Contracting muscle generates sounds which can be recorded easily by means of a microphone. To determine if a phomomyogram (PMG) can be used to monitor muscle force, a comparison was made between simultaneous recordings of PMG and monopolar electromyogram (EMG) from the isometrically contracting biceps brachii muscle and the external flexion force. Locations of the monopolar electrode and microphone were identified in relation to the motor point. Whatever the recording site, PMG amplitude was proportional to EMG amplitude and both showed a quadratic relationship to muscle force. Changes in the PMG spectrum with force were similar to those in EMG, i.e. the mean power frequency increased up to about 30% maximal voluntary contraction and then reached a plateau. Despite a slightly higher variability, PMG was shown to be a valid index of muscular isometric force. At the same force, the amplitude of both PMG and EMG was lower in the prone than in the supine position of the hand. This result indicated a selective recording of biceps brachii muscle activity.

Mechanical and neural activation changes that accompanied muscle isometric training were studied in males. Training and testing sessions consisted of right elbow isometric flexions. Each experimental group was trained during 5 wk at one of the following angles: 25, 80, and 120 degrees. Bipolar surface electromyogram (EMG) was recorded from the biceps brachii and brachioradialis muscles. An improvement of maximal voluntary contraction (MVC) was always found at the training angle and was systematically greater than at the other angles. Moreover, the shorter the muscle length at which the training has been carried out, the more the gain was limited to the training angle. An increase of the maximal integrated EMG of both biceps brachii and brachioradialis frequently accompanied the improvement of MVC at the training angle.

The recruitment and firing rate of biceps brachii (BB) and brachioradialis (BR) motor units (MUs) were studied in the course of fatiguing isometric contractions at 20%-30% of maximal voluntary contraction (MVC). MU recruitment generally occurred throughout the maintained contraction and was similar for BB and BR muscles. Newly recruited MUs started to discharge in the form of bursts, the duration of which increased until a continuous rhythmical firing was achieved. Within each burst, the first interval between two consecutive discharges was usually the shortest. MU threshold was lowered just after the limit time of the maintained contraction. The MU's firing rate either increased or remained stable as a function of the elapsed time. It is concluded that (1) in fatiguing isometric contractions at 20%-30% MVC contractile failure is mainly compensated for by MU recruitment and a lowered MU threshold and (2) differences between in surface changes in the electromyogram of BB and BR muscles cannot easily be explained by related differences in MU firing rate and recruitment.

Phonomyogram (PMG, or acoustic myogram) is known to increase with force in isometric contractions. We investigated this relationship for dynamic contractions against different inertias. PMG and surface electromyogram (EMG) from biceps brachii and brachioradialis muscles were simultaneously recorded with the angular acceleration of elbow flexions. These were self-initiated movements (30 degrees) toward a fixed target and performed against two different inertias. PMG and EMG were integrated from the onset of the signal to the end of the acceleration phase. Phono- and electromechanical delays were also measured. For integrated EMG (iEMG), there was a linear relationship between integrated PMG (iPMG) and force, the slope of which did not depend on inertia. There was also a linear relationship between iPMG or iEMG and angular acceleration, with a higher slope for the highest inertia condition. There was also a family of linear relationships between iPMG or iEMG and angular acceleration, and their slopes depended on inertia. Measurements of the phono- and electromechanical delays showed that onset of PMG followed that of EMG but preceded onset of acceleration. It is suggested that PMG expresses tension of the underlying muscle contractile elements. Given the simplicity of the PMG method, we conclude that PMG allows convenient evaluation of muscle tension during human dynamic contraction.

The purpose of this study was to determine the characteristics of the specific phonomyogram (PMG) of active motor units activated during voluntary isometric contractions. The electromyogram (EMG) and PMG were recorded from 87 anconeus motor units in 14 subjects. The elementary PMG from single motor units was analysed with a spike-triggered averaging technique. The electro-acoustical delay was 3.5 (SD 1.1) ms, which is within the range of values reported in the literature for PMG evoked by motor nerve stimulation. All motor units demonstrated a pattern of impulsive sounds with a duration of 87.2 (SD 10.7) ms. These results would imply that PMG is linked to the contractile activity of the motor units. These results also would suggest that PMG recorded from a contracting muscle in situ reflects the summation of elementary PMG during voluntary contraction more than the overall mechanical properties of the muscle.

During an external isometric constant torque (25% of the maximal voluntary contraction) maintained until the maximal endurance time (limit time), we analysed and compared the changes in electromyographic (EMG) activity illustrating muscular fatigue simultaneously with mechanical activity (the tangential acceleration theta") related to physiological tremor. The EMG activities recorded were of two agonistic flexors, the biceps brachii (BB) and the brachioradialis (BR) muscles and one of the main extensors, the triceps brachii (TB). The integrated EMG increase and the mean power frequency (MPF) of the power spectrum density function (PSDF) decrease were larger for BR than for BB activity. These two findings suggested a greater BR fatigability. However, it is shown that differences between BB and BR MPF changes could be related to differences in the PSDF upper frequency limit of the two muscles and also to the relative magnitude of their tremor component.

No abstract

The onset of fatigue has been studied by relating motor unit activity and surface myoelectric activity during constant force isometric contraction in man. The surface (global) EMG of the biceps brachii was recorded using bipolar electrodes. The global EMGs of the other elbow flexors were also recorded. Motor unit activity was recorded simultaneously at three points of the biceps brachii by using wire electrodes. The time course of the integrated surface EMG showed that fatigue occurred right at the beginning of the contraction. The increase of the integrated EMG was especially important when the value of the force maintenance was high. The time course of this increase was close to that of the output of the spikes recorded by the wire electrodes. No matter what the value of the force maintenance, MU recruitment occurred throughout the contraction. The existence of an increase in the MU firing frequently depended on the value of this force. Contractile element fatigue is believed to provide the explanation of these results, while neuromuscular junction fatigue may perhaps be more important near the time limit.