The aim of the present study was to examine the non-stationary mechanomyogram (MMG) during voluntary isometric ramp contractions of the biceps brachii muscles using the short-time Fourier transform, and to obtain more detailed information on the motor unit (MU) activation strategy underlying in the continuous MMG/force relationship. The subjects were asked to exert ramp contractions from 5% to 80% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. The root mean squared (RMS) amplitude of the MMG began to increase slowly at low levels of force, then there was a slight reduction between 12% and 20% MVC. After that, a progressive increase was followed by a decrease beyond 60% MVC. As to the mean power frequency (MPF), a relatively rapid increase up to 30% MVC was followed by a period of slow increment between 30% and 50% MVC. Then temporary reduction at around 50% MVC and a further rapid increase above 60% MVC was observed. The interaction between amplitude and MPF of the MMG in relation to the MU activation strategy is discussed for five force regions defined on the basis of their inflection points in the RMS-amplitude/force and MPF/force relationships. It was found that the MMG during ramp contractions enables deeper insights into the MU activation strategy than those determined during traditional separate contractions. In addition, this contraction protocol is useful not only to ensure higher force resolution in the MMG/force relationship, but also to markedly shorten the time taken for data acquisition and to reduce the risk of fatigue.

The aim of this study was to examine the mechanomyogram (MMG) and force relationship of the first dorsal interosseous (FDI) muscle as well as the biceps brachii (BB) muscle during voluntary isometric ramp contractions, and to elucidate the MMG responses resulting from the intrinsic motor unit (MU) activation strategy of FDI muscle with reference to the MMG of BB muscle. The subjects were asked to exert ramp contractions of FDI and BB muscle from 5% to 70% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. In FDI muscle, the root-mean-squared amplitude (RMS) of the MMG decreased slowly with force up to 21%, and then a progressive increase was followed by a relatively rapid decrease beyond 41% MVC. The RMS/%MVC relationship in BB muscle consisted of an initial slow increase followed by a rapid increase from 23% MVC and a progressive decrease beyond 61% MVC. With respect to the mean power frequency (MPF), FDI muscle demonstrated no obvious inflection point in the MPF/%MVC relationship compared with that in BB muscle. Namely, the MPF of FDI muscle increased linearly through the force levels exerted. In contrast to FDI muscle, the MPF/%MVC relationship in BB muscle was decomposed into four specific regions: (1) a relative rapidly increase (<34% MVC), (2) a slow increment (34-53% MVC), (3) a temporary reduction (53-62% MVC), and (4) a further rapid increase (>62% MVC). The different MMG responses between FDI and BB muscles are considered to reflect the fact that the MU activation strategy varies among different muscles in relation to their morphology and histochemical type. Namely, the rate coding of the MUs plays a more prominent role in force production in relatively small FDI muscle than does MU recruitment compared with their respective roles in the relatively large BB muscle.

Condenser microphones (MIC) have been widely used in mechanomyography, together with accelerometers and piezoelectric contact sensors. The aim of the present investigation was to clarify the mechanical variable (acceleration, velocity or displacement) indicated by the signal from a MIC transducer using a mechanical sinusoidal vibration system. In addition, the mechanomyogram (MMG) was recorded simultaneously with a MIC transducer and accelerometer (ACC) during voluntary contractions to confirm the mechanical variable reflected by the actual MMG and to examine the influence of motion artifact on the MMG. To measure the displacement-frequency response, mechanical sinusoidal vibrations of 3 to 300 Hz were applied to the MIC transducer with different sizes of air chambers (5, 10, 15 and 20 mm in diameter and 15, 20 or 25 mm long). The MIC transducer showed a linear relationship between the output amplitude and the vibration displacement, however, its frequency response declined with decreasing diameter and decreasing length of the air chamber. In fact, the cut-off frequency (-3dB) of the MIC transducer with the 5-mm-diameter chamber was 10, 8 and 4 Hz for the length 15, 20 and 25 mm, respectively. The air chamber with at least a diameter of 10 mm and a length of 15 mm is recommended for the MIC transducer. The sensitivity of this MIC transducer arrangement was 92 mV microm(-1) when excited at 100 Hz. During voluntary contraction, the amplitude spectral density function of the MMG from the MIC transducer resembled that of the double integral of the ACC transducer signal. The angle of the MIC transducer was delayed by 180 degrees in relation to the ACC transducer signal. The sensitivity of the MIC transducer was reduced to one-third because of the peculiar volume change of air chamber when the MMG was detected on the surface of the skin. In addition, the MIC transducer was contaminated by a smaller motion artifact than that from the ACC transducer. The maximal peak amplitude of the MIC and ACC transducer signal with the motion artifact was 7.7 and 12.3 times as much as the RMS amplitude of each signal without the motion artifact, respectively. These findings suggest that the MIC transducer acts as a displacement meter in the MMG. The MIC transducer seems to be a possible candidate for recording the MMG during dynamic muscle contractions as well as during sustained contractions.

This study was designed to examine the effect of increasing age on the recruitment and activation of motor units (MU) in the biceps brachii muscle, using the mechanomyogram (MMG)/force relationship during isometric ramp contractions. The relationships between the root mean squared amplitude (RMS) and mean power frequency (MPF) of the MMG and relative force (% MVC) in the elderly (male, n = 10, age = 69.8 +/- 4.7 years, mean +/- SD) were markedly different from those in the young group (male, n = 15, age = 22.7 +/- 1.8 years). In elderly individuals, the RMS increased progressively with force up to 57.6 +/- 3.4% MVC, when a brief rapid increase was followed by a stable trend beyond 63.6 +/- 3.7% MVC. The MPF increased slowly up to 59.4 +/- 2.3% MVC; after a temporary reduction from 59.4% to 64.3 +/- 2.0% MVC, it then increased progressively again. In conjunction with absolute force (F(abs)), both the RMS and MPF in the elderly were smaller than those in the young group throughout the submaximal levels of force exerted. The results reflect an alteration in MU activation strategy, with a predominant role for MUs with slow-twitch fibers and an effective fused tetanus induced at lower firing rate of the MUs, resulting from age-related neuromuscular changes.

The accelerometer is used for mechanomyogram (MMG) recordings of muscle contractions. Although the mechanical characteristics of other MMG transducers have been determined with reference to the accelerometer, mechanical aspects of the accelerometer itself, including the weight of the transducer, have not been verified. This study was designed to reinvestigate the mechanical variable of the MMG signal detected with an accelerometer, with reference to a laser distance sensor (LDS), and then to clarify the influence of the accelerometer weight on the MMG recording during muscle contractions. The study was performed during mechanical sinusoidal vibrations and during voluntary contractions of the quadriceps muscle. Maximum differences in the amplitude spectral density functions between the LDS signal and the double integral of the accelerometer signal were approximately 4 microm. The results verified that the MMG signal from the accelerometer accurately reflected the acceleration of body surface vibration. However, the MMG signal was gradually distorted when weight was added to the accelerometer: the addition of 4.0 g (total 6.0 g, including 2.0 g of accelerometer) substantially attenuated the MMG signal. The results suggest that the appropriate weight for the accelerometer should be less than 5.0 g for measurements of the quadriceps muscle and indicate that the transducer weight must be taken into account for accurate measurement of muscles of different sizes.

The piezoelectric contact sensor has been widely utilised in mechanomyography (MMG). The authors aim to clarify the mechanical variables (i.e. acceleration, velocity or displacement) reflected by the MMG signal detected with a piezoelectric contact sensor (PEC), and compare the results with those obtained simultaneously by an accelerometer (ACC). To measure the acceleration-frequency response, a mechanical sinusoidal excitation of 5 to 300 Hz at a constant magnitude of 0.01 G was applied to the two transducers. The acceleration-frequency response of the ACC transducer was confirmed to be almost flat. The PEC without any restriction of the transducer housing (including the combined seismic mass) demonstrated a similar response to the ACC transducer. The PEC transducer output with restricted housing decreased with increasing sinusoidal frequency and an attenuation slope of -40 dB/decade and phase angle of -180 degrees. The voluntary MMG signal during isometric knee extension was recorded simultaneously with the two transducers. The amplitude spectral density distribution of the MMG from the PEC transducer was narrow and the mean frequency was approximately one-half that obtained from the ACC transducer. The amplitude spectral density distribution with the PEC transducer resembled that of the double integral over time of the ACC transducer signal. The phase angle of the PEC transducer signal was different from that of the ACC transducer signal by approximately -180 degrees. These results suggest that the PEC transducer acts as a displacement meter of muscle vibration. In addition, differences in the MMG frequency components relating to the transducer type must be taken into consideration when investigating the mechanical activity of muscle.

This study was designed to compare electromyogram (EMG) and acoustic myogram (AMG) recordings of biceps brachii muscles in patients with spastic cerebral palsy (CP). The maximal voluntary contraction (MVC) in the CP group was approximately one half of that of the normal group even after being normalized by the muscle cross‐sectional area (CSA) (18.6 ± 5.9 kNm/m2 in CP, 37.3 ± 2.9 kNm/m2 in normal). Both CP and normal groups demonstrated a progressive increase in the root mean squared values per unit muscle CSA in the EMG (RMSEMG/CSA) as well as in the AMG (RMSAMG/CSA) with increasing force up to 50% MVC. The increasing magnitude of the RMSEMG/CSA with force was not significantly different between two subject groups. However, all the levels of force resulted in significantly smaller RMSAMG/CSA in the CP group compared to the normal group. The ratios of RMSAMG to RMSEMG in the CP group (0.75 ± 0.03 m/s2/mV) were significantly smaller than those in the normal group (1.37 ± 0.07 m/s2/mV) at force levels above 30% MVC. These results suggest that motor disabilities in CP patients are caused not only by primary neural impairment but also by secondary deterioration in muscular contractile properties, probably resulting from muscle fiber atrophy. This appears to be more selective in fast twitch fibers. © 1996 John Wiley & Sons, Inc.

SUMMARY Percentage body fat of 35 children with spina bifida and 129 age‐matched normal children was measured by underwater weighing and skinfold thickness to assess obesity. Percentage body fat of patients below five years was similar to that of controls; however, 58 per cent of patients above six years had an increased percentage of body fat. The neurological level and ambulatory ability were associated with percentage body fat. A significant correlation between percentage body fat and hydrocephalus suggests that the metabolic and nutritional maladaptation is caused not only by these patients' physical inactivity but also by the condition itself. Appropriate nutritional and mobility programmes should be started early to prevent the development of obesity. RÉSUMÉ Evaluation de l'obesité chez les enfants spina‐bifida Le pourcentage de graisse sur poids total fut mesure chez 35 enfants spina bifida et 129 enfants normaux apparies, utilisant les techniques de poids sous l'eau et de pli cutané pour apprecier l'obesité. Le pourcentage graisse/poids total des enfants en dessous de cinq and était identique dans les deux groups mais 58 pour cent des enfants spina de plus de six ans présentaient un excés de graisse. Le niveau neurologique et l'aptitude au déplacement étaient reliés au pourcentage de graisse. Une correlation significative a été trouvé entre le pourcentage de graisse et l'hydrocéphalie, suggerant que la mauvaise adaption métabolique et nutritionnelle n'est pas causée seulement par l'inactivité physique de ces patients, mais aussi par la condition morbide elle‐meme. Des programmes appropries d'alimentation et de mobilisation devraient débutés précocement pour éviter l'obésite. ZUSAMMENFASSUNG Adipositas bei Kindern mit Spina bifida Bei 35 Kindern mit Spina bifida und 129 altersentsprechenden Kontrollen wurde zur Beurteilung der Adipositas der Korperfettanteil mit Hilfe der unter Wasser Wiegemethode und der Messung der Hautfalten bestimmt. Bei Patienten under funf Jahren war der Korperfettanteil ahnlich wie bei den Kontrollen; 58 Patienten jedoch. die alter als sechs Jahre waren, hatten einen hoheren Korperfettanteil. Der neurologische Befund und die Gehfahigkeit korrelierten mit dem Korperfettanteil. Eine signifikante Korrelation zwischen Korperfettanteil und Hydrozephalus deutet darauf hin, daB die metabolische und ernahhrungsmaBige Fehlanpassung nicht nur durch die korperliche Inaktivitat dieser Patienten bedingt ist, sondern auch durch die Krankheit selbst. Man sollte fruhzeitig mit entsprechenden Ernahrungs‐ und Beratungsprogrammen beginnen, urn der Entwicklung der Adipositas entgegenzuwirken. RESUMEN Evaluacion de la obesidad en ninos con espina bifida Se midió el porcentaje de grasa corporal en 35 ninos con espina bifida y en 129 controles de la misma edad, utilizando las téchnicas del pesaje en el agua y del groso del pliegue cutaneo con el objeto de evaluar la obesidad. El porcentaje de grasa corporal de pacientes de menos de cinco anos era similar a1 de los controles. En cambio el 58 por ciento de ...