Trained slow tracking. II. Bidirectional discharge patterns of cerebellar nuclear, motor cortex, and spindle afferent neurons

Single-unit discharge was recorded in the dentate and interposed cerebellar nuclei, motor cortex, and C7 and C8 dorsal root ganglia during trained, slow hold-ramp-hold tracking, rapid alternating movement, torque-pulse perturbation, and action tremor of the monkey's wrist. Fifty-seven dentate and 45 interposed neurons were found in two monkeys that discharged in relation to slow tracking movement. Nearly all neurons had a distinct bidirectional pattern of discharge consisting of an abrupt increase (or decrease) in firing frequency at or before the onset of movement that was variably maintained throughout the ramp and was independent of movement direction. None of the neurons showed a clear relationship to direction, position, velocity, or load during the performance of this task. Nevertheless, many of these neurons discharged in relation to rapid alternation and (for interpositus) torque pulses in patterns that were directionally reciprocal. Some interpositus neurons showed a modulation related to tremor superimposed on the bidirectional discharge related to slow ramps. Twenty-nine neurons in motor cortex of one monkey discharged during slow hold-ramp-hold tracking in two patterns. Class I neurons (14 of 29) showed gradually changing, directionally reciprocal modulations of firing frequency for movements in opposite directions. These neurons were often related to torque load and/or to wrist position but not to velocity. The discharge pattern was similar to the pattern of activity of forearm muscles. Class II neurons (15 of 29) showed an abrupt change in firing frequency that was bidirectional. They were often related to torque load and/or to velocity but not to position. Motor cortex neurons discharged in relation to rapid alternating movements, torque pulses, and tremor in similar patterns that did not distinguish the two classes. Five units in dorsal root ganglia were identified as muscle spindle afferents. During ramps, their pattern of discharge was bidirectional and resembled the bidirectional discharge patterns of neurons in motor cortex (class II) and cerebellum. For some cells the bidirectional pattern varied slightly in relation to the direction and velocity of movement and the amount of torque load, but it was not related to the large changes in wrist position (muscle length). Modulation in relation to tremor was superimposed on the bidirectional pattern related to ramps. The comparison of spindle afferent discharge with the concurrent electromyogram (EMG) of the parent muscle suggested that spindles were driven by gamma-fusimotor activity dissociated from that of homonymous alpha-skeletomotor neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Independent control of reflex and volitional EMG modulation during sinusoidal pursuit tracking in humans

It is well known that during volitional sinusoidal tracking the long-latency reflex modulates in parallel with the volitional EMG activity. In this study, a series of experiments are reported demonstrating several conditions in which an uncoupling of reflex from volitional activity occurs. The paradigm consists of a visually guided task in which the subject tracked a sinusoid with the wrist. The movement was perturbed by constant torque or controlled velocity perturbations at 45° intervals of the tracking phase. Volitional and reflex-evoked EMG and wrist displacement as functions of the tracking phase were recorded. The relationship of both short-latency (30–60 ms) and longer-latency (60–100 ms) reflex components to the volitional EMG was evaluated. In reflex tracking, the peak reflex amplitude occurs at phases of tracking which correspond to a maximum of wrist joint angular velocity in the direction of homonymous muscle shortening and a minimum of wrist compliance. Uncoupling of the reflex and volitional EMG was observed in three situations. First, during passive movement of the wrist through the sinusoidal tracking cycle perturbation-evoked long-latency stretch reflex peak is modulated as for normal, volitional tracking. However, with passive joint movement the volitional EMG modulation is undetectable. Second, a subset of subjects demonstrate a normally modulated and positioned long-latency reflex with a single peak. However, these subjects have distinct bimodal peaks of volitional EMG. Third, the imposition of an anti-elastic load (positive position feedback) shifts the volitional EMG envelope by as much as 180° along the tracking phase when compared with conventional elastic loading. Yet the long-latency reflex peak remains at its usual phase in the tracking cycle, corresponding to the maximal velocity in the direction of muscle shortening. Furthermore, comparison of the results from elastic and anti-elastic loads reveals a dissociation of short- and long-latency reflex activity, with the short-latency reflex shifting with the volitional EMG envelope. Comparable results were also obtained for controlled velocity perturbations used to control for changes in joint compliance. The uncoupling of the reflex and volitional EMG activity in the present series of experiments points to a flexible relationship between reflex and volitional control systems, altered by peripheral input and external load.     

Arm muscle activation for static forces in three-dimensional space

1. Muscle activity was related to the direction of a static force at the human wrist. For each muscle the force direction of maximal activity and the directional tuning characteristics were determined. 2. Electromyographic (EMG) activity was recorded from nine superficial elbow and/or shoulder muscles while subjects held the right arm stationary in one of six postures. The direction of the force at the wrist was varied in two orthogonal planes. In each experiment a cable was attached to the subject's wrist, and a constant force magnitude was applied in various directions with the use of a pulley system. 3. The relationship between the averaged EMG level and the force direction was described for each muscle, in each posture, and in each plane. The EMG data were fit with a nonlinear, multiple cosine function, which allowed the identification of one, two, or sometimes three separate cosine peaks. 4. Two-cosine functions often provided the best fit to the EMG data. All nine muscles were best fit with a two-cosine function in at least two of the six postures. Four of the muscles had a second peak of activity in more than one-half of the experimental situations. The second peak was often in a direction that was nearly opposite the direction of the first peak and represented a negative contribution to the total force produced at the wrist ("coactivation"). We suggest that multimodal directional tuning results from the convergence of multiple sources of descending signals onto motoneurons. 5. The mechanical actions of nine elbow and/or shoulder muscles were estimated with the use of published data from a cadaver study by Wood and co-workers. Postural changes in the mechanical actions of muscles were substantial. A 45 degrees rotation of the shoulder, for example, might cause a 30-50 degrees change in the direction of force at the wrist that could be produced by the contraction of a given muscle. The magnitude of these postural changes suggests that arm position is an important determinate of EMG patterns. 6. Postural changes in the direction of maximal EMG activity usually paralleled the postural changes in mechanical pulling direction. Postural changes in EMG amplitudes usually covaried with postural changes in mechanical advantage. 7. The posterior deltoid (PD) was an exception to the general rule of covariation of mechanical actions and EMG activities. Instead of reflecting the muscle's mechanical action, the EMG activity of the PD closely resembled the EMG activity of the medial deltoid (MD).(ABSTRACT TRUNCATED AT 400 WORDS)     

Low Frequency Cortico-Muscular Coherence During Voluntary Rapid Movements of the Wrist Joint

Rapid voluntary point-to-point wrist tracking movements are generated by the co-operative action of a large number of wrist muscles activated in a stereotypic pattern. This pattern is composed of a two burst sequence occurring in synergist and antagonist muscles. The time course and duration of these bursts are relatively fixed but the burst magnitude in any one muscle varies in relation to the direction of movement and the preferred directional tuning characteristic of the muscle. This creates a highly adaptive method for generating fast movements to different positions in space. In this study we have examined the extent to which this adaptive burst behaviour can be associated with activity changes occurring in the contralateral motor cortex. Time dependent coherence estimates were obtained from simultaneous recordings of the electroencephalogram (EEG) made from the contralateral sensorimotor cortex and the electromyogram (EMG) from various wrist flexor and extensor muscles during fast point-to-point wrist tracking movements. Using the onset of movement as a trigger, event related coherence estimates reveal the presence of short lasting periods of low frequency (<12 Hz) coherence during the execution of fast wrist movements. The onset and duration of the periods of low frequency coherence vary with direction of movement and the temporal burst profile of a particular muscle's EMG activity. It is therefore likely that a significant low frequency activation of the motor cortex plays a part in the generation of the EMG burst patterns that underpin rapid point-to-point movements of the human wrist.     

Normalized lengthening peak torque is associated with temporal twitch characteristics in elderly women but not young women

Aim: To determine if greater normalized torque during maximal effort lengthening actions in elderly women compared with young women is related to age‐associated adjustments in neural activation and/or contractile function. Methods: The right knee extensors of 14 young women (21–30 years) and 12 elderly women (65–78 years) were assessed for isometric, shortening and lengthening peak torque, electromyography (EMG) activity, and isometric twitch contractile properties. Knee extensor contractile tissue volume was determined using magnetic resonance imaging. Normalized torque was determined as peak torque per unit of knee extensor contractile tissue volume. Results: Normalized torque during the isometric and shortening actions was similar between age groups (P > 0.05); however, lengthening normalized torque was significantly higher for the elderly women (P < 0.05). In the young women, a significant relationship existed between normalized torque and EMG for all muscle actions (P < 0.05), while no association was found between normalized torque and temporal twitch characteristics for any muscle action (P > 0.05). In the elderly women, a significant relationship existed between normalized torque and EMG for the isometric and shortening muscle actions (P < 0.05), but not for lengthening normalized torque and EMG (P > 0.05). Furthermore, no association existed between isometric and shortening normalized torque, and temporal twitch characteristics in the elderly women (P > 0.05); however, a significant relationship existed between lengthening normalized torque, and the rate of relaxation and contraction duration (P < 0.05). Conclusions: The greater capacity to develop lengthening peak torque relative to contractile tissue volume in the elderly women appeared to be associated with age‐related adjustments in the temporal twitch characteristics rather than neural activation.     

Short- and long-latency reflex responses during different motor tasks in elbow flexor muscles

 Stretch reflex responses in three elbow flexor muscles – the brachioradialis and the short and long heads of the biceps brachii – were studied during different motor tasks. The motor tasks were iso-velocity (8 deg/s) elbow flexion movements in which the muscles performed shortening or lengthening contractions, or were isometric contractions. Care was taken to maintain constant background electromyographic (EMG) activity in the brachoradialis muscle at a 50-deg elbow angle across the tasks by changing the magnitude of the initial load. During each task, mechanical perturbations (duration 170 ms) were applied at pseudorandom intervals when the elbow angle was 50 deg. The magnitude of the perturbation was varied across tasks in order to induce an elbow extension velocity of 80 deg/s over the first 50 ms after the onset of perturbation. The stretch reflex EMG responses in all muscles varied across the three tasks, despite a constant EMG level and similar perturbation-induced angular velocity in the direction of elbow extension. In particular, both the short- and long-latency reflex EMG components were reduced during the lengthening contractions. Further, the task-dependent variations in the early (M2) and the late (M3) components of the long-latency reflex were different, i.e., the magnitude of M3 was considerably enhanced during the shortening task as compared with that of M2. These findings suggest that central modification was responsible for the task-dependent modulation of late EMG responses. Received: 24 April 1996 / Accepted: 24 January 1997     

Correlation of neural discharge with pattern and force of muscular activity, joint position, and direction of intended next movement in motor cortex and cerebellum.

1. Monkeys were trained to grasp a rod movable in a horizontal arc (Fig. 1), and to hold the rod by angulation of the wrist in each of three positions (A,B, C). A maintained load was placed on the rod alternately to oppose flexion and extension. At a light signal, the monkey had to move to the next position in a prescribed sequence (ABCBABCBA, ETC.). The task was designed to dissociate, while holding in position, the following variables: 1) pattern of muscular activity in the forearm required to hold the wrist in position, determined by the direction of the load (flexor or extensor muscles); 2) position of the rod, and thus angulation of the wrist joint (A, B, and C); and 3) set for the direction of the intended next movement (flexor or extensor). These variables are subsequently referred to as MPAT, JPOS, and DSET, respectively. 2. After training, recordings were made of the EMG activity of muscles used in the task and of the discharge of single neurons in the motor cortex of the cerebrum and the interposed and dentate nuclei of the cerebellum. 3. While holding the wrist in position, EMG and interpositus behaved uniformly, with higher discharge frequency under load in one direction and lower discharge frequency under load in the opposite direction. This relation was relatively independent of the position held and of the direction of the intended next movement. Thus, interpositus and EMG both seemed best related to the MPAT variable, as opposed to JPOS and DSET variables. By contrast, neurons in motor cortex and in dentate fell into three categories: one category discharged in relation to the pattern of muscular activity (MPAT), a second to the position of the wrist (JPOS), and a third to the direction of the intended next movement (DSET). While MPAT neurons formed a distinct dissociated group, neurons that were best related to JPOS were often related to DSET, and vice versa. 4. A few of the MPAT neurons in interpositus and motor cortex were further studied by varying the magnitude (as well as the direction) of the loads. Both interpositus and motor cortex MPAT neurons changed firing frequency in relation to the magnitude of load, and though few neurons were thus studied, the relation seemed clearer for interpositus than for motor cortex. 5. Anatomically, the three types of neurons thus classified by firing pattern during the hold periods were intermixed in the arm area of motor cortex. In dentate and interpositus, those neurons thus related to the performance were localized to a narrow strip across the posterior part of both nuclei. Neurons apparently related to eye and drinking movements were located more posteriorly still, suggesting somatotopic representation.     

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)     

Reprogramming of muscle activation patterns at the wrist in compensation for elbow reaction torques during planar two-joint arm movements

The relationship between wrist kinematics, dynamics and the pattern of muscle activation were examined during a two-joint planar movement in which the two joints moved in opposite directions, i.e. elbow flexion/wrist extension and elbow extension/wrist flexion. Elbow movements (ranging from 10 to 70 deg) and wrist movements (ranging from 10 to 50 deg) were performed during a visual, step-tracking task in which subjects were required to attend to the initial and final angles at each joint. As the elbow amplitude increased, wrist movement duration increased and the wrist movement trajectories became quite variable. Analysis of the torques acting at the wrist joint showed that elbow movements produced reaction torques acting in the same direction as the intended wrist movement. Distinct patterns of muscle activation were observed at the wrist joint that were dependent on the relative magnitude of the elbow reaction torque in relation to the net wrist torque. When the magnitude of the elbow reaction torque was quite small, the wrist agonist was activated first. As the magnitude of the elbow reaction torque increased, activity in the wrist agonist decreased significantly. In conditions where the elbow reaction torque was much larger than the net wrist torque, the wrist muscle torque reversed direction to oppose the intended movement. This reversal of wrist muscle torque was directly associated with a change in the pattern of muscle activation where the wrist antagonist was activated prior to the wrist agonist. Our findings indicate that motion of the elbow joint is an important consideration in planning wrist movement. Specifically, the selection of muscle activation patterns at the wrist is dependent on the relative magnitude and direction of the elbow reaction torque in relation to the direction of wrist motion.     

Wrist muscle activation,interaction torque and mechanical properties in unskilled throws of different speeds

An unexpected property of unskilled overarm throws is that wrist flexion velocity at ball release does not increase in throws of increasing speed. We investigated the nature of the interaction torques and wrist mechanical properties that have been proposed to produce this property. Twelve recreational throwers made seated 2-D throws, which were used as a model for unskilled throwing. Joint motions were computed from recordings made with search coils; joint torques were calculated from inverse dynamics. Wrist flexion velocity at ball release was actually smaller in fast throws than in slow throws. This was associated in fast throws with the decrease in a large wrist flexor muscle torque (i.e., a calculated residual torque) in the last 40 ms before ball release, and its reversal to an extensor torque. Consequently, wrist flexor muscle torque was unable to oppose a small maintained wrist extensor interaction torque that arose from continuing elbow extension acceleration. The decrease in wrist flexor muscle torque was not associated with a decrease in wrist flexor EMG activity, nor with an increase in wrist extensor EMG activity. These findings support the hypothesis that the smaller wrist flexion velocity at ball release in fast 2-D throws results from a wrist extensor interaction torque and from a large wrist extensor viscoelastic torque. We propose that in fast 3-D throws skilled subjects decelerate elbow extension before ball release to help overcome these wrist extensor torques.     

Compensation for interaction torques during single- and multijoint limb movement

During multijoint limb movements such as reaching, rotational forces arise at one joint due to the motions of limb segments about other joints. We report the results of three experiments in which we assessed the extent to which control signals to muscles are adjusted to counteract these "interaction torques." Human subjects performed single- and multijoint pointing movements involving shoulder and elbow motion, and movement parameters related to the magnitude and direction of interaction torques were manipulated systematically. We examined electromyographic (EMG) activity of shoulder and elbow muscles and, specifically, the relationship between EMG activity and joint interaction torque. A first set of experiments examined single-joint movements. During both single-joint elbow (experiment 1) and shoulder (experiment 2) movements, phasic EMG activity was observed in muscles spanning the stationary joint (shoulder muscles in experiment 1 and elbow muscles in experiment 2). This muscle activity preceded movement and varied in amplitude with the magnitude of upcoming interaction torque (the load resulting from motion of the nonstationary limb segment). In a third experiment, subjects performed multijoint movements involving simultaneous motion at the shoulder and elbow. Movement amplitude and velocity at one joint were held constant, while the direction of movement about the other joint was varied. When the direction of elbow motion was varied (flexion vs. extension) and shoulder kinematics were held constant, EMG activity in shoulder muscles varied depending on the direction of elbow motion (and hence the sign of the interaction torque arising at the shoulder). Similarly, EMG activity in elbow muscles varied depending on the direction of shoulder motion for movements in which elbow kinematics were held constant. The results from all three experiments support the idea that central control signals to muscles are adjusted, in a predictive manner, to compensate for interaction torques-loads arising at one joint that depend on motion about other joints.     

The relative activation of muscles during isometric contractions and low-velocity movements against a load

Surface electromyographic (EMG) and motor unit activity were measured in human arm muscles during isometric contractions and during movements against an elastic load. The direction of force applied proximal to the wrist and movement direction of the wrist were varied in a horizontal plane. During isometric contractions the direction in which the largest EMG activity was measured corresponded to the direction in which motor units had the smallest recruitment threshold, for each muscle. The same was found for movements against an elastic load. However, this direction was different for isometric contractions and for movements. Because the magnitude and sign of these changes varied for different muscles, this resulted in a different relative activation of muscles for the two conditions. The amplitude of the surface EMG during contractions against an elastic load was generally significantly larger than that for isometric contractions against the same load. For m. brachioradialis isometric conditions yielded occasionally increased EMG activity. The change in EMG activity could be attributed completely to changes in motor unit recruitment thresholds leading to proportionate changes in the number of recruited motor units. However, the initial firing rate of motor units at recruitment was the same under both conditions and, therefore, did not contribute to changes in amplitude of surface EMG activity.     

Three-dimensional spatial tuning of neck muscle activation in humans

The complex structure of the neck musculoskeletal system poses challenges to understanding central nervous system (CNS) control strategies. Examining muscle activation patterns in relation to musculoskeletal geometry and three-dimensional mechanics may reveal organizing principles. We analyzed the spatial tuning of neck muscle electromyographic (EMG) activity while subjects generated moments in three dimensions. EMG tuning curves were characterized by their orientation (mean direction) and focus (spread of activity). For the four muscles that were studied (sternocleidomastoid, splenius capitis, semispinalis capitis and trapezius), EMG tuning curves exhibited directional preference, with consistent orientation and focus among 12 subjects. However, the directional preference (orientation) of three of the four neck muscles did not correspond to the muscle's moment arm, indicating that maximizing a muscle's mechanical advantage is not the only factor in determining muscle activation. The focus of muscle tuning did not change with moment magnitude, demonstrating that co-contraction did not increase with load. Axial rotation was found to have a strong influence on neck muscle spatial tuning. The uniform results among subjects indicate that the CNS has consistent strategies for selecting neck muscle activations to generate moments in specific directions; however, these strategies depend on three-dimensional mechanics in a complex manner. Electronic Publication     

Muscle strength and myoelectric activity in prepubertal and adult males and females

The purpose of this investigation was to compare children and adults of both genders with respect to torque-velocity, electromyogram (EMG)-velocity and torque-EMG relationships during maximal voluntary knee extensor muscle actions. Four groups of ten subjects each were studied comprising 11-year-old girls and boys and female and male physical education students (22–35 years). Maximal voluntary eccentric (lengthening) and concentric (shortening) actions of the knee extensors were performed at the constant velocities of 45, 90 and 180° · s^–1. Average values for torque and EMG activity, recorded by surface electrodes from the quadriceps muscle, were taken for the mid 40° of the 80° range of motion. The overall shapes of the torque- and EMG-velocity relationships were similar for all four groups, showing effects of velocity under concentric (torque decrease and EMG increase) but not under eccentric conditions. Eccentric torques were always greater than velocity-matched concentric ones, whereas the eccentric EMG values were lower than the concentric ones at corresponding velocities. Torque output per unit EMG activity was clearly higher for eccentric than for concentric conditions and the difference was of similar magnitude for all groups. Thus, the torque-EMG-velocity relationships would appear to have been largely independent of gender and to be fully developed at a prepubertal age.     

Isometric axial rotation of the trunk in the neutral posture

The purpose of this study was to measure the torque, the magnitude of the electromyogram (EMG) signal and the phase relationship of 14 muscles during trunk axial rotation. Fifty normal healthy volunteers (27 males and 23 females) with no lower-back injury participated in the study. The subjects were seated in an upright position in the axial rotation tester (AROT) after applying surface electrodes bilaterally to the following muscles: pectoralis major, rectus abdominis, external oblique, internal oblique, latissimus dorsi, and erector spinae at T₁₀ and L₃. They were stabilized from the hip down, and the shoulder harness of the AROT was applied to their shoulders. These subjects performed maximal isometric axial rotations to the left and right in a random order. The torque and 14 channels of EMG were monitored, and their magnitude, slope of the increase in magnitude, and timing of the anticipation and onset activity were determined. The results revealed that the females produced 65% of the torque of their male counterparts. The pattern and magnitude of EMG in performing these tasks were significantly different between males and females (P<0.01). Males generated the greatest activity in their ipsilateral latissimus dorsi followed by their contralateral external oblique muscles. In the females, maximal EMG activity was observed in their contralateral pectoralis muscle. Thus, under the current experimental conditions, the females employed a different muscle recruitment strategy compared to the males. Each muscle involved in axial rotation was significantly different from the other (P<0.01). The timing pattern for these activities was inconsistent, implying that there is no fixed-order phasic recruitment of the torso muscles during maximal isometric axial rotation. Electronic Publication     

Vastus lateralis surface and single motor unit electromyography during shortening,lengthening and isometric contractions corrected for mode‐dependent differences in force‐generating capacity

Aim: Knee extensor neuromuscular activity, rectified surface electromyography (rsEMG) and single motor unit EMG was investigated during isometric (60° knee angle), shortening and lengthening contractions (50–70°, 10° s^?1) corrected for force–velocity‐related differences in force‐generating capacity. However, during dynamic contractions additional factors such as shortening‐induced force losses and lengthening‐induced force gains may also affect force capacity and thereby neuromuscular activity. Therefore, even after correction for force–velocity‐related differences in force capacity we expected neuromuscular activity to be higher and lower during shortening and lengthening, respectively, compared to isometric contractions. Methods: rsEMG of the three superficial muscle heads was obtained in a first session [10 and 50% maximal voluntary contraction (MVC)] and additionally EMG of (46) vastus lateralis motor units was recorded during a second session (4–76% MVC). Using superimposed electrical stimulation, force‐generating capacity for shortening and lengthening contractions was found to be 0.96 and 1.16 times isometric (Iso) force capacity respectively. Therefore, neuromuscular activity during submaximal shortening and lengthening was compared with isometric contractions of respectively 1.04Iso (=1/0.96) and 0.86Iso (=1/1.16). rsEMG and discharge rates were normalized to isometric values. Results: rsEMG behaviour was similar (P > 0.05) during both sessions. Shortening rsEMG (1.30 ± 0.11) and discharge rate (1.22 ± 0.13) were higher (P < 0.05) than 1.04Iso values (1.05 ± 0.05 and 1.03 ± 0.04 respectively), but lengthening rsEMG (1.05 ± 0.12) and discharge rate (0.90 ± 0.08) were not lower (P > 0.05) than 0.86Iso values (0.76 ± 0.04 and 0.91 ± 0.07 respectively). Conclusion: When force–velocity‐related differences in force capacity were taken into account, neuromuscular activity was not lower during lengthening but was still higher during shortening compared with isometric contractions.     

Compensation for mechanically unstable loading in voluntary wrist movement

In order to study the roles of muscle mechanics and reflex feedback in stabilizing movement, experiments were conducted in which healthy human subjects performed targeted wrist movements under conditions where the damping of the wrist was reduced with a load having the property of negative viscosity. If the movement speed and negative viscosity were sufficiently high, the wrist oscillated for several hundred milliseconds about the final target position. Subjects increased the activation of both wrist flexor and extensor muscles to increase joint stiffness to damp the oscillations. With practice, both the tendency to oscillate and the level of muscle activation were reduced. A small bias torque in either direction, added to the negative viscosity, enhanced the oscillations as well as the amount of flexor and extensor muscle activation during the stabilization phase of fast movements. The tendency for the wrist to oscillate was also seen during slow movements where the oscillations were superimposed upon the voluntary movement. We suggest that this reduction in mechanical stability is primarily of reflex origin. As wrist stiffness increases, the natural frequency of the wrist also increases, which in turn produces an increase in the phase lag of the torque generated by the myotatic reflex with respect to wrist angular velocity, effectively reducing damping. The oscillation frequency was often close to a critical frequency for stability at which torque, due to the myotatic reflex, lagged angular velocity by 180° (6–7.5 Hz). Nevertheless, subjects were able to damp these oscillations, probably because the torque due to intrinsic muscle stiffness (in phase with position and hence lagging velocity by only 90°) dominated the torque contribution of the myotatic reflex. Increasing stiffness with declining oscillation amplitude may also have contributed significantly to damping.     

Tension regulation during lengthening and shortening actions of the human soleus muscle

In the present study we investigated tension regulation in the human soleus (SOL) muscle during controlled lengthening and shortening actions. Eleven subjects performed plantar flexor efforts on an ankle torque motor through 30° of ankle displacement (75°–105° internal ankle angle) at lengthening and shortening velocities of 5, 15 and 30° · s⁻¹. To isolate the SOL from the remainder of the triceps surae, the subject's knee was flexed to 60° during all trials. Voluntary plantar flexor efforts were performed under two test conditions: (1) maximal voluntary activation (MVA) of the SOL, and (2) constant submaximal voluntary activation (SVA) of the SOL. SVA trials were performed with direct visual feedback of the SOL electromyogram (EMG) at a level resulting in a torque output of 30% of isometric maximum. Angle-specific (90° ankle angle) torque and EMG of the SOL, medial gastrocnemius (MG) and tibialis anterior (TA) were recorded. In seven subjects from the initial group, the test protocol was repeated under submaximal percutaneous electrical activation (SEA) of SOL (to 30% isometric maximal effort). Lengthening torques were significantly greater than shortening torques in all test conditions. Lengthening torques in MVA and SVA were independent of velocity and remained at the isometric level, whereas SEA torques were greater than isometric torques and increased at higher lengthening velocities. Shortening torques were lower than the isometric level for all conditions. However, whereas SVA and SEA torques decreased at higher velocities of shortening, MVA torques were independent of velocity. These results indicate velocity- and activation-type-specific tension regulation in the human SOL muscle. Accepted: 11 October 1999     

Long-term adaptations differ for shortening and lengthening contractions

The purpose of this study was to determine whether practice of a sinusoidal task induces different neural adaptations for shortening and lengthening contractions performed within a task. Fourteen young adults were instructed to accurately match a sinusoidal target by lifting and lowering a light load (15% of 1 repetition maximum; 1-RM) with their index finger for 35?s. Each subject performed a total of 50 practice trials during the practice session. After 48?h, subjects performed five trials with the same sinusoidal target at each of three loading conditions: 15% (retention/savings), 7.5% (transfer to a lighter load), and 30% (transfer to a heavier load) of 1-RM. Movement error was quantified as the root mean square error of the movement trace from the target, while movement variability was quantified as the standard deviation of the acceleration of the index finger. First dorsal interosseus muscle activation was recorded using surface electromyography (EMG). The frequency structure of the acceleration and EMG signals were obtained using wavelets. Subjects were able to retain the trained task for both shortening and lengthening contractions; however, they exhibited better savings for the shortening contractions. Additionally, for the lowering segments of the task subjects exhibited better transfer to the lighter load. Short-term adaptation and transfer results may be related to changes in the agonist muscle neural activation. Finally, we found greater movement variability during lengthening contractions which was related to both the frequency structure of the acceleration and EMG signals.     

Proprioceptive population coding of limb position in humans

The present study investigates the coding of positions reached in a two-dimensional space by populations of muscle spindle afferents. The unitary activity of 35 primary muscle spindle afferents originating from the tibialis anterior, extensor digitorum longus, extensor hallucis longus, and peroneus lateralis muscles were recorded from the common peroneal nerve by the microneurographic technique. The steady mean frequency of discharge was analyzed during 16 passively maintained positions of the tip of the foot. These positions were equally distant from and circularly arranged around the "neutral" position of the ankle. The results showed that a same position of the foot was differently coded depending on whether it was maintained for several seconds or whether it was attained after a movement. Muscle spindle activity was increased or decreased, respectively, when the previous movement lengthened or shortened the parent muscle; the magnitude of change in activity depended on the amount of lengthening or shortening in relation to movement direction. Each muscle surrounding the ankle joint was shown to encode the different spatial positions following a directional tuning curve. Data were analyzed by using the "neuronal population vector model". This model consists of calculating population vectors representing the mean contribution of each muscle population of afferents to the coding of a particular position, and by finally calculating a sum vector. The direction of the sum vector was shown to accurately describe the direction of a given maintained position compared to the initial position. We conclude that muscle spindle position coding is based on afferent information coming from the whole set of muscles crossing a given joint. A given spatial position is associated with a stable muscle afferent inflow where each muscle makes an oriented and weighted contribution to its coding. Electronic Publication