Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

Cutaneous information processing differs with load type during isometric finger abduction

During submaximal isometric contraction, there are two different load types: maintenance of a constant limb angle while supporting an inertial load (position task) and maintenance of a constant force by pushing against a rigid restraint (force task). Previous studies demonstrated that performing the position task requires more proprioceptive information. The purpose of this study was to investigate whether there would be a difference in cutaneous information processing between the position and force tasks by assessing the gating effect, which is reduction of amplitude of somatosensory evoked potentials (SEPs), and cutaneomuscular reflex (CMR). Eighteen healthy adults participated in this study. They contracted their right first dorsal interosseous muscle by abducting their index finger to produce a constant force against a rigid restraint that was 20% maximum voluntary contraction (force task), or to maintain a target position corresponding to 10° abduction of the metacarpophalangeal joint while supporting a load equivalent to 20% maximum voluntary contraction (position task). During each task, electrical stimulation was applied to the digital nerves of the right index finger, and SEPs and CMR were recorded from C3' of the International 10-20 system and the right first dorsal interosseous muscle, respectively. Reduction of the amplitude of N33 component of SEPs was significantly larger during the force than position task. In addition, the E2 amplitude of CMR was significantly greater for the force than position task. These findings suggest that cutaneous information processing differs with load type during static muscle contraction.

Sex differences in fatigability after ischemic preconditioning of non-exercising limbs

Background

Ischemic preconditioning (IPC) is suggested to decrease fatigability in some individuals but not others. Sex differences in response to IPC may account for this variability and few studies systematically investigated the effects of IPC in men and women. The goal of this study was to determine if time to task failure, perception of pain, and neuromuscular mechanisms of fatigability were altered by IPC in men and women.

Methods

Ten women (29 ± 5 years old) and 10 men (28 ± 6 years old) performed isometric contractions with the plantar flexor muscles of the dominant leg at 20% of maximal voluntary contraction until task failure. We used a repeated measures design where each individual performed 3 randomized and counterbalanced test sessions: (A) IPC session, cuff inflation and deflation (5 min each repeated 3 times) performed before the exercise by inflating cuffs to the non-dominant leg and arm; (B) sham session, cuffs were inflated for a short period (1 min); and (C) control session, no cuffs were involved.

Results

Compared with control, IPC increased time to task failure in men (mean difference, 5 min; confidence interval (CI) of mean difference, 2.2; 7.8 min; P = 0.01) but not women (mean difference, − 0.6 min; CI of mean difference, − 3.5; 2.4 min; P = 0.51). In men, but not women, the IPC-induced increase in time to task failure was associated with lower response to pressure pain (r = − 0.79). IPC further exposed sex differences in arterial pressure during fatiguing contractions (session × sex: P < 0.05). Voluntary activation, estimated with the twitch interpolation technique, and presynaptic inhibition of leg Ia afferents were not altered after IPC for men and women. The tested variables were not altered with sham.

Conclusions

The ergogenic effect of IPC on time to task failure was observed only in men and it was associated with reductions in the perception of pain. This pilot data suggest the previously reported inter-individual variability in exercise-induced fatigability after IPC could be a consequence of the sex and individual response to pain.

Physiological tremor increases when skeletal muscle is shortened: implications for fusimotor control

KEY POINTS

In tonic, isometric, plantarflexion contractions, physiological tremor increases as the ankle joint becomes plantarflexed. Modulation of physiological tremor as a function of muscle stretch differs from that of the stretch reflex amplitude. Amplitude of physiological tremor may be altered as a function of reflex pathway gains. Healthy humans likely increase their γ-static fusimotor drive when muscles shorten. Quantification of physiological tremor by manipulation of joint angle may be a useful experimental probe of afferent gains and/or the integrity of automatic fusimotor control.

ABSTRACT

The involuntary force fluctuations associated with physiological (as distinct from pathological) tremor are an unavoidable component of human motor control. While the origins of physiological tremor are known to depend on muscle afferentation, it is possible that the mechanical properties of muscle-tendon systems also affect its generation, amplification and maintenance. In this paper, we investigated the dependence of physiological tremor on muscle length in healthy individuals. We measured physiological tremor during tonic, isometric plantarflexion torque at 30% of maximum at three ankle angles. The amplitude of physiological tremor increased as calf muscles shortened in contrast to the stretch reflex whose amplitude decreases as muscle shortens. We used a published closed-loop simulation model of afferented muscle to explore the mechanisms responsible for this behaviour. We demonstrate that changing muscle lengths does not suffice to explain our experimental findings. Rather, the model consistently required the modulation of  γ-static fusimotor drive to produce increases in physiological tremor with muscle shortening - while successfully replicating the concomitant reduction in stretch reflex amplitude. This need to control γ-static fusimotor drive explicitly as a function of muscle length has important implications. First, it permits the amplitudes of physiological tremor and stretch reflex to be decoupled. Second, it postulates neuromechanical interactions that require length-dependent γ drive modulation to be independent from α drive to the parent muscle. Lastly, it suggests that physiological tremor can be used as a simple, non-invasive measure of the afferent mechanisms underlying healthy motor function, and their disruption in neurological conditions.

Physiological mechanisms for stabilizing the limb when acting against physical constraints

Smooth physical interaction with our environment, such as when working with tools, requires adaptability to unpredictable perturbations that can be achieved through impedance control of multi-joint limbs. Modulation of arm stiffness can be achieved either increasing co-contraction of antagonistic muscles or by increasing the gain of spinal reflex loops. According to the "automatic gain scaling" principle, the spinal reflex gain, as measured via the H-reflex, scales with muscle activation. A previous experiment from our labs suggested, however, that reflex gains might instead be scaled to the force exerted by the limb, perhaps as a means to counteract destabilizing external forces. The goal of our experiment was to test whether force output, rather than the muscular activity per se, could be the critical factor determining reflex gain. Five subjects generated different levels of force at the wrist with or without assistance to dissociate applied force from agonist muscular activity. We recorded contact force, EMG and H-reflex response from a wrist flexor. We did not find a strict relationship between reflex gain and contact force but nor did we observe consistent modulation of reflex gain simply as a function of agonist muscle activity. These results are discussed in relation to the stability of the task constraints.

Maximal intermittent contractions of the first dorsal interosseous inhibits voluntary activation of the contralateral homologous muscle

The aim of this study was to investigate how maximal intermittent contractions for a hand muscle influence cortical and reflex activity, as well as the ability to voluntarily activate, the homologous muscle in the opposite limb. Twelve healthy subjects (age: 24 ± 3 years, all right hand dominant) performed maximal contractions of the dominant limb first dorsal interosseous (FDI), and activity of the contralateral FDI was examined in a series of experiments. Index finger abduction force, FDI EMG, motor evoked potentials and heteronomous reflexes were obtained from the contralateral limb during brief non-fatiguing contractions. The same measures, as well as the ability to voluntarily activate the contralateral FDI, were then assessed in an extended intermittent contraction protocol that elicited fatigue. Brief contractions under non-fatigued conditions increased index finger abduction force, FDI EMG, and motor evoked potential amplitude of the contralateral limb. However, when intermittent maximal contractions were continued until fatigue, there was an inability to produce maximal force with the contralateral limb (~30%) which was coupled to a decrease in the level of voluntary activation (~20%). These declines were present without changes in reflex activity, and regardless of whether cortical or motor point stimulation was used to assess voluntary activation. It is concluded that performing maximal intermittent contractions with a single limb causes an inability of the CNS to maximally drive the homologous muscle of the contralateral limb. This was, in part, mediated by mechanisms that involve the motor cortex ipsilateral to the contracting limb.

Motor unit activity in biceps brachii of left-handed humans during sustained contractions with two load types

The purpose of the study was to compare the discharge characteristics of single motor units during sustained isometric contractions that required either force or position control in left-handed individuals. The target force for the two sustained contractions (24.9 ± 10.5% maximal force) was identical for each biceps brachii motor unit (n = 32) and set at 4.7 ± 2.0% of maximal voluntary contraction (MVC) force above its recruitment threshold (range: 0.5-41.2% MVC force). The contractions were not sustained to task failure, but the duration (range: 60-330 s) was identical for each motor unit and the decline in MVC force immediately after the sustained contractions was similar for the two tasks (force: 11.1% ± 13.7%; position: 11.6% ± 9.9%). Despite a greater increase in the rating of perceived exertion during the position task (task × time interaction, P < 0.006), the amplitude of the surface-recorded electromyogram for the agonist and antagonist muscles increased similarly during the two tasks. Nonetheless, mean discharge rate of the biceps brachii motor units declined more during the position task (task × time interaction, P < 0.01) and the variability in discharge times (coefficient of variation for interspike interval) increased only during the position task (task × time interaction, P < 0.008). When combined with the results of an identical study on right-handers (Mottram CJ, Jakobi JM, Semmler JG, Enoka RM. J Neurophysiol 93: 1381-1392, 2005), the findings indicate that handedness does not influence the adjustments in biceps brachii motor unit activity during sustained submaximal contractions requiring either force or position control.

D1 and D2 Inhibitions of the Soleus H-Reflex Are Differentially Modulated during Plantarflexion Force and Position Tasks

Presynaptic inhibition (PSI) has been shown to modulate several neuronal pathways of functional relevance by selectively gating the connections between sensory inputs and spinal motoneurons, thereby regulating the contribution of the stretch reflex circuitry to the ongoing motor activity. In this study, we investigated whether a differential regulation of Ia afferent inflow by PSI may be associated with the performance of two types of plantarflexion sensoriomotor tasks. The subjects (in a seated position) controlled either: 1) the force level exerted by the foot against a rigid restraint (force task, FT); or 2) the angular position of the ankle when sustaining inertial loads (position task, PT) that required the same level of muscle activation observed in FT. Subjects were instructed to maintain their force/position at target levels set at ~10% of maximum isometric voluntary contraction for FT and 90° for PT, while visual feedback of the corresponding force/position signals were provided. Unconditioned H-reflexes (i.e. control reflexes) and H-reflexes conditioned by electrical pulses applied to the common peroneal nerve with conditioning-to-test intervals of 21 ms and 100 ms (corresponding to D1 and D2 inhibitions, respectively) were evoked in a random fashion. A significant main effect for the type of the motor task (FT vs PT) (p = 0.005, η²_p = 0.603) indicated that PTs were undertaken with lower levels of Ia PSI converging onto the soleus motoneuron pool. Additionally, a significant interaction between the type of inhibition (D1 vs D2) and the type of motor task (FT vs PT) (p = 0.038, η²_p = 0.395) indicated that D1 inhibition was associated with a significant reduction in PSI levels from TF to TP (p = 0.001, η²_p = 0.731), whereas no significant difference between the tasks was observed for D2 inhibition (p = 0.078, η²_p = 0.305). These results suggest that D1 and D2 inhibitions of the soleus H-reflex are differentially modulated during the performance of plantarflexion FT and PT. The reduced level of ongoing PSI during PT suggests that, in comparison to FT, there is a larger reliance on inputs from muscle spindles primary afferents when the neuromuscular system is required to maintain position-controlled plantarflexion contractions.

Learning to push and learning to move: the adaptive control of contact forces

To be successful at manipulating objects one needs to apply simultaneously well controlled movements and contact forces. We present a computational theory of how the brain may successfully generate a vast spectrum of interactive behaviors by combining two independent processes. One process is competent to control movements in free space and the other is competent to control contact forces against rigid constraints. Free space and rigid constraints are singularities at the boundaries of a continuum of mechanical impedance. Within this continuum, forces and motions occur in “compatible pairs” connected by the equations of Newtonian dynamics. The force applied to an object determines its motion. Conversely, inverse dynamics determine a unique force trajectory from a movement trajectory. In this perspective, we describe motor learning as a process leading to the discovery of compatible force/motion pairs. The learned compatible pairs constitute a local representation of the environment's mechanics. Experiments on force field adaptation have already provided us with evidence that the brain is able to predict and compensate the forces encountered when one is attempting to generate a motion. Here, we tested the theory in the dual case, i.e., when one attempts at applying a desired contact force against a simulated rigid surface. If the surface becomes unexpectedly compliant, the contact point moves as a function of the applied force and this causes the applied force to deviate from its desired value. We found that, through repeated attempts at generating the desired contact force, subjects discovered the unique compatible hand motion. When, after learning, the rigid contact was unexpectedly restored, subjects displayed after effects of learning, consistent with the concurrent operation of a motion control system and a force control system. Together, theory and experiment support a new and broader view of modularity in the coordinated control of forces and motions.

Sensorimotor modulation differs with load type during constant finger force or position

During submaximal isometric contraction, there are two different load types: production of a constant force against a rigid restraint (force task), and maintenance of position against a constant load (position task). Previous studies reported that the time to task failure during a fatigue task was twice as long in the force task compared with the position task. Sensory feedback processing may contribute to these differences. The purpose of the current study was to determine the influence of load types during static muscle contraction tasks on the gating effect, i.e., attenuation of somatosensory-evoked potentials (SEPs) and the cortical silent period (cSP). Ten healthy subjects contracted their right first dorsal interosseus muscle by abducting their index finger for 90 s, to produce a constant force against a rigid restraint that was 20% of the maximum voluntary contraction (force task), or to maintain a constant position with 10° abduction of the metacarpophalangeal joint against the same load (position task). Somatosensory evoked potentials (SEPs) were recorded from C3' by stimulating either the right ulnar or median nerve at the wrist while maintaining contraction. The cortical silent period (cSP) was also elicited by transcranial magnetic stimulation. Reduction of the amplitude of the P45 component of SEPs was significantly larger during the position task than during the force task and under control rest conditions when the ulnar nerve, but not the median nerve, was stimulated. The position task had a significantly shorter cSP duration than the force task. These results suggest the need for more proprioceptive information during the position task than the force task. The shorter duration of the cSP during the position task may be attributable to larger amplitude of heteronymous short latency reflexes. Sensorimotor modulations may differ with load type during constant finger force or position tasks.

Bilateral tremor responses to unilateral loading and fatiguing muscle contractions

Although physiological tremor has been extensively studied within a single limb, tremor relationships between limbs are not well understood. Early investigations proposed that tremor in each limb is driven by CNS oscillators operating in parallel. However, recent evidence suggests that tremor in both limbs arises from shared neural inputs and is more likely to be observed under perturbed conditions. In the present study, postural tremor about the elbow joint and elbow flexor EMG activity were examined on both sides of the body in response to unilateral loading and fatiguing muscle contractions. Applying loads of 0.5, 1.0, 1.5, and 3.0 kg to a single limb increased tremor and muscle activity in the loaded limb but did not affect the unloaded limb, indicating that manipulating the inertial characteristics of a limb does not evoke bilateral tremor responses. In contrast, maximal-effort unilateral isometric contractions resulted in increased tremor and muscle activity in both the active limb and the nonactive limb without any changes in between-limb tremor or muscle coupling. When unilateral contractions were repeated intermittently, to the extent that maximum torque generation about the elbow joint declined by 50%, different tremor profiles were observed in each limb. Specifically, unilateral fatigue altered coupling between limbs and generated a bilateral response such that tremor and brachioradialis EMG decreased for the fatigued limb and increased in the contralateral nonfatigued limb. Our results demonstrate that activity in the nonactive limb may be due to a "spillover" effect rather than directly coupled neural output to both arms and that between-limb coupling for tremor and muscle activity is only altered under considerably perturbed conditions, such as fatigue-inducing contractions.

PET/CT imaging of age- and task-associated differences in muscle activity during fatiguing contractions

The study compared positron emission tomography/computed tomography (PET/CT) of [(18)F]-2-fluoro-2-deoxy-D-glucose ([(18)F]-FDG) uptake by skeletal muscles and the amount of muscle activity as indicated by surface electromyographic (EMG) recordings when young and old men performed fatiguing isometric contractions that required either force or position control. EMG signals were recorded from thigh muscles of six young men (26 ± 6 yr) and six old men (77 ± 6 yr) during fatiguing contractions with the knee extensors. PET/CT scans were performed immediately after task failure. Glucose uptake in 24 leg muscles, quantified as standardized uptake values, was greater for the old men after the force task and differed across tasks for the young men (force, 0.64 ± 0.3 g/ml; position, 0.73 ± 0.3 g/ml), but not the old men (force, 0.84 ± 0.3 g/ml; position, 0.79 ± 0.26 g/ml) (age × task interaction; P < 0.001). In contrast, the rate of increase in EMG amplitude for the agonist muscles was greater for the young men during the two contractions and there was no difference for either group of subjects in the rate of increase in EMG amplitude across the two tasks. The imaging estimates of glucose uptake indicated age- and task-dependent differences in the spatial distribution of [(18)F]-FDG uptake by skeletal muscles during fatiguing contractions. The findings demonstrate that PET/CT imaging of [(18)F]-FDG uptake, but not surface EMG recordings, detected the modulation of muscle activity across the fatiguing tasks by the young men but not the old men.

Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

The purpose of the study was to compare the discharge characteristics of biceps brachii motor units of young and old adults when they performed steady, submaximal contractions while the arm supported different inertial loads. Young (28 ± 4 yr; n = 16) and old (75 ± 4 yr; n = 14) adults performed steady contractions with the elbow flexors at target forces set at either small (11.7 ± 4.4% maximum) or large (17.8 ± 6.5% maximum) differences below the recruitment threshold force of the motor unit (n = 40). The task was to maintain an elbow angle at 1.57 rad until the motor unit was recruited and discharged action potentials for ∼120 s. Time to recruitment was longer for the larger target force difference (187 ± 227 s vs. 23 ± 46 s, P < 0.001). Once recruited, motor units discharged action potentials either repetitively or intermittently, with a greater proportion of motor units exhibiting the repetitive pattern for old adults. Discharge rate at recruitment and during the steady contraction was similar for the two target force differences for old adults but was greater for the small target force difference for young adults. Discharge variability was similar at recruitment for the two age groups but less for the old adults during the steady contraction. The greatest difference between the present results and those reported previously when the arm pulled against a rigid restraint was that old adults modulated discharge rate less than young adults across the two contraction intensities for both load types.

Effects of fatigue on synergies in a hierarchical system

We investigated the effect of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of one of the hands on performance in MVC and accurate cyclic force production tasks in right-handed subjects. Based on earlier studies, we hypothesized that fatigue would produce an increase in the indices of force-stabilizing synergies in both hands as well as between the hands in two-hand tasks. Synergies were defined as co-varied adjustments of commands to fingers (modes) across cycles that stabilized total force. Fatigue caused a significant reduction in the MVC of the exercised as well as the non-exercised hand. Indices of finger enslaving (lack of individuation) increased with fatigue in both hands, although the increase was significant in the exercised hand only. In contrast to the significant effects of fatigue on MVC forces performed by the non-exercised hand, there were no comparable transfer effects on the root mean square errors during accurate force production. During one-hand tasks, both hands showed high indices of force-stabilizing synergies. These indices were larger in the left hand. Fatigue led to a general increase in synergy indices. Exercise by the left hand had stronger effects on synergy indices seen in both hands. Exercise by the right hand showed ipsilateral effects only. Smaller effects of fatigue were observed on accuracy of performance of the force-down segments of the force cycles compared to the force-up segments. For the bimanual tasks, synergies were analyzed at two hierarchical levels, two-hand (four-finger) and within-a-hand (two-finger). An increase in the synergy index with fatigue was observed at the lower (two-finger) level of the hierarchy only. We interpret the lack of effects of fatigue at the upper (two-hand) level as a consequence of a trade-off between synergies at different levels of the hierarchy. The differences between the hands are discussed within the dynamic dominance hypothesis.

Upper limb position control in fibromyalgia

BACKGROUND

Motor problems are reported by patients with fibromyalgia (FM). However, the mechanisms leading to alterations in motor performance are not well understood. In this study, upper limb position control during sustained isometric contractions was investigated in patients with FM and in healthy controls (HCs).

METHODS

Fifteen female FM patients and 13 HCs were asked to keep a constant upper limb position during sustained elbow flexion and shoulder abduction, respectively. Subjects received real-time visual feedback on limb position and both tasks were performed unloaded and while supporting loads (1, 2, and 3 kg). Accelerations of the dominant upper limb were recorded, with variance (SD of mean position) and power spectrum analysis used to characterize limb position control. Normalized power of the acceleration signal was extracted for three frequency bands: 1-3 Hz, 4-7 Hz, and 8-12 Hz.

RESULTS

Variance increased with load in both tasks (P < 0.001) but did not differ significantly between patients and HCs (P > 0.17). Power spectrum analysis showed that the FM patients had a higher proportion of normalized power in the 1-3 Hz band, and a lower proportion of normalized power in the 8-12 Hz band compared to HCs (P < 0.05). The results were consistent for all load conditions and for both elbow flexion and shoulder abduction.

CONCLUSION

FM patients exhibit an altered neuromuscular strategy for upper limb position control compared to HCs. The predominance of low-frequency limb oscillations among FM patients may indicate a sensory deficit.

Task- and time-dependent modulation of Ia presynaptic inhibition during fatiguing contractions performed by humans

Presynaptic modulation of Ia afferents converging onto the motor neuron pool of the extensor carpi radialis (ECR) was compared during contractions (20% of maximal force) sustained to failure as subjects controlled either the angular position of the wrist while supporting an inertial load (position task) or exerted an equivalent force against a rigid restraint (force task). Test Hoffmann (H) reflexes were evoked in the ECR by stimulating the radial nerve above the elbow. Conditioned H reflexes were obtained by stimulating either the median nerve above the elbow or at the wrist (palmar branch) to assess presynaptic inhibition of homonymous (D1 inhibition) and heteronymous Ia afferents (heteronymous Ia facilitation), respectively. The position task was briefer than the force task (P = 0.001), although the maximal voluntary force and electromyograph for ECR declined similarly at failure for both tasks. Changes in the amplitude of the conditioned H reflex were positively correlated between the two conditioning methods (P = 0.02) and differed between the two tasks (P < 0.05). The amplitude of the conditioned H reflex during the position task first increased (129 ± 20.5% of the initial value, P < 0.001) before returning to its initial value (P = 0.22), whereas it increased progressively during the force task to reach 122 ± 17.4% of the initial value at failure (P < 0.001). Moreover, changes in conditioned H reflexes were associated with the time to task failure and force fluctuations. The results suggest a task- and time-dependent modulation of presynaptic inhibition of Ia afferents during fatiguing contractions.

Voluntary breathing influences corticospinal excitability of nonrespiratory finger muscles

The present study aimed to investigate neurophysiologic mechanisms mediating the newly discovered phenomenon of respiratory-motor interactions and to explore its potential clinical application for motor recovery. First, young and healthy subjects were instructed to breathe normally (NORM); to exhale (OUT) or inhale (IN) as fast as possible in a self-paced manner; or to voluntarily hold breath (HOLD). In experiment 1 (n = 14), transcranial magnetic stimulation (TMS) was applied during 10% maximal voluntary contraction (MVC) finger flexion force production or at rest. The motor-evoked potentials (MEPs) were recorded from flexor digitorum superficialis (FDS), extensor digitorum communis (EDC), and abductor digiti minimi (ADM) muscles. Similarly, in experiment 2 (n = 11), electrical stimulation (ES) was applied to FDS or EDC during the described four breathing conditions while subjects maintained 10%MVC of finger flexion or extension and at rest. In the exploratory clinical experiments (experiment 3), four patients with chronic neurological disorders (three strokes, one traumatic brain injury) received a 30-min session of breathing-controlled ES to the impaired EDC. In experiment 1, the EDC MEP magnitudes increased significantly during IN and OUT at both 10%MVC and rest; the FDS MEPs were enhanced only at 10%MVC, whereas the ADM MEP increased only during OUT, compared with NORM for both at rest and 10%MVC. No difference was found between NORM and HOLD for all three muscles. In experiment 2, when FDS was stimulated, force response was enhanced during both IN and OUT, but only at 10%MVC. When EDC was stimulated, force response increased at both 10%MVC and rest, only during IN, but not OUT. The averaged response latency was 83 ms for the finger extensors and 79 ms for the finger flexors. After a 30-min intervention of ES to EDC triggered by forced inspiration in experiment 3, we observed a significant reduction in finger flexor spasticity. The spasticity reduction lasted for ≥ 4 wk in all four patients. TMS and ES data, collectively, support the phenomenon that there is an overall respiration-related enhancement on the motor system, with a strong inspiration-finger extension coupling during voluntary breathing. As such, breathing-controlled electrical stimulation (i.e., stimulation to finger extensors delivered during the voluntary inspiratory phase) could be applied for enhancing finger extension strength and finger flexor spasticity reduction in poststroke patients.

Unraveling the neurophysiology of muscle fatigue

Despite 100years of research since the seminal work of Angelo Mosso (1846-1910), our understanding of the interactions between the nervous system and muscle during the performance of fatiguing contractions remains rather rudimentary. Although the nervous system simply needs to provide an activation signal that will elicit the net muscle torque required for a prescribed action, changes in the number and diversity of synaptic inputs that must be integrated by the spinal motor neurons to accommodate the changes in the force-producing capabilities of the muscle fibers complicate the process of generating the requisite activation signal. This brief review examines two ways in which the activation signal can be compromised during sustained contractions and thereby contribute to the rate at which the muscles fatigue. These examples provide insight on the types of adjustments that occur in the nervous system during fatiguing contractions, but emphasize that much remains to be learned about the physiological processes that contribute to the phenomenon known as muscle fatigue.

Lack of hypertonia in thumb muscles after stroke

Despite the importance of the thumb to hand function, little is known about the origins of thumb impairment poststroke. Accordingly, the primary purpose of this study was to assess whether thumb flexors have heightened stretch reflexes (SRs) following stroke-induced hand impairment. The secondary purpose was to compare SR characteristics of thumb flexors in relation to those of finger flexors since it is unclear whether SR properties of both muscle groups are similarly affected poststroke. Stretch reflexes in thumb and finger flexors were assessed at rest on the paretic side in each of 12 individuals with chronic, severe, stroke-induced hand impairment and in the dominant thumb in each of eight control subjects also at rest. Muscle activity and passive joint flexion torques were measured during imposed slow (SS) and fast stretches (FS) of the flexors that span the metacarpophalangeal joints. Putative spasticity was then quantified in terms of the peak difference between FS and SS joint torques and electromyographic changes. For both the hemiparetic and control groups, the mean normalized peak torque differences (PTDs) measured in thumb flexors were statistically indistinguishable (P = 0.57). In both groups, flexor muscles were primarily unresponsive to rapid stretching. For 10 of 12 hemiparetic subjects, PTDs in thumb flexors were less than those in finger flexors (P = 0.03). Paretic finger flexor muscle reflex activity was consistently elicited during rapid stretching. These results may reflect an important difference between thumb and finger flexors relating to properties of the involved muscle afferents and spinal motoneurons.

Somatosensory comparison during haptic tracing

Active sensing involves memory retrieval and updating as well as mechanisms that trigger corrections to the ongoing exploratory movement. The present study examined this process in a task where human subjects moved the index fingertip clockwise around the circumference of a virtual sphere created by a robotic device. The fingertip pressed into the sphere during the movement, and the subjects were to report slight differences in sphere size (or surface curvature), which occurred from trial to trial. During each 2- to 3-s trial, subjects gradually adjusted their speed and pressure according to the current surface curvature, achieving a consistent level of contact force in the last half of the exploration. The results demonstrate that subjects were gradually accumulating haptic information about curvature and, at the same time, gradually changing the motor commands for the movement. When subjects encountered an unexpected transition in curvature (from circular to flat), they reacted by abruptly decreasing contact force at a latency of about 50 ms. This short latency indicates that spinally mediated corrections are engaged during this task. The results support the hypothesis that during haptic exploration, the neural comparison between expected and actual somatosensory feedback takes places at multiple levels, including the spinal cord.

Presynaptic modulation of Ia afferents in young and old adults when performing force and position control

The present work investigated presynaptic modulation of Ia afferents in the extensor carpi radialis (ECR) when young and old adults exerted a wrist extension force either to support an inertial load (position control) or to achieve an equivalent constant torque against a rigid restraint (force control) at 5, 10, and 15% of the maximal force. H reflexes were evoked in the ECR by stimulating the radial nerve above the elbow. A conditioning stimulus was applied to the median nerve above the elbow to assess presynaptic inhibition of homonymous Ia afferents (D1 inhibition) or at the wrist (palmar branch) to assess the ongoing presynaptic inhibition of heteronymous Ia afferents that converge onto the ECR motor neuron pool (heteronymous Ia facilitation). The young adults had less D1 inhibition and greater heteronymous Ia facilitation during the position task (79 and 132.1%, respectively) compared with the force task (69.1 and 115.1%, respectively, P < 0.05). In contrast, the old adults exhibited no difference between the two tasks for either D1 inhibition ( approximately 72%) or heteronymous Ia facilitation ( approximately 114%). Contraction intensity did not influence the amount of D1 inhibition or heteronymous Ia facilitation for either group of subjects. The amount of antagonist coactivation was similar between tasks for young adults, whereas it was greater in the position task for old adults (P = 0.02). These data indicate that in contrast to young adults, old adults did not modulate presynaptic inhibition of Ia afferents when controlling the position of a compliant load but rather increased coactivation of the antagonist muscle.

Interactions between limb and environmental mechanics influence stretch reflex sensitivity in the human arm

Stretch reflexes contribute to arm impedance and longer-latency stretch reflexes exhibit increased sensitivity during interactions with compliant or unstable environments. This increased sensitivity is consistent with a regulation of arm impedance to compensate for decreased stability of the environment, but the specificity of this modulation has yet to be investigated. Many tasks, such as tool use, compromise arm stability along specific directions, and stretch reflexes tuned to those directions could present an efficient mechanism for regulating arm impedance in a task-appropriate manner. To be effective, such tuning should adapt not only to the mechanical properties of the environment but to those properties in relation to the arm, which also has directionally specific mechanical properties. The purpose of this study was to investigate the specificity of stretch reflex modulation during interactions with mechanical environments that challenge arm stability. The tested environments were unstable, having the characteristics of a negative stiffness spring. These were either aligned or orthogonal to the direction of maximal endpoint stiffness for each subject. Our results demonstrate preferential increases in reflexes, elicited within 50-100 ms of perturbation onset, to perturbations applied specifically along the direction of the destabilizing environments. This increase occurred only when the magnitude of the environmental instability exceeded endpoint stiffness along the same direction. These results are consistent with task-specific reflex modulation tuned to the mechanical properties of the environment relative to those of the human arm. They demonstrate a highly adaptable, involuntary mechanism that may be used to modulate limb impedance along specific directions.

Muscle inhibition following tendon stimulation is reduced in chronic stroke

OBJECTIVE

Electrical tendon stimulation elicits reflex inhibition in the homonymous muscle that is thought to be mediated by group III afferents. The study goals were to: evaluate group III-mediated reflex inhibition in people with post-stroke hemiparesis; determine the presence of heteronymous group III pathways; investigate the relevance of reflex inhibition to arm function.

METHODS

Reflex responses were recorded in wrist, elbow, and shoulder muscles following stimulation of the extensor carpi radialis (ECR) tendon in 16 people with post-stroke hemiparesis and 16 control subjects. In control subjects, reflex inhibition also was compared between static and dynamic muscle activation.

RESULTS

Reflex inhibition following ECR tendon stimulation was present in heteronymous muscles of most, but not all, stroke and control subjects. The level of reflex inhibition was significantly reduced in stroke subjects. In the controls, reflex inhibition was greater during dynamic activation of elbow muscles compared to static activation.

CONCLUSIONS

Evidence that reflex inhibition projects to heteronymous muscles and is modulated during movement suggests a role for the reflex in multi-joint coordination. The reflex is impaired in post-stroke hemiparesis.

SIGNIFICANCE

Abnormalities in the regulation of group III-mediated muscle inhibition in the stroke population may contribute to impaired muscle activation patterns during movement.

Influence of load type on presynaptic modulation of Ia afferent input onto two synergist muscles

The present work was designed to investigate presynaptic modulation of Ia afferents in the extensor (ECR) and flexor carpi radialis (FCR) when the two muscles acted as synergists during radial deviation to either support an inertial load (position task) or exert an equivalent constant torque against a rigid restraint (force task). H reflexes were evoked in the ECR and FCR by stimulating at the elbow level (1-ms duration) the radial and median nerves, respectively. Conditioning stimulation was applied to the median and radial nerves at the elbow level to assess presynaptic inhibition of homonymous Ia afferent input (D1 inhibition) from the ECR and FCR, respectively. The ongoing presynaptic inhibition of heteronymous Ia afferents that converges onto ECR and FCR motor neuron pools (heteronymous Ia facilitation) was assessed by stimulating the median nerve at the wrist level (palmar branch) prior to the stimulus applied over the radial or median nerve. The heteronymous monosynaptic Ia facilitation was greater (P < 0.05) during the position task (ECR 121%; FCR 147%) compared with the force task (ECR 115%; FCR 132%), and was paralleled by the depression of D1 inhibition (P < 0.05) during the position task (ECR 75.4%; FCR 79.0%) compared with force task (ECR 58.7%; FCR 58.8%). These data indicate that Ia presynaptic inhibition is reduced during the position task relative to the force task. Such differential modulation of Ia afferent input onto the motor neuron pool likely reflects the requirement to heighten reflex responsiveness during the unstable task of maintaining limb position.

Load type influences motor unit recruitment in biceps brachii during a sustained contraction

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task (P=0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% (P=0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (-16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.

The separate neural control of hand movements and contact forces

To manipulate an object, we must simultaneously control the contact forces exerted on the object and the movements of our hand. Two alternative views for manipulation have been proposed: one in which motions and contact forces are represented and controlled by separate neural processes, and one in which motions and forces are controlled jointly, by a single process. To evaluate these alternatives, we designed three tasks in which subjects maintained a specified contact force while their hand was moved by a robotic manipulandum. The prescribed contact force and hand motions were selected in each task to induce the subject to attain one of three goals: (1) exerting a regulated contact force, (2) tracking the motion of the manipulandum, and (3) attaining both force and motion goals concurrently. By comparing subjects' performances in these three tasks, we found that behavior was captured by the summed actions of two independent control systems: one applying the desired force, and the other guiding the hand along the predicted path of the manipulandum. Furthermore, the application of transcranial magnetic stimulation impulses to the posterior parietal cortex selectively disrupted the control of motion but did not affect the regulation of static contact force. Together, these findings are consistent with the view that manipulation of objects is performed by independent brain control of hand motions and interaction forces.

Spinal mechanisms contribute to differences in the time to failure of submaximal fatiguing contractions performed with different loads

This study compared the mechanisms that limit the time to failure of a sustained submaximal contraction at 20% of maximum when the elbow flexors either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). The surface electromyogram (EMG), the motor-evoked potential (MEP) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus were recorded in biceps brachii during the two tasks. Although the time to failure for the position task was only 44% of that for the force task, the rate of increase of the average EMG (aEMG; % initial MVC) and MEP area (% Mmax) did not differ significantly during the two tasks. At task failure, however, the increases in normalized aEMG and MEP area were significantly (P < 0.05) greater for the force task (36.4 and 219.9%) than for the position task (22.4 and 141.7%). Furthermore, the superimposed mechanical twitch (% initial MVC), evoked by TMS during a brief MVC of the elbow flexors immediately after task failure, was increased similarly in both tasks. Although the normalized H-reflex area (% Mmax) decreased during the two fatiguing contractions, the reduction was more rapid and greater during the position task (59.8%) compared with the force task (34.7%). Taken together, the results suggest that spinal mechanisms were a major determinant of the briefer time to failure for the position task.

Muscle fatigue: what, why and how it influences muscle function

Much is known about the physiological impairments that can cause muscle fatigue. It is known that fatigue can be caused by many different mechanisms, ranging from the accumulation of metabolites within muscle fibres to the generation of an inadequate motor command in the motor cortex, and that there is no global mechanism responsible for muscle fatigue. Rather, the mechanisms that cause fatigue are specific to the task being performed. The development of muscle fatigue is typically quantified as a decline in the maximal force or power capacity of muscle, which means that submaximal contractions can be sustained after the onset of muscle fatigue. There is even evidence that the duration of some sustained tasks is not limited by fatigue of the principal muscles. Here we review experimental approaches that focus on identifying the mechanisms that limit task failure rather than those that cause muscle fatigue. Selected comparisons of tasks, groups of individuals and interventions with the task-failure approach can provide insight into the rate-limiting adjustments that constrain muscle function during fatiguing contractions.

A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions

Magnetic and electrical stimulation at different levels of the neuraxis show that supraspinal and spinal factors limit force production in maximal isometric efforts ("central fatigue"). In sustained maximal contractions, motoneurons become less responsive to synaptic input and descending drive becomes suboptimal. Exercise-induced activity in group III and IV muscle afferents acts supraspinally to limit motor cortical output but does not alter motor cortical responses to transcranial magnetic stimulation. "Central" and "peripheral" fatigue develop more slowly during submaximal exercise. In sustained submaximal contractions, central fatigue occurs in brief maximal efforts even with a weak ongoing contraction (<15% maximum). The presence of central fatigue when much of the available motor pathway is not engaged suggests that afferent inputs contribute to reduce voluntary activation. Small-diameter muscle afferents are likely to be activated by local activity even in sustained weak contractions. During such contractions, it is difficult to measure central fatigue, which is best demonstrated in maximal efforts. To show central fatigue in submaximal contractions, changes in motor unit firing and force output need to be characterized simultaneously. Increasing central drive recruits new motor units, but the way this occurs is likely to depend on properties of the motoneurons and the inputs they receive in the task. It is unclear whether such factors impair force production for a set level of descending drive and thus represent central fatigue. The best indication that central fatigue is important during submaximal tasks is the disproportionate increase in subjects' perceived effort when maintaining a low target force.