Sensory control of target acquisition.

Recent research has expanded our understanding of how the nervous system uses visual and kinesthetic input to move the arm to a target. In this review, we present data to show how the nervous system can rapidly use sensory input to control impending or ongoing motor activity. We contrast visual control with kinesthetic control to show how these two sources of sensory input are used to control parameters of motor command such as amplitude and direction, to trigger the motor commands, and then to correct errors in trajectory. Despite many differences in the organization of the visual and the kinesthetic sensory systems, the nervous system appears to process these two types of sensory input similarly.

Interaction of involuntary post-contraction activity with locomotor movements.

Involuntary post-contraction muscle activity may occur after performing a strong long-lasting (about 30 s) isometric muscle contraction (Kohnstamm phenomenon). Here we examined how this putative excitatory state may interact with a locomotor movement. The subjects stood upright and were asked to oppose a rotational force applied to the pelvis for about 30 s either in the clockwise or in the counterclockwise direction. After that, they were asked to perform various motor tasks with the eyes closed. During quiet standing, we observed an involuntary post-contraction torsion of the trunk. During walking, the post-contraction facilitatory effect of body torsion was not overridden by the voluntary activity, but instead significantly influenced the forward locomotor program such that subjects walked along a curved trajectory in the direction of the preceding torsion. In contrast, we did not observe any rotational component when subjects were asked to step in place. We conclude that the post-contraction rotational aftereffect does not transfer to just any motor task but apparently manifests itself in those movements that incorporate the activated axial muscle synergy or rotational component. We argue that central excitability changes following the voluntary effort may contribute to the phenomenon and highlight the role of tonic influences in fine-tuning of the spinal cord.

The measurement of tremor using simple laser systems.

Quantification of movement pathologies is an important challenge of the clinical and research laboratories today. Basically, two problems must be addressed. The first one is to find the appropriate technology; the second is to develop adequate measures from the raw data which will best discriminate between health and pathology. In this paper, we propose a simple method to record and analyse tremor and other microdisplacements of the upper extremities based on the recording of position by laser analog sensors. Any microcomputer equipped to perform analog-digital conversion is compatible for use with this system. The performance of the laser system is examined and compared with the performance of accelerometers. Finally, data recorded with a laser analog sensor from a patient with Parkinson's disease and a control subject are presented. This new laser-based quantitative method may prove to be an important tool in early and differential diagnosis of neurodegenerative diseases of the central nervous system.

The effect of aging on dynamic position sense at the ankle.

The present study addressed whether dynamic position sense at the ankle--or sense of position and velocity during movement--shows a similar decline as a result of aging as previously described for static position sense and movement detection threshold. Additionally, the involvement of muscle spindle afferents in the possible age-related decline was studied. To assess dynamic position sense, blindfolded subjects had to open the hand briskly when the right ankle was rotating passively through a prescribed target angle. To assess the involvement of muscle spindles, the effect of tibialis anterior vibration was studied. The results showed that aging lead to a significant increase in deviation from the target angle at hand opening as well as in variability of performance. Vibration resulted in larger undershoot errors in the elderly compared to the young adults, suggesting that the age-related decline in performance on the dynamic position sense task is not (solely) due to muscle spindle function changes. Alternatively, this degeneration might be due to altered input from other sources of proprioceptive input, such as skin receptors. The elderly subjects did show a beneficial effect of practice with the task, which may provide solid fundaments for rehabilitation.

Kinesthetic coordination of a movement sequence in humans.

This experiment examined whether kinesthetic input could be used to coordinate a 'movement sequence', a series of sequentially executed joint rotations. In this experiment, human subjects extended the elbow at a constant velocity and opened the hand as the elbow passed through a predetermined angle, as in throwing. Subjects were able to open the hand at the prescribed elbow angle, even though they received no visual feedback and they could not predict when the elbow would reach this angle. The only remaining source of information about elbow angle was kinesthetic input. Being able to control the internal timing of our movement sequences with kinesthetic input may be important to the way we move.

The role of paraspinal muscle spindles in lumbosacral position sense in individuals with and without low back pain.

STUDY DESIGN: A two-group experimental design with repeated measures on one factor was used. OBJECTIVES: To investigate the role of paraspinal muscle spindles in lumbosacral position sense in individuals with and without low back pain. SUMMARY OF BACKGROUND DATA: Proprioceptive deficits have been identified in patients with low back pain. The underlying mechanisms, however, are not well documented. METHODS: Lumbosacral position sense was determined before, during, and after lumbar paraspinal muscle vibration in 23 young patients with low back pain and in 21 control subjects. Position sense was estimated by calculating the mean absolute error, constant error, and variable error between six criterion and reproduction sacral tilt angles. RESULTS: Repositioning accuracy was significantly lower in the patient group than in healthy individuals (absolute error difference between groups = 2.7 degrees, P < 0.0001). Multifidus muscle vibration induced a significant muscle-lengthening illusion that resulted in an undershooting of the target position in healthy individuals (constant error = -3.1 degrees, P < 0.0001). Conversely, the position sense scores of the patient group did not display an increase in negative directional error but a significant improvement in position sense during muscle vibration (P < 0.05). No significant differences in absolute error were found between the first and last trial in the healthy individuals (P >/= 0.05) and in the patient group (P > 0.05). CONCLUSIONS: Patients with low back pain have a less refined position sense than healthy individuals, possibly because of an altered paraspinal muscle spindle afference and central processing of this sensory input. Furthermore, muscle vibration can be an interesting expedient for improving proprioception and enhancing local muscle control.

The sit-up: complex kinematics and muscle activity in voluntary axial movement.

This paper describes the kinematics and muscle activity associated with the standard sit-up, as a first step in the investigation of complex motor coordination. Eight normal human subjects lay on a force table and performed at least 15 sit-ups, with the arms across the chest and the legs straight and unconstrained. Several subjects also performed sit-ups with an additional weight added to the head. Support surface forces were recorded to calculate the location of the center of pressure and center of gravity; conventional motion analysis was used to measure segmental positions; and surface EMG was recorded from eight muscles. While the sit-up consists of two serial components, 'trunk curling' and 'footward pelvic rotation', it can be further subdivided into five phases, based on the kinematics. Phases I and II comprise trunk curling. Phase I consists of neck and upper trunk flexion, and phase II consists of lumbar trunk lifting. Phase II corresponds to the point of peak muscle contraction and maximum postural instability, the 'critical point' of the sit-up. Phases III-V comprise footward pelvic rotation. Phase III begins with pelvic rotation towards the feet, phase IV with leg lowering, and phase V with contact between the legs and the support surface. The overall pattern of muscle activity was complex with times of EMG onset, peak activity, offset, and duration differing for different muscles. This complex pattern changed qualitatively from one phase to the next, suggesting that the roles of different muscles and, as a consequence, the overall form of coordination, change during the sit-up.

Time-dependent effects of kinesthetic input.

Sensory input can be used by the nervous system to control the spatial parameters of motor responses (e.g., distance, velocity, and direction) by initializing these parameters before movement onset and then by adjusting these parameters during movement. Sensory input can also be used to trigger movements. In the experiments reported in this paper, we compared the effects of kinesthetic input on a triggered motor response when the kinesthetic input was generated at different times relative to the onset of the motor response. Human subjects responded to a visual stimulus by intentionally increasing elbow torque to a target level. Kinesthetic input was generated by unexpectedly rotating each subject's elbow 100 ms before the onset of the intentional torque response (early) or coincident with the onset of the intentional torque response (late). The effect of early kinesthetic input on the intentional torque response markedly differed from the effect of late kinesthetic input. The effect of early kinesthetic input was relatively independent of the direction of elbow rotation, had a different dependence on the amplitude of rotation, and required a shorter duration of rotation compared to the effect of late kinesthetic input. These differences in the effects of early and late kinesthetic input might be related to the initialization, triggering, and adjustment of motor responses.

Interaction between visually and kinesthetically triggered voluntary responses.

A voluntary motor response that is prepared in advance of a stimulus may be triggered by any sensory input. This study investigated the combination of visual and kinesthetic inputs in triggering voluntary torque responses. When a visual stimulus was presented alone, subjects produced a fast and accurate increase in elbow flexion torque. When a kinesthetic stimulus was presented instead of the visual stimulus, subjects produced a similar response with a reduced response latency. When a visual stimulus was presented in combination with a kinesthetic stimulus, subjects initiated their responses after either a visual or a kinesthetic response latency, depending on the relative timing of the two stimuli. An analysis of response amplitude suggested that when visual and kinesthetic stimuli were combined, both stimuli triggered a response. The results are more consistent with a simple behavioral model of addition of visual and kinesthetic responses (which predicts that the response to combined stimuli should be the sum of individual responses) than with a model of exclusion of one response (which predicts that the response to combined stimuli should be identical to either the visual or the kinesthetic response). Because addition of visually and kinesthetically triggered responses produced a response with an erroneously large amplitude, it is suggested that visual and kinesthetic inputs are not always efficiently integrated.

Haptic touch reduces sway by increasing axial tone.

It is unclear how haptic touch with a stable surface reduces postural sway. We hypothesized that haptic input enhances postural stability due to alterations in axial postural tone. We measured the influence of heavy and light touch (LT) of the hands on a stable bar on axial postural tone and postural sway during stance in 14 healthy adults. A unique "Twister" device measured hip torque by fixing the upper body in space while oscillating the surface in yaw ±10 at 1 deg/s. Subjects were tested while: (1) standing quietly with their arms at their sides, (2) lightly touching a rigid bar in front of them and (3) firmly gripping the bar. Horizontal and vertical sway was not restricted by the device's yaw fixation, therefore, the subjects remained in a state of active postural control during the three touch conditions. Haptic touch significantly increased hip postural tone by 44% during light touch, from 2.5±0.9 to 3.6±1.0 Nm (P=0.005), and by 40% during firm grip to 3.5±0.8 Nm (P=0.005). Increases in hip postural tone were associated with a reduction in postural sway (r=-0.55, P=0.001). This is the first study showing that axial postural tone can be modified by remote somatosensory input and provides a potential explanation for how light touch improves postural stability. Changes in subjects' perception from trunk to surface rotation when changing from no touch (NT) to haptic touch, suggests that the CNS changes from using a global, to a local, trunk reference frame for control of posture during touch. The increase of hip postural tone during touching and gripping can be explained as a suppression of hip muscle shortening reactions that normally assist axial rotation.

Increased dynamic regulation of postural tone through Alexander Technique training.

Gurfinkel and colleagues (2006) recently found that healthy adults dynamically modulate postural muscle tone in the body axis during anti-gravity postural maintenance and that this modulation is inversely correlated with axial stiffness. Our objective in the present study was to investigate whether dynamic modulation of axial postural tone can change through training. We examined whether teachers of the Alexander Technique (AT), who undergo "long-term" (3-year) training, have greater modulation of axial postural tone than matched control subjects. In addition, we performed a longitudinal study on the effect of "short-term" (10-week) AT training on the axial postural tone of individuals with low back pain (LBP), since short term AT training has previously been shown to reduce LBP. Axial postural tone was quantified by measuring the resistance of the neck, trunk and hips to small (±10°), slow (1°/s) torsional rotation during stance. Modulation of tone was determined by the torsional resistance to rotation (peak-to-peak, phase-advance, and variability of torque) and axial muscle activity (EMG). Peak-to-peak torque was lower (∼50%), while phase-advance and cycle-to-cycle variability were enhanced for AT teachers compared to matched control subjects at all levels of the axis. In addition, LBP subjects decreased trunk and hip stiffness following short-term AT training compared to a control intervention. While changes in static levels of postural tone may have contributed to the reduced stiffness observed with the AT, our results suggest that dynamic modulation of postural tone can be enhanced through long-term training in the AT, which may constitute an important direction for therapeutic intervention.

Axial kinesthesia is impaired in Parkinson's disease: effects of levodopa.

Integration of sensory and motor inputs has been shown to be impaired in appendicular muscles and joints of Parkinson's disease (PD) patients. As PD advances, axial symptoms such as gait and balance impairments appear, which often progresses to complete inability stand or walk unaided. The current study evaluates kinesthesia in the axial musculature of PD patients during active postural control to determine whether impairments similar to those found in the appendages are also present in the hip and trunk. Using axial twisting, we quantified the detection threshold and directional accuracy of the hip relative to the feet (i.e. Hip Kinesthesia) and the hip relative to the shoulders (i.e. Trunk Kinesthesia). The relation of kinesthetic threshold to disease progression as measured by UPDRS and the effect of levodopa treatment on kinesthesia were assessed in 12 PD compared to age-matched controls. Subjects stood unaided while passively twisted at a very low constant rotational velocity (1 degrees /s). The results showed that accuracy in determining the direction of axial twisting was reduced in PD relative to healthy control subjects in the hip (PD-ON: 81%; PD-OFF: 91%; CTL=96%) and trunk (PD-ON: 81%; PD-OFF: 88%; CTL=95%). Thresholds for perception of axial twisting were increased when PD subjects were ON levodopa versus OFF in both the hip (p<0.01) and the trunk (p<0.05). The magnitude of decrease in sensitivity due to being ON levodopa was significantly correlated with the increase in UPDRS motor scores (Hip: r=0.90, p<0.01 and Trunk: r=0.60, p<0.05). This effect was not significantly correlated with equivalent levodopa dosage. PD subjects with disease onset on the left side of their body showed significantly higher axial thresholds than subjects with right PD onset (p<0.05). In conclusion, deficits in axial kinesthesia seem to contribute to the functional impairments of posture and locomotion in PD. Although levodopa has been shown to improve appendicular kinesthesia, we observed the opposite in the body axis. These findings underscore the dissociable neurophysiological circuits and dopaminergic pathways that are known to innervate these functionally distinct muscle groups.

Axial hypertonicity in Parkinson's disease: direct measurements of trunk and hip torque.

A cardinal feature of Parkinson's disease (PD) is muscle hypertonicity, i.e. rigidity. Little is known about the axial tone in PD or the relation of hypertonia to functional impairment. We quantified axial rigidity to assess its relation to motor symptoms as measured by UPDRS and determine whether rigidity is affected by levodopa treatment. Axial rigidity was measured in 12 PD and 14 age-matched controls by directly measuring torsional resistance of the longitudinal axis to twisting (+/-10 degrees ). Feet were rotated relative to fixed hips (Hip Tone) or feet and hips were rotated relative to fixed shoulders (Trunk Tone). To assess tonic activity only, low constant velocity rotation (1 degrees /s) and low acceleration (<12 degrees /s(2)) were used to avoid eliciting phasic sensorimotor responses. Subjects stood during testing without changing body orientation relative to gravity. Body parts fixed against rotation could translate laterally within the boundaries of normal postural sway, but could not rotate. PD OFF-medication had higher axial rigidity (p<0.05) in hips (5.07 N m) and trunk (5.30 N m) than controls (3.51 N m and 4.46 N m, respectively), which did not change with levodopa (p>0.10). Hip-to-trunk torque ratio was greater in PD than controls (p<0.05) and unchanged by levodopa (p=0.28). UPDRS scores were significantly correlated with hip rigidity for PD OFF-medication (r values=0.73, p<0.05). Torsional resistance to clockwise versus counter-clockwise axial rotation was more asymmetrical in PD than controls (p<0.05), however, there was no correspondence between direction of axial asymmetry and side of disease onset. In conclusion, these findings concerning hypertonicity may underlie functional impairments of posture and locomotion in PD. The absence of a levodopa effect on axial tone suggests that axial and appendicular tones are controlled by separate neural circuits.

Effect of slow, small movement on the vibration-evoked kinesthetic illusion.

The study reported in this paper investigated how vibration-evoked illusions of joint rotation are influenced by slow (0.3 degrees /s), small (2-4 degrees ) passive rotation of the joint. Normal human adults (n=15) matched the perceived position of the left ("reference") arm with the right ("matching") arm while vibration (50 pps, 0.5 mm) was applied for 30 s to the relaxed triceps brachii of the reference arm. Both arms were constrained to rotate horizontally at the elbow. Three experimental conditions were investigated: (1) vibration of the stationary reference arm, (2) slow, small passive extension or flexion of the reference arm during vibration, and (3) slow, small passive extension or flexion of the reference arm without vibration. Triceps brachii vibration at 50 pps induced an illusion of elbow flexion. The movement illusion began after several seconds, relatively fast to begin with and gradually slowing down to a stop. On average, triceps vibration produced illusory motion at an average latency of 6.3 s, amplitude of 9.7 degrees , velocity of 0.6 degrees /s, and duration of 16.4 s. During vibration, slow, small ( approximately 0.3 degrees /s, 1.3 degrees ) passive rotations of the joint dramatically enhanced, stopped, or reversed the direction of illusory movement, depending on the direction of the passive joint rotation. However, the subjects' perceptions of these passive elbow rotations were exaggerated: 2-3 times the size of the actual movement. In the absence of vibration, the subjects accurately reproduced these passive joint rotations. We discuss whether the exaggerated perception of slow, small movement during vibration is better explained by contributions of non muscle spindle Ia afferents or by changes in the mechanical transmission of vibration to the receptor.

Mechanisms controlling accurate changes in elbow torque in humans.

This paper addresses a fundamental question of how motor commands specify target torque levels. Human subjects produced fast and accurate changes in torque with the isometric elbow joint. Visual stimuli were used to indicate target torque levels as well as to cue subjects to initiate their responses. During rapid changes in torque from one steady-state level to another, target torque was achieved through a sequence of approximations. During the first 200-250 msec of responses produced in the presence of visual feedback, 3 distinct control mechanisms were recruited to guide torque to the target level. The timing and accuracy of each control mechanism were evaluated. The first control mechanism was triggered by the visual stimulus and produced the initial rise in torque. Target torque predictability was found to strongly influence the accuracy of this control mechanism. The second control mechanism produced a corrective adjustment in torque within roughly the first 100 msec of responses. This mechanism incorporated target torque information provided by the stimulus into the response. The third control mechanism began 200-250 msec after response onset and produced corrective adjustments based on visual feedback of torque errors. The stability of the visual feedback mechanism was evaluated because of a long loop delay. Two strategies were used to control stability: low gain and information transfer between the visual feedback mechanism and the preceding (second) control mechanism.

Kinesthetic control of a multijoint movement sequence.

1. The individual joint rotations of a movement sequence might be controlled either by a central motor plan or by motion-dependent (i.e., kinesthetic) sensory input. Most previous research has focused on how the nervous system uses central motor plans to control movement sequences. This study examined how the nervous system uses kinesthetic input to control a multijoint movement sequence. 2. Human subjects were trained to extend the elbow horizontally at 22 degrees/s and to open the hand as the elbow passed through a 2 degrees-wide target zone. Different distances to the target zone were used to examine a wide range of movement times of the elbow to target zone (i.e., 150-1,500 ms). 3. A hydraulic apparatus simulated a spring resistance to the elbow extension. In some trials, the spring constant was unexpectedly increased or decreased just before the subject initiated the elbow extension, causing the elbow to slow down or speed up. Because these changes in spring constant were randomly imposed and because no visual feedback was available, subjects had to use kinesthetic input to control this motor task. 4. The experimental subjects employed two different strategies for the use of kinesthetic input to control this motor task. In the first strategy, the subjects used kinesthetic input related to the elbow rotation to detect and correct velocity errors caused by the changes in spring constant. The onset of error correction varied between 92 and 196 ms after the appearance of velocity errors. The proportion of the error corrected by the time the elbow reached the target zone varied between 31 and 78%, depending on the movement time to the target zone. However, because this correction for velocity errors was neither instantaneous nor complete, the changes in spring constant caused leads and lags in the time that the elbow reached the target zone. 5. In the second strategy, subjects used kinesthetic input related to the elbow rotation to advance or delay the onset of the hand movement, thereby compensating for leads and lags in the arrival of the elbow at the target zone. These adjustments in the triggering time of the hand movement allowed subjects to open the hand while the elbow was in the target zone. This kinesthetic triggering mechanism was effective for elbow rotations reaching the target zone within 150-1,500 ms. 6. These results suggest that, to fully understand how multijoint movement sequences are controlled by the nervous system, sensory mechanisms must be considered in addition to central mechanisms.

Kinesthetic and visual control of a bimanual task: specification of direction and amplitude.

Kinesthetic information about a perturbation can quickly modify motor activity by producing reflexive compensation. The purpose of the present study was to determine how quickly kinesthetic information about target movement can modify motor activity. Visual information about target movement is known to guide motor activity both quickly and accurately. Therefore, we compared the speed and accuracy of responses to kinesthetically and visually presented targets. Human subjects produced changes in elbow torque as quickly and accurately as possible after the random presentation of 1 of 8 target torques. Information about the direction and amplitude of the target torque was provided either kinesthetically or visually. Responses to kinesthetic targets started at an average latency of 150 msec, and after an additional 159 msec, these responses became accurately graded according to target amplitude. Responses to visual targets started at an average latency of 250 msec, and after an additional 208 msec, these responses became accurately graded according to target amplitude. The accuracy of responses to kinesthetic targets was very similar to the accuracy of responses to visual targets. We conclude that the neural processing of kinesthetic information about target movement is sufficiently fast and accurate to guide typical motor activities.

Properties of postural adjustments associated with rapid arm movements.

1. We have examined rapid postural adjustments associated with a class of voluntary movements that disturb postural equilibrium. In the text that follows, these motor activities are termed associated postural adjustments and voluntary focal movements, respectively. Standing human subjects performed a variety of movement tasks on a hand-held manipulandum, resulting in disturbances to their postural equilibrium. The experimental use of movements that interact with the subject's environment in a relatively simple was permitted a more precise comparison of the postural adjustments with their associated focal movements. 2. Subjects either pulled or pushed on a stiff interface (the handle) or they responded in a predetermined way to handle perturbations. These activities were carried out with various degrees of steady-state postural stability. Prior to and during these movements, support surface and handle forces, electromyographic (EMG) signals, and body sway were monitored. 3. In addition to previously shown postural adjustments associated with reaction-time armed movements, we have demonstrated these postural activities occur in concept with segmental stretch reflexes and self-initiated (untriggered) movements. Postural adjustments were initiated shortly before all focal movements tested except the short-latency component of the biceps stretch reflex (25- to 30-ms latency). However, this reflex component was rarely elicited by handle perturbations in free-standing subjects; therefore, postural adjustments usually preceded any biceps activity under this condition. 4. By varying the degree of steady-state postural equilibrium, a reciprocal gain/threshold relationship between postural and focal components was documented, i.e., when stability was high, postural activity was reduced or absent and focal activity enhanced. Conversely, the biceps stretch reflex was difficult to elicit under any condition where the subjects was not fully supported in the direction of movement and reaction times of focal movements were prolonged. 5. Postural activities associated with focal movements were found to share a number of organizational properties with automatic postural adjustments to support surface movements. Specifically, the postural muscle synergies were equivalent in muscle composition, relative activation magnitudes, and relative temporal sequencing. Furthermore, both types of postural adjustments were highly specific in locus and magnitude to the quality of steady-state postural equilibrium (e.g., postural "set"). 6. A conceptual model is proposed that suggests one simple way in which the reciprocal influence of postural set on postural and focal movement components and their temporal sequencing might be accomplished. Furthermore, we propose in this model a common central organization of postural adjustments associated with focal movements and those elicited by support-surface movements.

Relation of automatic postural responses and reaction-time voluntary movements of human leg muscles.

This study contrasts the properties of compensatory postural adjustments in response to movements of the support surface with those of reaction-time voluntary movements in human subjects. Subjects stood upon a six degrees-of-freedom movable platform and performed tone and movement-triggered voluntary sways about the ankle joints both under conditions of postural stability and instability. These triggered movements could be executed as rapidly as postural adjustments to support surface perturbations (80-120 ms), but only when the former were well practiced, single-choice (direction) and were performed under conditions of postural stability. Evaluation of the properties of postural adjustments and reaction-time voluntary movements revealed a number of clear organizational differences between the two categories of movement, but most interesting was the finding that, when reaction-time movements were triggered by or at the onset of platform movement, the postural adjustments always occurred first. Only when subjects were given a tone trigger 50 ms in advance of platform movement were they able to execute the reaction-time movement first. We found that the dichotomous voluntary/reflexive classification of movements was not consistent with all of the identified properties of postural adjustments and reaction-time movements. Instead, we find a system which classifies movements by function, as either stabilizating or orientational adjustments, to be more useful. In the context of whole-body movement then, intentional focal components would be closely associated with others directed towards postural stabilization.

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.