Morihiko Okada

Muscle function in 164 men and women aged 20-84 yr

PURPOSE The purpose of the present study was to investigate the effect of aging in men and women on muscle functional properties, i.e., muscle force and force per unit of cross-sectional area (force/CSA). METHODS A total of 164 volunteers participated in this study and were divided into five groups according to their chronological age as follows: 20s (20--39 yr old), 40s, 50s, 60s, and 70s (70--84 yr old). Isokinetic (0, 60, 180, and 300 degrees.s(-1)) knee extensor and flexor peak torque, and CSA of the quadriceps femoris (QF) muscle of the mid-thigh were measured. RESULTS Peak torque during knee extension and flexion was inversely related to age in both men and women. This was the case irrespective of the speed of contraction in both genders (men: r = -0.797 to -0.756, all P < 0.001, women: r = -0.639 to -0.530, all P < 0.001). A significant correlation was observed between CSA of QF and peak torque during isometric knee extension in men (r = 0.827, P < 0.001) and women (r = 0.657, P < 0.001). During isometric contraction, the force/CSA exhibited a significant decrease with increasing age in men (r = -0.518, P < 0.001) but not in women (r = -0.207, NS). CONCLUSION These results thus suggest that muscle strength losses would be mainly due to a decline in muscle mass in both genders, whereas age-related decline in muscle function in men may also be the result of neural factors, such as muscle recruitment and/or specific tension.

Biomechanical analysis of primate bipedal walking by computer simulation

Abstract A computer simulation technique was applied to make clear the mechanical characteristics of primate bipedal walking. A primate body and the walking mechanism were modeled mathematically with a set of dynamic equations. Using a digital computer, the following were calculated from these equations by substituting measured displacements and morphological data of each segment of the primate: the acceleration, joint angle, center of gravity, foot force, joint moment, muscular force, transmitted force at the joint, electric activity of the muscle, generated power by the leg and energy expenditure in walking. The model was evaluated by comparing some of the calculated results with the experimental results such as foot force and electromyographic data, and improved in order to obtain the agreement between them. The level bipedal walking of man, chimpanzee and Japanese monkey and several types of synthesized walking were analyzed from the viewpoint of biomechanics. It is concluded that the bipedal walking of chimpanzee is nearer to that of man than to that of the Japanese monkey because of its propulsive mechanism, but it requires large muscular force for supporting the body weight.

Influence of contraction force and speed on muscle fiber conduction velocity during dynamic voluntary exercise.

Before using electromyographic (EMG) variables such as muscle fiber conduction velocity (MFCV) and the mean or median frequency (MDF) of an EMG power spectrum as indicators of muscular fatigue during dynamic exercises, it is necessary to determine the influence of a joint angle, contraction force and contraction speed on the EMG variables. If these factors affect the EMG variables, their influence must be removed or compensated for before discussing fatigue. The vastus lateralis of eight normal healthy male adults was studied. EMG signals during non-fatiguing dynamic knee extension exercises were detected with a three-bar active surface electrode array. EMG variables were calculated from the detected signals and compared with the angle of the knee joint, the extension torque and the extension speed. The extension torque was set at four levels with 10% intervals between 40 and 70% of the maximum voluntary contraction. The extension speed was set at five levels with 60 degrees /s intervals between 0 and 240 degrees /s. Because the joint angle unsystematically affected the MFCV, EMG variables at a given joint angle were extracted for comparison. The influence of the extension torque and speed on the extracted EMG variables was clarified with an ANOVA and a regression analysis. The statistical analyses showed that MFCV increased with the extension torque but did not depend on the extension speed. In contrast, MDF was independent of the extension torque but was dependent on the extension speed. MDF thus showed a behavior different from that of MFCV. It became clear that if MFCV is used as an indicator of muscular fatigue during dynamic exercises, it is at least necessary to extract MFCV at a predetermined joint angle and then remove the influence of extension torque on MFCV.

A muscular fatigue index based on the relationships between superimposed M wave and preceding background activity

A practical muscle fatigue index is studied in this paper using the correlation between the instantaneous frequencies (IFs) of the superimposed M wave and the mean power frequency (MPF) of the preceding background activity. A superimposed M wave is an M wave elicited during a sustained contraction and was recently introduced for studying muscle fatigue. The authors investigated the details of the distribution of a feature vector (mpf, if) in two-dimensional space. Their experimental results showed that MPF and IFs were closely correlated during the first phase of a short-term high-level sustained voluntary contraction and then became uncorrelated or sometimes showed negative correlation as muscular fatigue progressed. Combining the correlation coefficients and conventional myoelectric (ME) parameters, such as the MPF and the average rectified value of ME signals, the authors propose a fuzzy rule based muscular fatigue index that can be used for managing the inevitable variability among individual subjects collected as a group. Introducing fuzzy inference seemed effective, but further studies including detailed investigation of the level of voluntary effort, the muscle fiber type composition, and metabolic by-products will be needed to customize the membership functions and fuzzy rules more appropriately in each practical field.

Innervation Zones in the Back Muscles Investigated by Multichannel Surface EMG

By detecting the propagation of motor unit action potentials (MUAPs) with a linear surface electrode array we can locate the innervation zones (motor endplate regions) and estimate the muscle fibre conduction velocity. This technique has mostly been applied to muscles in the upper and lower extremities. The purpose of the present study was to clarify whether the same technique is applicable to three back muscles: the trapezius, the latissimus dorsi and the erector spinae, which are related to application fields including ergonomic problems and low back pain studies. The results indicated that the trapezius showed clear propagation of MUAPs and the innervation zones defined by the source of the propagation were distributed in a narrow band running along the midpoint of muscle fibres. Propagation of MUAPs in the latissimus dorsi was not so clear as that in the trapezius, but it was still possible to locate the innervation zones, which were scattered in a large area and distributed concentrically with respect to the fossa axillaris. The erector spinae, which consisted of many distinct muscles, showed no clear propagation of MUAPs and its innervation zones could not be defined by the surface electrode array technique.

Distribution of muscle fibre types in the thoracic and lumbar epaxial muscles of Japanese macaques (Macaca fuscata).

Histochemical properties are investigated in the thoracic and lumbar epaxial muscles of Japanese macaques, Macaca fuscata. Whole transverse sections of the muscles were removed at the different levels of the thoracic and lumbar vertebrae and muscle fibre types were classified into type I and type II fibres on the basis of their histochemical profiles. In the lumbar region the iliocostalis and longissimus muscles have many type II fibres. In the thoracic region the iliocostalis muscle has more type I fibres than the longissimus muscle. The intermammillares and mammilloaccessorii muscles which lie medially and deep to the longissimus muscle in the lumbar region have an extremely large number of type I fibres. These differences in the distribution of muscle fibre types among and within muscles suggest that functional differentiation exists among and within the muscles.

AGE-RELATED EMG VARIABLES DURING MAXIMUM VOLUNTARY CONTRACTION

We studied the neuromuscular adaptation that occurs with aging by comparing changes in surface electromyography (EMG) variables from the tibialis anterior muscle in 12 young (21.4 ± 1.7 yr. old) and 13 older subjects (70.8 ± 3.0 yr. old). EMG variables such as the muscle fiber conduction velocity, median frequency, and averaged rectified value were calculated during maximum voluntary contraction for 5-sec, isometric contractions. The dorsiflexion force, muscle fiber conduction velocity, median frequency, and average rectified value during maximum voluntary contraction were significantly smaller in the older than in the younger group (p <.05). These results suggested char the neuromuscular system in older subjects is affected by the selective atrophy of fast twitch fibers and differences in motor unit firing statistics. Our results suggest the utility of applying the EMG observed during maximum voluntary contraction to the noninvasive evaluation of neuromuscular function in elderly persons.

Human Posture in Zero Gravity [and Comments and Reply]

New, unpublished data from the Skylab Experiment is presented which provides a metric description of mans posture in space where there is no gravity to activate the postural reflexes. Some of the implications are discussed.

Evaluation of fatigue using heart rate variability and myoelectric signals during skiing

We studied the change in functional activities during skiing exercise for a whole day, in terms of the interactions between cardiovascular and neuromuscular activities with the progression of fatigue. To evaluate functional activities at each phase, we developed a radio controlled remote data acquisition system for measuring multichannel biosignals. Then we used the wavelet analysis for analyzing the details in the heart rate variability. Based on the previous results, we proposed a scatter graph between an autonomic nervous activity related index and a muscular fatigue related index. The scatter graph was effective as a tool for a snapshot evaluation of fatigue or degeneration of functional activities during long-term physical exercise.

Depth and intensity of equivalent current dipoles estimated through an inverse analysis of surface electromyograms using the image method

The depth and intensity of equivalent current dipoles that can create the surface potentials of active motor units in human skeletal muscles are estimated through an inverse analysis of surface electromyographic (EMG) potentials in an attempt to measure detailed muscular activity non-invasively. The inverse analysis is conducted by repetition of forward analyses. In the study, the image method is used for forward analysis, because it is the simplest potential calculation method for electric currents in a semi-infinite volume conductor. Using this method, surface EMG potentials are calculated for current sources assumed to be located in a muscle. An inverse analysis is then carried out by searching for the depth and intensity of such current sources that would minimise the sum of squares difference between measured and calculated surface EMG potentials. The inverse analysis is applied to surface EMG potentials measured from the biceps brachii of three healthy subjects. As a result, the individual current sources are estimated to be 2.7±1.6 mm deep and 0.5±0.9 nAm in intensity, whereas the total current intensity for individual motor units is 2.4±2.9 nAm.