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Prompted by the observation that the slope of the relationship between average rectified electromyography (EMG) and the ensemble activation rate of a pool of motor units progressively decreased (showing a downward nonlinearity), an experimental study was carried out to test the widely held notion that the EMG is the simple algebraic sum of motor-unit action-potential trains. The experiments were performed on the cat soleus muscle under isometric conditions, using electrical stimulation of alpha-motor axons isolated in ventral root filaments. The EMG signals were simulated experimentally under conditions where the activation of nearly the entire pool of motor units or of subsets of motor units was completely controlled by the experimenter. Sets of individual motor units or of small groups of motor units were stimulated independently, using stimulation profiles that were strictly repeatable between trials. This permitted a rigorous quantitative comparison of EMGs that were recorded during combined activation of multiple motor filaments with EMGs that were synthesized from the algebraic summation of motor unit action potential trains generated by individual nerve filaments. These were recorded separately by individually stimulating the same filaments with the same activation profiles that were employed during combined stimulation. During combined activation of up to 10 motor filaments, experimentally recorded and computationally synthesized EMGs were virtually identical. This indicates that EMG signals indeed are the outcome of the simple algebraic summation of motor-unit action-potential trains generated by concurrently active motor units. For both recorded and synthesized EMGs, it was confirmed that EMG magnitude increased nonlinearly with the ensemble activation rate of a pool of motor units. The nonlinearity was largely abolished when EMG magnitude was estimated as the sum of rectified, instead of raw, motor-unit action-potential trains. This suggests that the downward nonlinearity in the EMG-ensemble activation rate relation is due to signal cancellation arising from the perfectly linear summation of positive and negative components of action-potential waveforms. The findings provide a much needed post hoc validation of the concept of EMG generation by strict algebraic summation of motor unit action potentials that is generally relied on in theoretical modeling studies of EMG and in EMG decomposition algorithms.

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The purpose of this study was to investigate, theoretically, to what extent muscle properties could contribute to recovery from perturbations during locomotion. Four models with different actuator properties were created: the FLVT model, which encompassed force-length (FL) and force-velocity (FV) characteristics of human muscles as well as muscle stimulation inputs as functions of time (T); the FLT model, which had muscles without force-velocity characteristics; the FVT model, which had muscles without specific force-length characteristics; and the MT model, which had no muscles but was driven by joint moments (M) as a function of time. Each model was exposed to static and dynamic perturbations and its response was examined. FLVT showed good resistance to both static and dynamic perturbations. FLT was resistant to static perturbation but could not counteract dynamic perturbation, whereas the opposite was found for FVT. MT could not counteract either of the perturbations. Based on the results of the simulations, skeletal muscle force-length-velocity properties, although interactively complex, contribute substantially to the dynamic stability of the musculoskeletal system.

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1. Afferent activity of 111 single units from the glabrous skin area was recorded percutaneously in the median nerve of human subjects, using tungsten electrodes. 2. The majority of the units (103) were classified as low‐threshold mechano‐sensitive units belonging to one of the four categories previously described: rapidly adapting with small receptive fields (RA), rapidly adapting with large receptive fields (PC, presumed Pacinian corpuscle units), slowly adapting with small fields (SA I), and slowly adapting with large fields (SA II). The size of the responses (in number of impulses) to indentation and stretching of the skin was compared with that of the responses elicited during voluntary isotonic finger movements, which avoided trivial excitation of the units by direct touch. 3. All four types of units, and 77% of the single units, were activated by isotonic movements. The decreasing order of responsiveness was PC, SA II, SA I, RA. 4. Almost all responsive units were excited during the dynamic phase of ramp and smooth oscillatory movements. Static responses, on the other hand, occurred only with 50% of the slowly adapting units, corresponding to a third of the total sample (SA II, 81%; SA I, 17%. 5. For all four types of units the dynamic responses to movements were of similar size as the responses to localized skin indentation with a von Frey hair at five times threshold. 6. The results are discussed with regard to the possible implications for kinaesthesia and motor control.

SUMMARY1. Single fusimotor fibres were stimulated repetitively to test their action on the responsiveness of muscle spindle primary endings in the cat soleus to sinusoidal stretching of both large and small amplitude. Frequencies of 0-06-4 Hz were used at amplitudes from 10 jttm to 3 mm.2. The response was assessed by fitting a sinusoid to the cycle histogram of the afferent firing throughout the course of the cycle; this linear approximation measures the fundamental of the response and ignores any harmonics. The sine was allowed to project to negative values and any empty bins in the histogram were ignored when fitting.3. With small amplitudes of stretching the histograms were reasonably sinusoidal, but with large amplitudes they showed appreciable distortion of the wave form for the passive ending and during dynamic fusimotor stimulation. Non-linearity of response manifested itself also, with increasing amplitude of stretching, by an increase in the phase advance of the response, by increasing r.m.s. deviation of the histogram points from the fitted sine and (for dynamic stimulation) by an increase in the mean value of the fitted sine.4. With increasing amplitude the response modulation ceased to increase proportionately with the stimulus, so that the sensitivity of the ending to a large stretch (defined as afferent modulation/stretch amplitude) was appreciably less than for a small stretch. This effect was most pronounced for the passive ending.5. Whatever the amplitude of movement the modulation during static stimulation was less than that for the passive or during dynamic In reaching these conclusions more attention was paid to the slope of the sensitivity lines than to the values of phase.7. It appears that the major effect of fusimotor action, whether static or dynamic, is to regulate the sensitivity ofthe primary ending to stretching for all amplitudes of movement (i.e. gain) rather than to control the relative values of its sensitivity to length and to velocity (i.e. crudely, the damping in a feed-back loop).