New transducers for measuring muscle length in unrestrained animals.

Muscle length in unrestrained, chronically implanted animals is conventionally measured with gauges consisting of a compliant silicone rubber tube filled with either hypertonic saline or mercury, the measurement principle being a continuous change in the electrical resistance of the fluid column inside the tubing with stretch. These gauges have two major disadvantages: (1) changes in resistance that are not related to changes in length, such as those produced by changes in temperature or osmotic dilution of the hypertonic saline, cause the measurements to drift, and (2) there is no direct and accurate way to calibrate the measurements. In this communication two new types of muscle length gauge are described that eliminate both problems. Both types make use of the principle of sonomicrometry, i.e., the measurement of distances with pulsed ultrasound. Both types have been successfully used to measure the length of the medial gastrocnemius muscle in chronically implanted cats during treadmill locomotion.

Effect of velocity and mechanical history on the forces of motor units in the cat medial gastrocnemius muscle.

1. Two fundamental aspects of the dynamic behavior of motor units of the cat medial gastrocnemius (MG) muscle were measured. Force-velocity (FV) relationships were measured with the use of constant velocity shortening and lengthening movements. Effects of mechanical history were assessed via comparisons of forces immediately after or during slow movements with standard isometric forces. Isometric force-length (FL) relations were also measured, and the effect of different stimulation rates on both FV and FL data was assessed. 2. Prior or concurrent movement greatly potentiated motor-unit force, but this movement potentiation was highly dependent on the amplitude of the unit's force. The smallest twitch forces of type S units (< 10 mN) were potentiated more than threefold, but no potentiation occurred for unit forces > 200 mN. It was tentatively concluded that movement potentiation may play little role in normal movements because it does not occur at forces > 1% of maximal isometric force of the MG. 3. During shortening, the normalized FV relations of type S units were relatively steeper than those of type FR or FF units. For lengthening, there was no evident relation between FV steepness and motor-unit type. 4. Stimulation rate affected both the FV and FL relationships of the motor units. The peak of the FL relationship (Lo) clearly shifted to shorter muscle lengths as stimulation rate was increased. The steepness of the FV relationship for shortening was decreased by increasing stimulation rate, but this effect was modest. 5. The shift in motor-unit Lo and the differences in motor-unit FV relationships were hypothesized to play significant roles during normal motor behavior. Realistic computer simulations of FL and FV functions for a population of motor units undergoing normal steady-state recruitment and rate modulation supported these hypotheses. As the level of simulated neural drive increased, the population Lo shifted to considerably shorter lengths, and the normalized FV function became much less steep. The significance of these results for models of muscle are discussed.

The masseteric post-stimulus electromyographic-complex in people with dysfunction of the mandibular joint.

In mandibular joint-symptom-free subjects, post-stimulus EMG complexes (PSEC) were derived by standardized mechanical stimulation of an upper central incisor during clenching at a constant level of 5 per cent of maximal masseteric EMG activity. Seventy-two sweeps per subject were processed by means of a computer program, and different morphologies of the PSEC were seen. There appears to be no correlation with the Helkimo-index as a whole or its specific components. There is a strong correlation between tooth-grinding habits and the occurrence of a single inhibitory period. Patients with myofascial pain dysfunction consistently had a single inhibitory wave. When there was one affected side, the end latency of the second inhibitory wave was significantly shorter.

The effects of motor unit synchronization on the power spectrum of the electromyogram.

A realistic model for two synchronized motor unit action potential trains (MUAPT) is presented in which the variability of the time difference between corresponding action potentials (hereafter denoted by delay) is taken into account. Specifically, this delay is modeled as a continuous random variable that may assume both positive and negative values. Expressions are derived for the auto- and cross-power spectra of two such trains using their relations with the auto- and cross-correlation functions, respectively, with which they form Fourier transform pairs. The results show that the auto- and the cross-power spectra of two such synchronized MUAPTs differ from the auto- and the cross-spectra of two independent MUAPTs. The contribution of the statistics of the interpulse intervals to one of the auto-power spectra is smaller and the cross-power spectra no longer reduce to a Dirac sigma-function at the origin but are now determined by the other auto-power spectrum and by the Fourier transform of the density function associated with the time difference between corresponding action potentials. As a consequence of this change in the cross-power spectra synchronization leads to an absolute increase of power at low frequencies and to a relative decrease of power at high frequencies. The results are then generalized to electromyograms (EMG) composed of more than just two MUAPTs and illustrated with simulated power spectra with which the theory shows excellent agreement.

Spectral analysis of the surface electromyogram as a tool for studying rate modulation: a comparison between theory, simulation, and experiment.

Theoretical work suggests that if the interpulse intervals ( IPIs ) of motor unit action potential trains ( MUAPTs ) are independently and normally distributed, then spectral analysis of the electromyogram could be a useful tool for studying rate modulation by virtue of the presence of a peak in the power spectrum at the average firing frequency of all active motor units. It is shown in this paper that IPIs need not be normally distributed, specifically that the results are very much the same if the IPIs are distributed according to a Gamma probability density function ( PDF ). Simulation of the electromyogram based on this theory proved the applicability of the method. Experimental results obtained for the masseter, biceps brachii and first dorsal interosseus (FDI) muscles, however, were in disagreement with both theory and simulation except for the biceps muscle at force levels up to 20% of the maximal force and for the masseter and FDI muscles in 1 out of 5 subjects. This indicates that the models for MUAPTs hitherto used might not be generally correct. Apart from this discrepancy, our results reveal differences between masseter and FDI muscles on the one hand and the biceps brachii on the other, which indicate that motor unit synchronisation is much more pronounced in the latter muscle.

The influence of clenching level on the post-stimulus EMG complex, including silent periods, of the masseter muscles in man.

Standardized mechanical taps were delivered on an upper central incisor, while the nine subjects investigated maintained constant clenching levels of 5, 10, 20, 40 and 60 per cent of their maximal EMG outputs. Using statistical criteria, a computer program enabled an objective determination of the interaction between the clenching level and the sequence of upward and downward going waves following the stimulus in full-wave rectified and superimposed EMG, called a post-stimulus EMG complex (PSEC). The morphology of the PSEC, including one or two silent periods, was subject-specific and reproducible over one year. The surfaces of the waves were greatly influenced by the clenching level. In general, their total surface decreased as a function of the clenching level. However, the individual waves decreased selectively. The results suggest that the sequence of waves results from overlapping of inhibitory and excitatory inputs. The inhibitory waves, constituting the silent periods, are largely cancelled by the excitatory reflexes of the PSEC.

Development of Na inactivation in motor and sensory myelinated nerve fibres of Rana esculenta.

Development of Na permeability inactivation was investigated in myelinated motor (N = 12) and sensory (N = 12) nerve fibres of Rana esculenta at 20 degrees C. The K currents were blocked by 10 mM tetraethylammonium chloride, added to the superfusing solution. Additionally, in 4 fibres of each group internal CsCl was applied by diffusion from the cut internodes. Development of Na inactivation was approximated by the sum of two exponentials. The time constants of the fast and slow inactivation phase (tau h1 and tau h2) were dependent on membrane potential (E) with similar values in both fibre types for a given E. In contrast, significant differences were found in the contribution of both phases. In motor nerve fibres the amplitude of the fast phase was g = 0.70 (mean value of 12 fibres) throughout the potential range investigated (-30 mV less than or equal to E less than or equal to 40 mV). In sensory fibres g was potential dependent, increasing from 0.76 (mean value of 12 fibres) at E = -30 mV to unity at E greater than 30 mV. This difference in Na permeability inactivation is a further distinguishing property between motor and sensory nerve fibres.