Characteristics of synergic relations during isometric contractions of human elbow muscles

We studied the patterns of EMG activity in elbow muscles in three normal human subjects. The myoelectrical activity of 7-10 muscles that act across the human elbow joint was simultaneously recorded with intramuscular electrodes during isometric joint torques exerted over a range of directions. These directions included flexion, extension, varus (internal humeral rotation), valgus (external humeral rotation), and several intermediate directions. The forces developed at the wrist covered a range of 360 degrees, all orthogonal to the long axis of the forearm. The levels of EMG activity were observed to increase with increasing joint torque in an approximately linear manner. All muscles were active for ranges less than 360 degrees and most were active for less than 180 degrees. The EMG activity was observed to vary in a systematic manner with changes in torque direction and, when examined over the full angular range at a variety of torque levels, is simply scaled with increasing torque magnitude. There were no torque directions or torque magnitudes for which a single muscle was observed to be active alone. In all cases, joint torque appeared to be produced by a combination of muscles. The direction for which the EMG of a muscle reached a maximum value was observed to correspond to the direction of greatest mechanical advantage as predicted by a simple mechanical model of the elbow and relevant muscles. Muscles were relatively inactive during varus torques. This implies that the muscles were not acting to stabilize the joint in this direction and could have been allowing ligaments to carry the load. Plots of EMG activity in one muscle against EMG activity in another demonstrate some instances of pure synergies, but patterns of coactivation for most muscles are more complicated and vary with torque direction. The complexity of these patterns raises the possibility that synergies are determined by the task and may have no independent existence. Activity in two heads of triceps brachii (medial head--a single-joint muscle and long head--a two-joint muscle) covaried closely for a range of torque magnitudes and directions, though shoulder torque and hence the forces experienced by the long head of the triceps undoubtedly varied. The similarity of activation patterns indicates that elbow torque was the principal determining factor. The origins of muscle synergies are discussed. It is suggested that they are best understood on the basis of a model which encodes limb torque in premotor neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement by serotonin of tonic vibration and stretch reflexes in the decerebrate cat

The effects of pharmacological manipulation of serotonergic systems on spinal reflexes were determined in the unanesthetized decerebrate cat. The prolonged motor output that continues after cessation of high frequency longitudinal tendon vibration was strongly enhanced by the serotonin reuptake blocker fluoxetine and the serotonin precursor 5-hydroxytryptophan, and was decreased by the serotonin receptor antagonist methysergide. In addition, both dynamic and static stretch reflex stiffness was markedly increased by fluoxetine and 5-hydroxytryptophan, while methysergide produced a decrease in stretch reflex stiffness. These powerful effects on tonic vibration and stretch reflexes could not be explained by drug-induced alterations in muscle spindle primary afferent discharge. In light of other recent results on serotonin-mediated effects on motoneurons, we believe that the effects of these agents result from modification of an intrinsically mediated prolonged depolarization of spinal neurons. However, the possibility that these drugs modify longlasting discharge in associated interneuronal pathways cannot be ruled out.

Reflex actions of muscle afferents on fusimotor innervation in decerebrated cats: an assessment of beta contributions

The existence of beta innervation in many cat muscles raises the possibility that spindle afferent discharge will excite beta motoneurons, augmenting spindle afferent discharge and thereby closing a positive feedback loop. In order to evaluate the strength of such a loop through beta motoneurons and muscle spindles, the stretch responses of muscle spindle afferents from medial gastrocnemius (MG) and soleus (SOL) muscles were studied in decerebrated cats before and after dorsal root section. If a positive feedback loop were operational, the spindle afferent stretch response should be diminished following dorsal root section by an amount related to the magnitude of positive feedback. After dorsal root section, the static positional sensitivities of our MG spindle afferent sample were significantly reduced for 72% (13/18, p less than 0.05) of the afferents, and dynamic rate/length slopes were decreased for 88% (8/9) of a subset of the afferents studied. Similar reductions for 6 afferents from SOL were not found. To apportion these afferent changes to reflex excitation of either gamma or beta motoneurons, we recorded the stretch responses of gamma and alpha-type fibers in the same preparation. (We assume that the population of alpha-type fibers includes beta fibers). In keeping with other reports, alpha fibers were much more responsive to stretch than gamma fibers. The mean positional sensitivity for alpha fibers (1.29 +/- 0.92 pps/mm, n = 15) was greater (p less than 0.05) than that of gamma fibers (0.49 +/- 0.93 pps/mm, n = 18). Because of these differences in sensitivity, beta motoneurons are more likely (than gamma motoneurons) to be involved in a positive feedback loop, although some gamma contribution is also likely. Using equations based on a beta position regulating scheme, differences in spindle positional sensitivity were used to estimate beta loop gain. The average loop gain was estimated to be 0.41 (n = 18). The contribution of such a beta configuration to reducing the sensitivity of muscle to changes in load and muscle properties is evaluated.

Force-sensitive interneurons in the spinal cord of the cat

The input-output properties of interneurons mediating spinal reflexes were investigated by extracellularly recording the response of interneurons to excitation from muscle receptors in the ankle extensor muscles of decerebrated, spinal cats. A population ofinterneurons in the intermediate region ofthe spinal cord is potently excited by increases in muscle force. Unlike the discharge of Golgi tendon organs, which accurately encodes moment-to-moment variations in the force of a single muscle, the discharge of these interneurons depends in a dynamic and usually nonlinear way on the force in several muscles. Powerful input from unidentified mechanoreceptors in muscle, presumably free nerve endings, is at least partly responsible for these properties. These force-sensitive interneurons are more likely to mediate clasp knife-type inhibition than simple negative force feedback.

Motor-unit activation patterns in lengthening and isometric contractions of hindlimb extensor muscles in the decerebrate cat

1. Multiunit integrated electromyographic (EMG) signals and single-unit EMG potentials were recorded during isometric and lengthening (stretch reflex) contractions of soleus and medial gastrocnemius (MG) muscles in 20 decerebrate cats. Patterns of motor-unit recruitment and rate modulation were examined in isometric muscles and during constant-velocity stretches. 2. Analysis of multiunit EMG activity and its relationship to active force revealed a marked difference between isometric and lengthening contractions. While the force-EMG relationship for isometric contractions was characteristically linear, the relation recorded during stretch-reflex responses showed a disproportionate early EMG increase, which was most obvious at low force levels, suggesting that the efficacy of force production is reduced in lengthening muscle. 3. Single-unit recruitment patterns were found to be qualitatively similar in isometric and lengthening contractions. In each case, motor units were recruited in order of increasing spike voltage. The numbers of newly recruited units declined steeply with each successive increment in active force. For a given unit, the force at which recruitment occurred was found to be greater in lengthening contractions than in isometric contractions, and in lengthening contractions it was also found to depend on the level of initial force. 4. Two patterns of motor-unit rate modulation were observed during muscle stretch, depending on whether a given unit was firing before the beginning of stretch or whether it was recruited during the course of stretch. Motor units that were active prior to stretch were found to increase firing rate at stretch onset and to vary their rate very little thereafter. Motor units recruited in the course of stretch began firing at an initial rate proportional to their force threshold, gradually increased their firing rate with increasing force, and sometimes reached an apparent maximum rate. 5. These results are discussed in terms of the mechanical properties of lengthening muscle and reflex regulation of these properties. Each identified pattern of motor-unit recruitment and rate modulation is evaluated for its potential contribution to the regulation of muscle properties, especially the prevention of muscle yield. We conclude that at low to moderate levels of initial force, recruitment of new motor units is likely to be the most effective compensatory mechanism.

Contributions of motor-unit recruitment and rate modulation of compensation for muscle yielding

1. Subdivided portions of the cut ventral root innervation of the soleus muscle were electrically stimulated in 14 anesthetized cats. The stimulus trains imposed on these nerves simulated the recruitment and rate-modulation patterns of single motor units recorded during stretch-reflex responses in decerebrate preparations. Each activation pattern was evaluated for its ability to prevent muscle yield. 2. Three basic stimulus patterns, recruitment, step increases in stimulus rate, and doublets were imposed during the course of ramp stretches applied over a wide range of velocities. The effect of each stimulus pattern on muscle force was compared to the force output recorded without stretch-related recruitment or rate modulation. 3. Motor-unit recruitment was found to be most effective in preventing yield during muscle stretch. Newly recruited motor units showed no evidence of yielding for some 250 ms following activation, at which time muscle stiffness declined slightly. This time-dependent resistance to yield was observed regardless of whether the onset of the neural stimulus closely preceded or followed stretch onset. 4. Step increases in stimulus rate arising shortly after stretch onset did not prevent the occurrence of yield at most stretch velocities, but did augment muscle stiffness later in the stretch. Doublets in the stimulus train were found to augment muscle stiffness only when they occurred in newly recruited motor units. 5. These results suggest that at low or moderate initial forces, the prevention of yield in lengthening, reflexively intact muscle results primarily from rapid motor-unit recruitment. To a lesser extent, the spring-like character of the stretch-reflex response also derives from step increases in firing rate of motor units active before stretch onset and doublets in units recruited during the course of stretch. Smooth rate increases appear to augment muscle force later in the course of the reflex response.

Prolonged time course for vibratory suppression of stretch reflex in the decerebrate cat

We studied the effects of longitudinal tendon vibration on the stretch reflex of the soleus and gastrocnemius muscles in 11 decerebrate cats. Vibration was applied at amplitudes (40-80 micrometer) and frequencies (120-250 HZ) sufficient to provide a strong tonic vibration reflex. In keeping with previous reports, we found that during an established tonic vibration reflex, the force and emg response to superimposed ramp and hold stretch are largely suppressed. This suppression is most obvious during the dynamic phase of stretch where it gives rise to a complex force response resembling that of active areflexic muscle. If stretch initiation is delayed until after vibration is terminated, the suppressed effects of vibration persist for 5 s or more. These suppressive effects are marked in the first 200 ms, and then decay gradually over the ensuing time period, paralleling the decline in emg and force which follows vibration offset. Simultaneous recordings from homonymous Ia afferents showed that this suppression persists even though the stretch responsiveness of primary spindle endings has returned to normal immediately following the end of vibration. When stretch is initiated shortly after vibration commences, the suppressive effects are first evident at 50-100 ms latency, but are not well established until 1 s or more after vibration onset. Tests of monosynaptic transmission using small amplitude tendon taps or electrical stimulation of synergist nerves to activate Ia fibers revealed that reductions in the magnitude of the response following vibration are usually modest (12% mean reduction at 50 ms, n = 5), and they are quite sensitive to the initial level of excitation of the motoneuron pool. These reductions were also rather shortlived, being largely completed within 500 ms of vibration offset. Although the relative contributions of presynaptic and postsynaptic inhibition are not readily dissociated in this type of experiment, it is likely that the magnitude of presynaptic inhibition is quite small. We argue that the effects of vibration on the stretch reflex are best explained by invoking an excitatory autogenetic projection from Ia interneurons to extensor motor neurons, which lies in parallel with the Ia monosynaptic projection. In order to account for the vibratory suppression, we propose that these interneurons are driven to saturation by vibration. When vibration ceases, the discharge rate of these interneurons declines with a prolonged time-course that coincides with the recovery of stretch responsiveness. This recovery would contribute to the return of stretch reflex force.

Abnormal force--EMG relations in paretic limbs of hemiparetic human subjects

The relations between surface EMG and isometric force generated by elbow flexor muscles were compared in normal and paretic limbs of 17 hemiparetic human subjects. Similar analyses were performed on both arms of 11 normal subjects. In almost half of the hemiparetic subjects examined (8/17), the slope of the relation between elbow flexion force and surface EMG, measured over the biceps-brachialis and brachioradialis muscle groups was increased in the paretic limb. A mechanism based on anomalous reductions in mean motor unit discharge rate in paretic muscles is advanced as the most likely cause of the findings.

Joint position sense: the effects of muscle contraction

The effects of isometric and isotonic force production on perceived static joint position were investigated in 12 adult subjects. The joint examined was the proximal interphalangeal joint of one index finger, and its perceived position was determined from matching movements of the equivalent joint on the other hand. When the perturbed joint was free to move, even when supporting substantial loads, the position was accurately estimated; however, when the subject was required to exert substantial isometric force against the device imposing the joint movement, significant errors occurred: these errors were in the direction of the increasing force. Similar effects were evident during increasing isometric flexion force in anaesthetized fingers. It is suggested that force-related afferent discharge from muscle, presumably originating in tendon organ receptors, contributes to static joint position sense. This force-feedback may allow the nervous system to accommodate for the effects of changing fusimotor bias, but it also appears to induce errors when afferent information of force and length provide potentially conflicting information.

Mechanisms of the clasp-knife reflex studied in an animal model

The mechanisms of the clasp-knife reflex were studied in the soleus muscle of an animal model, the decerebrate cat with a dorsal hemisection of the lower thoracic cord. The reflex is shown to be autogenetic, and to depend on muscle length in keeping with previous suggestions. However, the magnitude of the inhibition increases with increasing initial force, and the inhibition is mimicked by gentle manipulation of the muscle and tendon surface. Concurrent muscle afferent recordings showed that the electromyogram (emg) reduction was not a result of a decline in Ia afferent input and was not well related to secondary or tendon organ afferent discharge. It is now known that many group III and some group IV muscle afferents are also activated by muscle stretch and contraction, and we here report limited stretch sensitivity in four non-spindle group II afferents. Since these fiber groups each include afferents that produce inhibition of extensor motoneurons, it is proposed that the clasp-knife reflex may result from the activation of these slowly conducting afferent fibers.

Effect of compensatory hypertrophy studied in individual motor units in medial gastrocnemius muscle of the cat

1. Compensatory hypertrophy of the medial gastrocnemius (MG) muscle was produced by denervating or removing its synergists (i.e., the lateral gastrocnemius, soleus, and plantaris muscles) in adult cats. Following survival times of 14-32 wk, intracellular recording and stimulation techniques were used to study the motor-unit population in MG. The data obtained were compared with results from MG motor units in normal unoperated cats of the same body size and weight. 2. Using criteria employed for normal motor units, the units in hypertrophic MG muscles were readily classified into the same groups (types FF, F(int), FR, and S) as in normal MG. There was no detectable difference in the distribution of motor-unit types after hypertrophy. 3. When compared with a normal motor-unit sample, there was a large increase in mean tetanic tension, but no significant change in twitch tension, for each motor-unit type in the hypertrophied muscles. The most marked increase was found among the fatigue-resistant type S and type FR motor units. There was no alteration of twitch contraction times or fatigue resistance in any unit type after hypertrophy. 4. For each motor-unit type, the mean homonymous (MG) group Ia EPSP amplitude was the same in normal and hypertrophic MG populations. There was, however, a significant increase in the average conduction velocity of MG motor axons in the animals with uncomplicated MG synergist removal and maximal MG hypertrophy. 5. On the basis of histochemical staining, muscle fibers from comparable sections of hypertrophic and contralateral (unoperated) MG muscles were presumptively identified as belonging to FF, FR, or S units. There was no significant difference between hypertrophic and contralateral MG muscles in the percentage of each fiber type, although there was some variability in muscle composition from one cat to another. One muscle pair was studied in detail for fiber cross-sectional area. In this cat, with marked hypertrophy by muscle weight, there was a modest increase in the mean fiber areas of histochemical S and FR muscle fibers, but no evident change in FF fibers, on the hypertrophic side. 6. MG motor units were examined in several cats in which synergist removal resulted in scarring and marked limitation of passive ankle mobility, and no evident weight gain in MG. Motor units of all types in these animals showed a decrease in twitch tension and in mean twitch/tetanus ratios, with little alteration in mean tetanic tensions. 7. The main effect of compensatory hypertrophy under the present conditions was a large increase in tetanic tension output from individual motor units due, at least in part, to an increase in fiber cross-sectional area. There was no evidence indicating any "conversion" of motor units or of their muscle fibers from one type to another.

An estimate of the secondary spindle receptor afferent contribution to the stretch reflex in extensor muscles of the decerebrate cat

1. Vibration or stretch of the medial gastrocnemius muscle in the decerebrate cat each caused a significant increase in the tension of a synergist, the lateral gastrocenmius. 2. Simultaneous vibration and stretch of the medial gastrocnemius resulted in a substantial increase of lateral gastrocnemius tension which was greater that that produced by medial gastrocnemius vibration alone. The size of this force increase was proportional to the amplitude of medial gastrocnemius stretch, for the limited range of amplitudes examined. 3. Since the discharge of the medial gastrocnemius I a afferent fibres was held constant by vibration, the additional tension in lateral gastrocnemius provoked by medial gastrocnemius stretch must have resulted from the activation of an excitatory pathway separate from the I a afferent system. The secondary spindle afferent pathway was considered to be the most likely candidate. 4. The contributions of the Ia afferents and the additional stretch induced excitation to the stretch reflex were compared. The Ia potency was calculated from the ration of tonic vibration reflex force and the vibration frequency. The total Ia contribution to the stretch reflex, which was estimated from the product of this ratio and the primary ending stretch sensitivity, seemed modest, and was consistently smaller than the proposed secondary contribution. 5. The medial gastrocnemius nerve was subjected to anodal blockade at a strength sufficient to eliminate Ia transmission. Under these conditions, the lateral gastrocnemius excitation produced by medial gastrocnemius stretch or vibration was largely eliminated. When lateral gastrocnemius vibration was superimposed, the excitatory effect of medial gastrocnemius stretch was partly restored suggesting that some central facilitation by group Ia afferents may be necessary for group II pexcitatory effects to be manifested. 6. Although the additional excitatory actions of medial gastrocnemius stretch were examined exclusively in a synergist, it is suggested that similar effects are likely to occur in the homonymous stretch reflex.

Relative strength of synaptic input from short-latency pathways to motor units of defined type in cat medial gastrocnemius

1. Intracellular recording and stimulation techniques were used in anesthetized cats to study the interrelations between amplitudes of PSPs produced by electrical stimulation of several short-latency pathways to MG alpha motoneurons and the mechanical properties of muscle units innervated by the same cells. Motor-unit types were identified by muscle-unit properties.2. The maximum amplitudes of monosynaptic EPSPs produced in MG motoneurons by activation of homonymous (MG) and heteronymous (LGS) group Ia afferents were clearly related to motor-unit type, being, on the average, largest in type S units, somewhat smaller in type FR and F(int) units, and smallest in type FF units. Correspondingly, group Ia EPSP amplitudes were inversely correlated with muscle-unit tension production and directly correlated with resistance to fatigue. The same input distribution was true for disynaptic IPSPs produced by group Ia afferents from antagonist ankle flexors.3. The amplitudes of monosynaptic EPSPs produced by fibers descending in the ipsilateral ventral funiculi of the low thoracic cord were not clearly related to MG motor-unit type or (therefore) to muscle-unit properties.4. A quantitative input-output model of the MG motor-unit pool, based in part on the present results, suggests that overall characteristics of MG motor units, and their relative numbers in the MG pool, reflect functional specializations determined by specific mechanical demands placed on the MG muscle by the usual motor behavior of the animal.