (L) of the tendon and aponeurosis of vastus lateralis (VL) and medial gastrocnemius muscles (MG) during isometric knee extension and plantar flexion, respectively, were determined using real-time ultrasonic apparatus, while the subjects performed ramp isometric contraction up to the voluntary maximum, followed by ramp relaxation. The relationship between estimated muscle force (Fm) and tendon elongation (L) was fitted to a linear regression, the slope of which was defined as stiffness. The hysteresis was calculated as the ratio of the area within the Fm-L loop to the area beneath the load portion of the curve. Results: L values above 100 N were significantly greater after bed rest for VL, while there were no significant differences in L values between before and after for MG. The stiffness decreased after bed rest for VL (70.3±27.4 v 50.1±24.8 N/mm, before and after bed rest, respectively; p = 0.003) and MG (29.4±7.5 v 25.6±7.8 N/mm, before and after bed rest, respectively; p = 0.054). In addition, hysteresis increased after bed rest for VL (16.5±7.1% v 28.2±12.9%, before and after bed rest, respectively; p = 0.017), but not for MG (17.4±4.4% v 17.7±6.1%, before and after bed rest, respectively; p = 0.925). Conclusions: These results suggested that bed rest decreased the stiffness of human tendon structures and increased their hysteresis, and that these changes were found in knee extensors, but not the plantar flexors.

Ubiquitin ligase gene expression in healthy volunteers with 20-day bedrest.

In animal models, several ubiquitin ligases play an important role in skeletal muscle atrophy caused by unloading. In this study we examined protein ubiquitination and ubiquitin ligase gene expression in quadriceps femoris muscle from healthy volunteers after 20‐day bedrest to clarify ubiquitin‐dependent proteolysis in human muscles after unloading. During bedrest, thickness and cross‐sectional area of the quadriceps femoris muscle decreased significantly by 4.6% and 3.7%, respectively. Ubiquitinated proteins accumulated in these atrophied human muscles. A real‐time reverse transcription–polymerase chain reaction system showed that bedrest significantly upregulated expression of two ubiquitin ligase genes, Cbl‐b and atrogin‐1. We also performed DNA microarray analysis to examine comprehensive gene expression in the atrophied muscle. Bedrest mainly suppressed the expression of muscle genes associated with control of gene expression in skeletal muscle. Our results suggest that, in humans, Cbl‐b– or atrogin‐1–mediated ubiquitination plays an important role in unloading‐induced muscle atrophy, and that unloading stress may preferentially inhibit transcriptional responses in skeletal muscle. Muscle Nerve, 2006

Effects of isometric training at different knee angles on the muscle–tendon complex in vivo

The purpose of this study was to investigate the influences of isometric training at different joint angles on the muscle size and function of the human muscle–tendon complex in vivo. Furthermore, we tried to gain a better understanding of the mechanisms involved in angle specificity after isometric training from the aspect of neuromuscular adaptation and the changes in the properties of the muscle–tendon complex. Nine males completed 12‐week unilateral training program (70% of maximal voluntary contraction (MVC) × 15 s × six sets) on the knee extensors at 50° (shorter muscle length: ST) and 100° (longer muscle length: LT). The internal muscle force (mechanical stress) is higher at 100° than at 50° because of the difference in the moment arm length, although there were no difference in the relative torque level, contraction and relaxation times, and repetition between ST and LT. Before and after training, isometric strength at eight angles and muscle volume were determined. Tendon elongation of knee extensors was measured by ultrasonography. There was no significant difference in the rate of increment of muscle volume between the protocols. Tendon stiffness increased significantly for LT, but not for ST. Although significant gain was limited to angles at or near the training angle for ST, increases in MVC at all angles were found for LT. These results suggest that only mechanical stress (internal muscle force imposed on muscle and tendon) contributes to adaptation in the tendon stiffness, although metabolic (relative torque level, etc.) and mechanical stress relate to muscle hypertrophy. Furthermore, increment of tendon stiffness for LT might contribute to increase torque output at smaller angles other than the training angle.

Effects of viscoelastic properties of tendon structures on stretch – shortening cycle exercise in vivo

The aim of the present study was to examine the effects of viscoelastic properties of human tendon structures during stretch – shortening cycle exercise. The elongation of tendon and aponeurosis of the medial gastrocnemius muscle of 26 participants was measured by ultrasonography while they performed ramp isometric plantar flexion up to the voluntary maximum, followed by a ramp relaxation. The relationship between estimated muscle force and tendon elongation during the ascending phase was fitted to a linear regression, the slope of which was defined as stiffness. The percentage of the area within the muscle force – tendon elongation loop relative to the area beneath the curve during the ascending phase was defined as hysteresis. In addition, maximal voluntary concentric contractions at 2.09 and 3.14 rad · s−1 with and without prior eccentric contractions were performed. The difference in the concentric torque at equivalent joint angles with and without prior eccentric contractions (i.e. pre-stretch augmentation) was negatively correlated with stiffness (P  < 0.05) and hysteresis (P  < 0.05). Furthermore, there was a higher correlation between the pre-stretch augmentation and the viscoelastic properties index – that is, the sum of normalized score values of stiffness and hysteresis (P  < 0.01) – than with either stiffness or hysteresis alone. The results of this study suggest that performance during stretch – shortening cycle exercise is significantly affected by the viscoelastic properties of the tendon structures.

The Relationship Between Passive Ankle Plantar Flexion Joint Torque and Gastrocnemius Muscle and Achilles Tendon Stiffness: Implications for Flexibility

STUDY DESIGN Experimental laboratory study. OBJECTIVES We tested the hypothesis that the muscle fibers and the connective tendinous structures, combined in series, provide the resistance to passive joint movement at the ankle. We also determined the relative association between passive joint torque and each of these 2 elements. BACKGROUND The reason for individual variation in joint flexibility or tightness is not clearly understood, but the influence of musculotendinous stiffness has been inferred. METHODS AND MEASURES Each of the subjects (6 women and 6 men) was seated with the right knee extended and right ankle positioned at a 30 degrees , 20 degrees , 10 degrees , 0 degrees , -10 degrees , -20 degrees , and -30 degrees (0, neutral position, positive values reflecting plantar flexion) angle while passive plantar flexion torque was measured. The distal muscle-tendon junction of the medial gastrocnemius was visualized by ultrasonography, and its positional change was defined as muscle belly length change. The whole muscle-tendon unit length change was estimated from joint angle changes, from which Achilles tendon length change was estimated. RESULTS Both the muscle belly and tendon were significantly elongated as the ankle was dorsiflexed (at 0 degrees the mean +/- SD muscle belly elongation was 10.3% +/- 1.8 %, and the tendon elongation was 2.8% +/- 1.2 %, of the initial length at 30 degrees of ankle plantar flexion), from which stiffness indices were determined both for muscle belly and tendon. The passive torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees was significantly correlated with the stiffness indices of the Achilles tendon (at 0 degrees , r2 = 0.70 and 0.62 for overall and specific stiffness, respectively; P<.05). A tendon stiffness index, separately obtained from tendon lengthening during maximal isometric contraction, was also correlated with passive ankle plantar flexion torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees (at 0 degrees , r2 = 0.76; P<.05). The specific stiffness index of the muscle belly was correlated (r2 = 0.47, P<.05) with the passive ankle plantar flexion torque at 0 degrees , but its overall stiffness index was not (r2 = 0.32, P>.05). CONCLUSION Results suggest that extensibility of the muscle-tendon unit of the Achilles tendon for the most part is related to passive ankle plantar flexion joint torque.

Establishing a new index of muscle cross-sectional area and its relationship with isometric muscle strength

The present study aimed i) to establish an index of muscle cross-sectional area (CSA) based on muscle thickness and circumference through a comparison with muscle CSA determined by magnetic resonance imaging (MRI), and ii) to examine the relationships between muscle strength and the index determined at rest and during the maximal isometric contraction. The muscle CSA of elbow flexors at 60% of the upper arm length (CSA60) and the maximal CSA of elbow flexors (CSAmax) were measured using MRI in 26 men and 8 women. The muscle thickness (MT) of elbow flexors and the circumference (C) of upper arm at 60% of the upper arm length were measured using ultrasonography and anthropometry, respectively, in 29 men and 9 women. The measurements of MT and C were performed in the resting (MTr and Cr) and contracted condition (MTm and Cm), where the subjects performed maximal voluntary contraction (MVC) of isometric elbow joint flexion. The torque developed during MVC was converted into the muscle force (F) of elbow flexors. The MTr × Cr was significantly correlated both with CSA60 and CSAmax (P < 0.001). The F was significantly correlated with MTm × Cm (r = 0.847, P < 0.001) and MTr × Cr (r = 0.839, P < 0.001). However, stepwise multiple regression analysis selected only MTm × Cm as a significant contributor for estimating F. The present study indicates that MT × C reflects muscle CSA, and can be an index for assessing muscle CSA. In addition, the findings obtained here showed a possibility that MT × C during MVC is more closely related to F than that at rest.