Joint sense, muscle sense, and their combination as position sense, measured at the distal interphalangeal joint of the middle finger

Proprioception: An evidence-based review

BACKGROUND

Proprioception is an essential sensory function of the body. Proprioception is defined as one's awareness of their body's position and movement through space. It contributes to both the conscious and unconscious awareness of limb and trunk position and movement. The purpose of this review is to provide an evidence-based review of proprioception and conditions that interfere with proprioceptive acuity.

PURPOSE

The purpose of this review is to provide an evidence-based review of proprioception and conditions that interfere with proprioceptive acuity.

STUDY DESIGN

This narrative literature review examines studies that determine proprioceptive systems and their implication for rehabilitation.

METHODS

Relevant study data were extracted as part of this review.

RESULTS

Types of proprioceptive interventions can include active or passive movement training, somatosensory stimulation training, force reproduction, and somatosensory discrimination training. Joint position sense error is the most widely used objective measure of proprioception.

CONCLUSIONS

Therapists should consider using a standardized measure to ascertain proprioceptive deficits in their patients following upper extremity injury or disease to determine the deficits and measure change. There are a variety of interventions that can be used in hand rehabilitation to restore proprioceptive acuity, and active movement interventions have been found to be the most effective.

Misencoding of ankle joint angle control system via cutaneous afferents reflex pathway in chronic ankle instability

This study aimed to investigate how the cutaneous reflexes in the peroneus longus (PL) muscle are affected by changing the ankle joint position in patients with chronic ankle instability (CAI). We also investigated the correlation between the degree of reflex modulation and angle position sense of the ankle joint. The participants were 19 patients with CAI and 20 age-matched controls. Cutaneous reflexes were elicited by applying non-noxious electrical stimulation to the sural nerve at the ankle joint in the neutral standing and eversion/inversion standing positions. The suppressive middle latency cutaneous reflex (MLR; ~ 70-120 ms) and angle position sense of the ankle joint were assessed. During neutral standing, the gain of the suppressive MLR was more prominent in the CAI patients than in controls, although no significant difference was seen during 30° inversion standing. In addition, the ratios of the suppressive MLR and background electromyography in a neutral position were significantly larger than those at the 15°, 25°, and 30° inversion positions in CAI patients. No such difference was seen in control individuals. Furthermore, the correlations between reflex modulation degree and position sense error were quite different in CAI patients compared to controls. These findings suggest that the sensory-motor system was deteriorated in CAI patients due to changes in the PL cutaneous reflex pathway excitability and position sense of the ankle joint.

Proprioceptive sensitivity to imposed finger deflections

Hand proprioception, the sense of the posture and movements of the wrist and digits, is critical to dexterous manual behavior and to stereognosis, the ability to sense the three-dimensional structure of objects held in the hand. To better understand this sensory modality and its role in hand function, we sought to characterize the acuity with which the postures and movements of finger joints are sensed. To this end, we measured the ability of human subjects to discriminate changes in posture and speed around the three joints of the index finger. In these experiments, we isolated the sensory component by imposing the postures on an otherwise still hand, to complement other studies in which subjects made judgments on actively achieved postures. We found that subjects could reliably sense 12-16% changes in joint angle and 18-32% changes in joint speed. Furthermore, the acuity for posture and speed was comparable across the three joints of the finger. Finally, task performance was unaffected by the presence of a vibratory stimulus, calling into question the role of cutaneous cues in hand proprioception.NEW & NOTEWORTHY Manual dexterity and stereognosis are supported by two exquisite sensory systems, namely touch and proprioception. Here, we measure the sensitivity of hand proprioception and show that humans can sense the posture and movements of the fingers with great accuracy. We also show that application of a skin vibration does not impair sensitivity, suggesting that proprioceptive acuity relies primarily on receptors in the muscles (and possibly tendons) rather than the skin.

Proprioception, the regulator of motor function

In animals, proper locomotion is crucial to find mates and foods and avoid predators or dangers. Multiple sensory systems detect external and internal cues and integrate them to modulate motor outputs. Proprioception is the internal sense of body position, and proprioceptive control of locomotion is essential to generate and maintain precise patterns of movement or gaits. This proprioceptive feedback system is conserved in many animal species and is mediated by stretch-sensitive receptors called proprioceptors. Recent studies have identified multiple proprioceptive neurons and proprioceptors and their roles in the locomotion of various model organisms. In this review we describe molecular and neuronal mechanisms underlying proprioceptive feedback systems in C. elegans, Drosophila, and mice.

Interjoint coupling of position sense reflects sensory contributions of biarticular muscles

Perception of limb position and motion combines sensory information from spindles in muscles that span one joint (monoarticulars) and two joints (biarticulars). This anatomical organization should create interactions in estimating limb position. We developed two models, one with only monoarticulars and one with both monoarticulars and biarticulars, to explore how biarticulars influence estimates of arm position in hand (x, y) and joint (shoulder, elbow) coordinates. In hand coordinates, both models predicted larger medial-lateral than proximal-distal errors, although the model with both muscle groups predicted that biarticulars would reduce this bias. In contrast, the two models made significantly different predictions in joint coordinates. The model with only monoarticulars predicted that errors would be uniformly distributed because estimates of angles at each joint would be independent. In contrast, the model that included biarticulars predicted that errors would be coupled between the two joints, resulting in smaller errors for combinations of flexion or extension at both joints and larger errors for combinations of flexion at one joint and extension at the other joint. We also carried out two experiments to examine errors made by human subjects during an arm position matching task in which a robot passively moved one arm to different positions and the subjects moved their other arm to mirror-match each position. Errors in hand coordinates were similar to those predicted by both models. Critically, however, errors in joint coordinates were only similar to those predicted by the model with monoarticulars and biarticulars. These results highlight how biarticulars influence perceptual estimates of limb position by helping to minimize medial-lateral errors.NEW & NOTEWORTHY It is unclear how sensory information from muscle spindles located within muscles spanning multiple joints influences perception of body position and motion. We address this issue by comparing errors in estimating limb position made by human subjects with predicted errors made by two musculoskeletal models, one with only monoarticulars and one with both monoarticulars and biarticulars. We provide evidence that biarticulars produce coupling of errors between joints, which help to reduce errors.

A closed-loop hand prosthesis with simultaneous intraneural tactile and position feedback

Current myoelectric prostheses allow transradial amputees to regain voluntary motor control of their artificial limb by exploiting residual muscle function in the forearm. However, the overreliance on visual cues resulting from a lack of sensory feedback is a common complaint. Recently, several groups have provided tactile feedback in upper limb amputees using implanted electrodes, surface nerve stimulation, or sensory substitution. These approaches have led to improved function and prosthesis embodiment. Nevertheless, the provided information remains limited to a subset of the rich sensory cues available to healthy individuals. More specifically, proprioception, the sense of limb position and movement, is predominantly absent from current systems. Here, we show that sensory substitution based on intraneural stimulation can deliver position feedback in real time and in conjunction with somatotopic tactile feedback. This approach allowed two transradial amputees to regain high and close-to-natural remapped proprioceptive acuity, with a median joint angle reproduction precision of 9.1° and a median threshold to detection of passive movements of 9.5°, which was comparable with results obtained in healthy participants. The simultaneous delivery of position information and somatotopic tactile feedback allowed both amputees to discriminate the size and compliance of four objects with high levels of performance (75.5%). These results demonstrate that tactile information delivered via somatotopic neural stimulation and position information delivered via sensory substitution can be exploited simultaneously and efficiently by transradial amputees. This study paves a way to more sophisticated bidirectional bionic limbs conveying richer, multimodal sensations.

Elbow proprioception is normal in patients with a congenital absence of functional muscle spindles

KEY POINTS

Individuals with hereditary sensory and autonomic neuropathy type III (HSAN III), also known as Riley-Day syndrome or familial dysautonomia, do not have functional muscle spindle afferents but do have essentially normal cutaneous mechanoreceptors. Lack of muscle spindle feedback from the legs may account for the poor proprioception at the knee and the ataxic gait typical of HSAN III. Given that functional muscle spindle afferents are also absent in the upper limb, we assessed whether proprioception at the elbow was likewise compromised. Passive joint angle matching showed that proprioception was normal at the elbow, suggesting that individuals with HSAN III rely more on cutaneous afferents around the elbow.

ABSTRACT

Hereditary sensory and autonomic neuropathy type III (HSAN III) is a rare neurological condition that features a marked ataxic gait that progressively worsens over time. We have shown that functional muscle spindle afferents are absent in the upper and lower limbs in HSAN III, and we have argued that this may account for the ataxia. We recently used passive joint angle matching to demonstrate that proprioception of the knee joint is very poor in HSAN III but can be improved towards normal by application of elastic kinesiology tape across the knee joints, which we attribute to the presence of intact cutaneous mechanoreceptors. Here we assessed whether proprioception was equally compromised at the elbow joint, and whether it could be improved through taping. Proprioception at the elbow joint was assessed using passive joint angle matching in 12 HSAN III patients and 12 age-matched controls. There was no difference in absolute error, gradient or correlation coefficient of the relationship between joint angles of the reference and indicator arms. Unlike at the knee, taping did not improve elbow proprioception in either group. Clearly, the lack of muscle spindles compromised proprioception at the knee but not at the elbow, and we suggest that the HSAN III patients rely more on proprioceptive signals from the skin around the elbow.

Neural correlates of proprioceptive upper limb position matching

Proprioceptive information allows humans to perform smooth coordinated movements by constantly updating one's mind with knowledge of the position of one's limbs in space. How this information is combined with other sensory modalities and centrally processed to form conscious perceptions of limb position remains relatively unknown. What has proven even more elusive is pinpointing the contribution of proprioception in cortical activity related to motion. This study addresses these gaps by examining electrocortical dynamics while participants performed an upper limb position matching task in two conditions, namely with proprioceptive feedback or with both visual and proprioceptive feedback. Specifically, we evaluated the reduction of the electroencephalographic power (desynchronization) in the μ frequency band (8-12 Hz), which is known to characterize the neural activation associated with motor control and behavior. We observed a stronger desynchronization in the left motor and somatosensory areas, contralateral to the moving limb while, parietal and occipital regions, identifying association and visual areas, respectively, exhibited a similar activation level in the two hemispheres. Pertaining to the influence of the two experimental conditions it affected only movement's offset, and precisely we found that when matching movements are performed relying only on proprioceptive information, a lower cortical activity is entailed. This effect was strongest in the visual and association areas, while there was a minor effect in the hand motor and somatosensory areas.

Finger Posture and Finger Load are Perceived Independently

The ability to track the time-varying postures of our hands and the forces they exert plays a key role in our ability to dexterously interact with objects. However, how precisely and accurately we sense hand kinematics and kinetics has not been completely characterized. Furthermore, the dominant source of information about hand postures stems from muscle spindles, whose responses can also signal isometric force and are modulated by fusimotor input. As such, one might expect that changing the state of the muscles – for example, by applying a load – would influence perceived finger posture. To address these questions, we measure the acuity of human hand proprioception, investigate the interplay between kinematic and kinetic signals, and determine the extent to which actively and passively achieved postures are perceived differently. We find that angle and torque perception are highly precise; that loads imposed on the finger do not affect perceived joint angle; that joint angle does not affect perceived load; and that hand postures are perceived similarly whether they are achieved actively or passively. The independence of finger posture and load perception contrasts with their interdependence in the upper arm, likely reflecting the special functional importance of the hand.

Fingertip-Coupled Spindle Signaling Does Not Contribute to Reduce Postural Sway Under Light Touch

The details of how light touch (LT) of a stable surface reduces postural sway are still not well known. We hypothesized that removal of feedback provided by muscle afferents of the touching fingertip would increase postural sway in standing subjects. Eleven participants stood upright on a force plate with eyes closed and on an unstable surface. The experimental conditions involved two different finger positions: with partial muscle afferents (PMA), which includes sensory information from the fingertip flexor muscles, and no muscle afferents (NMA), without information from either fingertip flexor or extensor muscles. In the control condition, the participants kept the same posture, but with no finger touch (NT). Postural sway in both anteroposterior (AP) and mediolateral (ML) axes were recorded. Results showed that LT decreased all sway quantifiers as compared with the NT condition. The withdrawal of information from the touch finger muscle afferents (NMA condition) did not increase postural sway. Actually, there was a small, albeit statistically significant, decrease in the variability of center of pressure displacement in the AP direction. These results indicate that in some cases, muscle afferent input may either not contribute or even worsen the overall quality of sensory feedback from a given body segment, leading to no improvement or even a slightly decreased performance of the motor control system (evaluated by means of levels of postural sway in the present investigation). The results suggest that non-spindle fingertip afferents provide the bulk of the sensory feedback associated with the fingertip that is touching a ground-referenced object during quiet standing under LT.

Passive changes in muscle length

This review, the first in a series of minireviews on the passive mechanical properties of skeletal muscles, seeks to summarize what is known about the muscle deformations that allow relaxed muscles to lengthen and shorten. Most obviously, when a muscle lengthens, muscle fascicles elongate, but this is not the only mechanism by which muscles change their length. In pennate muscles, elongation of muscle fascicles is accompanied by changes in pennation and changes in fascicle curvature, both of which may contribute to changes in muscle length. The contributions of these mechanisms to change in muscle length are usually small under passive conditions. In very pennate muscles with long aponeuroses, fascicle shear could contribute substantially to changes in muscle length. Tendons experience moderate axial strains even under passive loads, and, because tendons are often much longer than muscle fibers, even moderate tendon strains may contribute substantially to changes in muscle length. Data obtained with new imaging techniques suggest that muscle fascicle and aponeurosis strains are highly nonuniform, but this is yet to be confirmed. The development, validation, and interpretation of continuum muscle models informed by rigorous measurements of muscle architecture and material properties should provide further insights into the mechanisms that allow relaxed muscles to lengthen and shorten.

Effect of Sensory Deprivation on Maximal Force Abilities from Local to Non-local Digits

The present study investigates the effect of sensory deprivation of the index and middle finger on motor function of all digits during maximal voluntary force production tasks. A total of 27 subjects performed maximal isometric pressing tasks by using different instructed finger combinations. Subjects completed the same tasks in two visits: a control visit when they had normal sensory feedback in all fingers, and an anesthesia visit when digital nerve blocks were performed on their right index and middle fingers. We evaluated three aspects of motor adaptation on both local (anesthetized) and non-local (non-anesthetized) digits during maximal force production: (1) task-relevant and overall force magnitude, (2) force directional application, and (3) digital individuation and force sharing. Our results indicate that selective digital anesthesia resulted in decreased maximal force magnitude, changed direction of force production, and significant changes extended to non-local digits. The motor weakness and inefficiency revealed in the non-local digits implies that sensory information from each digit can be shared across the digits to assist motor execution within the same hand.

Relative Motion Flexion Splinting for Flexor Tendon Lacerations: Proof of Concept

BACKGROUND

The principle of relative motion has allowed patients to regain a higher degree of hand function, while protecting extensor tendon repairs. The purpose of this study was to determine whether the principle of relative motion could be a viable method to protect a flexor tendon repair.

METHODS

Four fresh-frozen cadaver arms were each mounted on a testing apparatus (wrist in 30° of extension, metacarpophalangeal [MCP] joints blocked to 70°-80°). A minimum of 11 N was used to cyclically load the flexor digitorum profundus and extensor digitorum communis tendons to maximum allowable flexion and extension for 25 cycles. Measurements of elongation of the tendons were obtained through the use of differential variable reluctance transducers. Testing was performed in both intact and repaired (single 6-0 nylon suture) middle finger tendons (zone 3) with and without a relative motion flexion splint (RMFS), which placed the affected finger in 15° to 25° of relative flexion at the MCP joint.

RESULTS

In all 4 hands, elongation was restricted to less than 1.3 mm in repaired tendon in the RMFS compared with elongation >2 mm in the nonsplinted condition. Average elongation was 0.86 mm (SD = 0.45). Visual examination of the tendons demonstrated no gapping with the use of the RMFS in any of the hands. All repairs had suture breakage and repair rupture without the RMFS.

CONCLUSIONS

This study demonstrates that the RMFS decreases elongation and eliminates tendon-repair gapping after flexion/extension cycling in a cadaver model. It provides proof of concept that the RMFS may be a viable protective mechanism for flexor tendon repairs in zone 3.

Does the Improvement of Position Sense Following Motor Imagery Practice Vary as a Function of Age and Time of Day?

The effectiveness of motor imagery practice is known to depend on age and on the ability to form motor images. In the same individual, motor imagery quality changes during the day, being better late in the morning for older adults and in the afternoon for younger adults. Does this mean that motor imagery practice should be done at specific time of the day depending on the age of participants to maximize motor learning? To examine whether the effect of motor imagery practice varies as a function of time of day and age, the authors used an arm configuration reproduction task and measured position sense accuracy before and after 135 kinesthetic motor imagery trials. Younger and older participants were randomly assigned to either a morning or an afternoon session. Data showed that the accuracy for reproducing arm configurations improved following imagery practice regardless of time of day for both younger and older adults. Moreover, the authors observed that the position sense was less accurate in the afternoon than in the morning in older participants (before and after motor imagery practice), while performance did not change during the day in younger participants. These results may have practical implications in motor learning and functional rehabilitation programs. They highlight the effectiveness of motor imagery practice for movement accuracy in both younger and older adults regardless of time of day. By contrast, they reveal that the assessment of position sense requires that the time of day be taken into account when practitioners want to report on the older patients' progress without making any mistakes.

Quantification of upper limb position sense using an exoskeleton and a virtual reality display

Background

Proprioceptive sense plays a significant role in the generation and correction of skilled movements and, consequently, in most activities of daily living. We developed a new proprioception assessment protocol that enables the quantification of elbow position sense without using the opposite arm, involving active movement of the evaluated limb or relying on working memory. The aims of this descriptive study were to validate this assessment protocol by quantifying the elbow position sense of healthy adults, before using it in individuals who sustained a stroke, and to investigate its test-retest reliability.

Methods

Elbow joint position sense was quantified using a robotic device and a virtual reality system. Two assessments were performed, by the same evaluator, with a one-week interval. While the participant’s arms and hands were occluded from vision, the exoskeleton passively moved the dominant arm from an initial to a target position. Then, a virtual arm representation was projected on a screen placed over the participant’s arm. This virtual representation and the real arm were not perfectly superimposed, however. Participants had to indicate verbally the relative position of their arm (more flexed or more extended; two-alternative forced choice paradigm) compared to the virtual representation. Each participant completed a total of 136 trials, distributed in three phases. The angular differences between the participant’s arm and the virtual representation ranged from 1° to 27° and changed pseudo-randomly across trials. No feedback about results was provided to the participants during the task. A discrimination threshold was statistically extracted from a sigmoid curve fit representing the relationship between the angular difference and the percentage of successful trials. Test-retest reliability was evaluated with 3 different complementary approaches, i.e. a Bland-Altman analysis, an intraclass correlation coefficient (ICC) and a standard error of measurement (SEm).

Results

Thirty participants (24.6 years old; 17 males, 25 right-handed) completed both assessments. The mean discrimination thresholds were 7.0 ± 2.4 (mean ± standard deviation) and 5.9 ± 2.1 degrees for the first and the second assessment session, respectively. This small difference between assessments was significant (− 1.1 ± 2.2 degrees), however. The assessment protocol was characterized by a fair to good test-retest reliability (ICC = 0.47).

Conclusion

This study demonstrated the potential of this assessment protocol to objectively quantify elbow position sense in healthy individuals. Futures studies will validate this protocol in older adults and in individuals who sustained a stroke.

The Influence of External Forces on Wrist Proprioception

Proprioception combines information from cutaneous, joint, tendon, and muscle receptors for maintaining a reliable internal body image. However, it is still a matter of debate, in both neurophysiology and psychology, to what extent such body image is modified or distorted by a changing haptic environment. In particular, what is worth investigating is the contribution of external forces on our perception of body and joint configuration. The proprioceptive acuity of fifteen young participants was tested with a Joint Position Matching (JPM) task, performed with the dominant wrist under five different external forces, in order to understand to what extent they affect proprioceptive acuity. Results show that accuracy and precision in target matching do not change in a significant manner as a function of the loading condition, suggesting that the multi-sensory integration process is indeed capable of discriminating different sub-modalities of proprioception, namely the joint position sense and the sense of force. Furthermore, results indicate a preference for target undershooting when movements are performed in a viscous or high resistive force field, rather than passive or null fields in which subjects did not show any predominance for under/over estimation of their position.

Age-based model for metacarpophalangeal joint proprioception in elderly

Neurological injuries such as stroke can lead to proprioceptive impairment. For an informed diagnosis, prognosis, and treatment planning, it is essential to be able to distinguish between healthy performance and deficits following the neurological injury. Since there is some evidence that proprioception declines with age and stroke occurs predominantly in the elderly population, it is important to create a healthy reference model in this specific age group. However, most studies investigate age effects by comparing young and elderly subjects and do not provide a model within a target age range. Moreover, despite the functional relevance of the hand in activities of daily living, age-based models of distal proprioception are scarce. Here, we present a proprioception model based on the assessment of the metacarpophalangeal joint angle difference threshold in 30 healthy elderly subjects, aged 55-80 years (median: 63, interquartile range: 58-66), using a robotic tool to apply passive flexion-extension movements to the index finger. A two-alternative forced-choice paradigm combined with an adaptive algorithm to define stimulus magnitude was used. The mixed-effects model analysis revealed that aging has a significant, increasing effect on the difference threshold at the metacarpophalangeal joint, whereas other predictors (eg, tested hand or sex) did not show a significant effect. The adaptive algorithm allowed reaching an average assessment duration <15 minutes, making its clinical applicability realistic. This study provides further evidence for an age-related decline in proprioception at the level of the hand. The established age-based model of proprioception in elderly may serve as a reference model for the proprioceptive performance of stroke patients, or of any other patient group with central or peripheral proprioceptive impairments. Furthermore, it demonstrates the potential of such automated robotic tools as a rapid and quantitative assessment to be used in research and clinical settings.

Upper Extremity Proprioception After Stroke: Bridging the Gap Between Neuroscience and Rehabilitation

Proprioception is an important aspect of function that is often impaired in the upper extremity following stroke. Unfortunately, neurorehabilitation has few evidence based treatment options for those with proprioceptive deficits. The authors consider potential reasons for this disparity. In doing so, typical assessments and proprioceptive intervention studies are discussed. Relevant evidence from the field of neuroscience is examined. Such evidence may be used to guide the development of targeted interventions for upper extremity proprioceptive deficits after stroke. As researchers become more aware of the impact of proprioceptive deficits on upper extremity motor performance after stroke, it is imperative to find successful rehabilitation interventions to target these deficits and ultimately improve daily function.

Measurement of voluntary activation based on transcranial magnetic stimulation over the motor cortex

This article reviews the use of transcranial magnetic stimulation (TMS) over the motor cortex to make estimates of the level of voluntary drive to muscles. The method, described in 2003 (Todd et al. J Physiol 551: 661-671, 2003), uses a TMS pulse to produce descending corticospinal volleys that synaptically activate motoneurons, resulting in a muscle twitch. Linear regression of the superimposed twitch amplitude and voluntary force (or torque) can generate an "estimated" resting twitch for muscles involved in a task. This procedure has most commonly been applied to elbow flexors but also to knee extensors and other muscle groups. Data from 44 papers using the method were tabulated. We identify and discuss five major technical challenges, and the frequency with which they are addressed. The technical challenges include inadvertent activation of the cortical representation of antagonist muscles, the role of antagonist torques at the studied joint, uncertainty about the effectiveness of the TMS pulse in activating the motoneuron pool, the linearity of the voluntary force (or torque) and superimposed twitch relationship, and variability in the TMS-evoked EMG and force/torque responses. The ideal situation in which the descending corticospinal volleys recruit all of the agonist motoneurons and none of the antagonist motoneurons is unlikely to ever occur, and hence results must be carefully examined to assess the authenticity of the voluntary activation estimates in the context of the experimental design. A partial compromise lies in the choice of stimulus intensity. We also identify aspects of the procedure that require further investigation.

Error signals driving locomotor adaptation: cutaneous feedback from the foot is used to adapt movement during perturbed walking

KEY POINTS

Sensory input from peripheral receptors are important for the regulation of walking patterns. Cutaneous input mediates muscle responses to deal with immediate external perturbations. In this study we focused on the role of cutaneous feedback in locomotor adaptation that takes place over minutes of training. We show that interfering with cutaneous feedback reduced adaptation to ankle perturbations during walking. These results help us understand the neural mechanisms underlying walking adaptation, and have clinical implications for treating walking impairments after neurological injuries.

ABSTRACT

Locomotor patterns must be adapted to external forces encountered during daily activities. The contribution of different sensory inputs to detecting perturbations and adapting movements during walking is unclear. In the present study, we examined the role of cutaneous feedback in adapting walking patterns to force perturbations. Forces were applied to the ankle joint during the early swing phase using an electrohydraulic ankle-foot orthosis. Repetitive 80 Hz electrical stimulation was applied to disrupt cutaneous feedback from the superficial peroneal nerve (foot dorsum) and medial plantar nerve (foot sole) during walking (Choi et al. 2013). Sensory tests were performed to measure the cutaneous touch threshold and perceptual threshold of force perturbations. Ankle movement were measured when the subjects walked on the treadmill over three periods: baseline (1 min), adaptation (1 min) and post-adaptation (3 min). Subjects (n = 10) showed increased touch thresholds measured with Von Frey monofilaments and increased force perception thresholds with stimulation. Stimulation reduced the magnitude of walking adaptation to force perturbation. In addition, we compared the effects of interrupting cutaneous feedback using anaesthesia (n = 5) instead of repetitive nerve stimulation. Foot anaesthesia reduced ankle adaptation to external force perturbations during walking. The results of the present study suggest that cutaneous input plays a role in force perception, and may contribute to the 'error' signal involved in driving walking adaptation when there is a mismatch between expected and actual force.

Increasing cutaneous afferent feedback improves proprioceptive accuracy at the knee in patients with sensory ataxia

Hereditary sensory and autonomic neuropathy type III (HSAN III) features disturbed proprioception and a marked ataxic gait. We recently showed that joint angle matching error at the knee is positively correlated with the degree of ataxia. Using intraneural microelectrodes, we also documented that these patients lack functional muscle spindle afferents but have preserved large-diameter cutaneous afferents, suggesting that patients with better proprioception may be relying more on proprioceptive cues provided by tactile afferents. We tested the hypothesis that enhancing cutaneous sensory feedback by stretching the skin at the knee joint using unidirectional elasticity tape could improve proprioceptive accuracy in patients with a congenital absence of functional muscle spindles. Passive joint angle matching at the knee was used to assess proprioceptive accuracy in 25 patients with HSAN III and 9 age-matched control subjects, with and without taping. Angles of the reference and indicator knees were recorded with digital inclinometers and the absolute error, gradient, and correlation coefficient between the two sides calculated. Patients with HSAN III performed poorly on the joint angle matching test [mean matching error 8.0 ± 0.8° (±SE); controls 3.0 ± 0.3°]. Following application of tape bilaterally to the knee in an X-shaped pattern, proprioceptive performance improved significantly in the patients (mean error 5.4 ± 0.7°) but not in the controls (3.0 ± 0.2°). Across patients, but not controls, significant increases in gradient and correlation coefficient were also apparent following taping. We conclude that taping improves proprioception at the knee in HSAN III, presumably via enhanced sensory feedback from the skin.

Baseline skin information from the foot dorsum is used to control lower limb kinematics during level walking

The aim of the current study was to explore the role of dorsal foot skin on the joint kinematics of gait during level walking. Twelve volunteers experienced sensory perturbations with either reduced dorsal skin feedback using topical anesthetic, reduced visual feedback of the lower visual field, or a combination of both cutaneous and visual reductions (paired). The visual condition was introduced to impose a greater reliance on skin input (goggles occluded lower visual field input). Our results showed that a reduction in skin input, alone, resulted in significant angular position changes at both the ankle and knee joints through swing (increased flexion, p < 0.010), despite preservation of minimal toe clearance (MTC; p = 0.908). Conversely, a reduction in lower visual field input resulted in a greater minimal toe clearance affect (MTC; p < 0.001), a slight increase in dorsiflexion at the ankle (p = 0.046), yet no effect on angular position changes for the knee (p = 0.110). The locomotor changes observed following a reduction in cutaneous feedback from the foot dorsum suggest an important role of the skin over this region for the regulation of level ground walking. Interestingly, it appears that these healthy young adults were able to compensate for the reduced skin information while preserving locomotor efficiency via a maintained ground clearance (MTC). Our data also demonstrated an interaction between skin and visual inputs; vision appears to have a less dominant role compared to skin in controlling the joint positions through swing phase of gait. This work is the first to highlight the influence of reduced cutaneous input from the dorsum of the foot on locomotor strategies.

Activity in the brain network for dynamic manipulation of unstable objects is robust to acute tactile nerve block: An fMRI study

OBJECTIVE

To study whether a temporary block of the tactile afferents from the fingers causes altered activity in the neural network for dexterous manipulation.

METHODS

Whole-brain functional Magnetic Resonance Imaging (fMRI) was conducted in 18 healthy subjects, while they compressed an unstable spring between the thumb and index finger of the right hand. Two sensory conditions--with and without tactile input from the fingers--were employed. In the latter condition the digital nerves were blocked by local anesthesia.

RESULTS

Compression of the unstable spring was associated with activity in an earlier described network for object manipulation. We found that this entire network remained active after a nerve block, and the activity was increased in the dorsal premotor cortex.

CONCLUSIONS

The neural network for dexterous manipulation is robust with only minor alterations after acute loss of tactile information from the fingers. There was no loss of activity, but, unexpectedly, an increased activity in some parts of the network.

SIGNIFICANCE

This study gives new insights to possible neural compensatory mechanisms that make fine motor control possible after acute disruption of tactile information in natural situations like cold weather or wearing surgical gloves.

Reliability and validity of using the Lokomat to assess lower limb joint position sense in people with incomplete spinal cord injury

Background

Proprioceptive sense (knowing where the limbs are in space) is critical for motor control during posture and walking, and is often compromised after spinal cord injury (SCI). The purpose of this study was to assess the reliability and validity of using the Lokomat, a robotic exoskeleton used for gait rehabilitation, to quantitatively measure static position sense of the legs in persons with incomplete SCI.

Methods

We used the Lokomat and custom software to assess static position sense in 23 able-bodied (AB) subjects and 23 persons with incomplete SCI (American Spinal Injury Association Impairment Scale level B, C or D). The subject’s leg was placed into a target position (joint angle) at either the hip or knee and asked to memorize that position. The Lokomat then moved the test joint to a “distractor” position. The subject then used a joystick controller to bring the joint back into the memorized target position. The final joint angle was compared to the target angle and the absolute difference was recorded as an error. All movements were passive. Known-groups validity was determined by the ability of the measure to discriminate between able-bodied and SCI subjects. To evaluate test-retest reliability, subjects were tested twice and intra-class correlation coefficients comparing errors from the two sessions were calculated. We also performed a traditional clinical test of proprioception in subjects with SCI and compared these scores to the robotic assessment.

Results

The robot-based assessment test was reliable at the hip and knee in persons with SCI (P ≤ 0.001). Hip and knee angle errors in subjects with SCI were significantly greater (P ≤ 0.001) and more variable (P < 0.0001) than in AB subjects. Error scores were significantly correlated to clinical measure of joint position sense (r ≥ 0.507, P ≤ 0.013).

Conclusions

This study shows that the Lokomat may be used as a reliable and valid clinical measurement tool for assessing joint position sense in persons with incomplete SCI. Quantitative assessments of proprioceptive deficits after neurological injury will help in understanding its role in the recovery of skilled walking and in the development of interventions to aid in the return to safe community ambulation.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-0003-11-167) contains supplementary material, which is available to authorized users.

Decoding of the spike timing of primary afferents during voluntary arm movements in monkeys

Understanding the mechanisms of encoding forelimb kinematics in the activity of peripheral afferents is essential for developing a somatosensory neuroprosthesis. To investigate whether the spike timing of dorsal root ganglion (DRG) neurons could be estimated from the forelimb kinematics of behaving monkeys, we implanted two multi-electrode arrays chronically in the DRGs at the level of the cervical segments in two monkeys. Neuronal activity during voluntary reach-to-grasp movements were recorded simultaneously with the trajectories of hand/arm movements, which were tracked in three-dimensional space using a motion capture system. Sixteen and 13 neurons, including muscle spindles, skin receptors, and tendon organ afferents, were recorded in the two monkeys, respectively. We were able to reconstruct forelimb joint kinematics from the temporal firing pattern of a subset of DRG neurons using sparse linear regression (SLiR) analysis, suggesting that DRG neuronal ensembles encoded information about joint kinematics. Furthermore, we estimated the spike timing of the DRG neuronal ensembles from joint kinematics using an integrate-and-fire model (IF) incorporating the SLiR algorithm. The temporal change of firing frequency of a subpopulation of neurons was reconstructed precisely from forelimb kinematics using the SLiR. The estimated firing pattern of the DRG neuronal ensembles encoded forelimb joint angles and velocities as precisely as the originally recorded neuronal activity. These results suggest that a simple model can be used to generate an accurate estimate of the spike timing of DRG neuronal ensembles from forelimb joint kinematics, and is useful for designing a proprioceptive decoder in a brain machine interface.

Elbow joint position sense after total elbow arthroplasty

BACKGROUND

Multiple human experiments have shown that articular lesions can have a negative effect on proprioception. The influence of total elbow arthroplasty on joint position sense has not been reported so far. The purpose of the study was to evaluate proprioception, defined as a joint position sense, after total elbow arthroplasty.

METHODS

The study included 16 patients with unilateral semiconstrained linked total elbow arthroplasty and 21 healthy volunteers. The evaluation included measurement of active and passive reproduction of joint position sense of both elbows after surgery and the control groups. Reference angles included extension to 50° and 70° and flexion to 110°. We also assessed function of the elbow in arthroplasty group using the Mayo Elbow Performance Score, the Disability of the Arm, Shoulder and Hand score, and a visual analog scale for pain level.

RESULTS

The average value of error of passive reproduction of joint position for elbows after arthroplasty was significantly inferior for all evaluated positions compared with the contralateral elbow and with the control group, respectively, at 110° flexion: 4.3°, 2.7°, and 3.2°; at 70° extension: 4.9°, 2.9°, and 2.7°; and at 50° extension: 6.3°, 3.8°, and 3.8°. The average value of error of active reproduction of joint position for the arthroplasty group was also significantly inferior, respectively, at 110° flexion: 3.5°, 1.9° and 2°; and at 50° extension: 4.4°, 3.3°, and 3°.

CONCLUSION

Proprioception in elbows that undergo total arthroplasty is significantly inferior compared with the contralateral site of the patient and in the healthy control group.

Effects of surgical repair on active shoulder position sense and 3-dimensional reaching accuracy in a patient with anterior shoulder instability

The purpose of this report was to investigate whether a subject with anterior shoulder instability exhibited better active shoulder position sense and 3-dimensional (3-D) reaching accuracy after a surgical repair. The 19-year-old male subject underwent an open Bankart repair procedure for his left shoulder followed by a standardized post-operative rehabilitation program. Shoulder position sense was examined with traditional passive matching and active positioning protocols. Reaching accuracy in space with the unrestricted arm motion was also examined. The subject was tested 5 months prior to the surgery and re-tested 6 months after the surgery. With the traditional passive matching protocol, shoulder position sense improved <2° after surgery. However, shoulder position sense improved greatly after surgery with active shoulder abduction (up to 4.25°) and active shoulder rotation (up to 5.87°) testing protocols. In addition, reaching accuracy also greatly improved after surgery (up to 10.97 cm) with the most significant improvement when reaching to targets located in the frontal plane. Data suggest that anterior shoulder repair with rehabilitation can improve both active shoulder position sense and reaching accuracy, especially in shoulder positions involving abduction with external rotation.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

Enhanced proprioceptive acuity at the knee in the competitive athlete

STUDY DESIGN

Controlled laboratory study: cross-sectional.

OBJECTIVE

To determine if proprioception, measured by the threshold to detection of passive motion (TDPM), differed in individuals who regularly participate in moderate-intensity exercise for fitness as compared to individuals involved in high-intensity skilled exercise.

BACKGROUND

Previous research has been equivocal as to whether exercise training is associated with superior proprioceptive acuity, in particular, exercise that includes dynamic postural challenges such as cutting and pivoting.

METHODS

Two groups of 25 healthy individuals (18-32 years old) were recruited. One group consisted of individuals who performed moderate-activity level exercises for 5 to 10 hours per week. Participants in the other group performed high-activity level exercises, including high-speed cutting and pivoting activities, at least 10 hours per week. Proprioception was determined using TDPM, in which the knee was slowly extended or flexed at an angular velocity of 0.5°/s or less from a starting position of 40° of knee flexion.

RESULTS

Individuals participating in competitive, high-intensity skilled exercise demonstrated better acuity (average of both limbs) of TDPM (mean ± SD, 0.81° ± 0.38°; P<.001) than those participating in moderate-intensity exercise for fitness (1.53° ± 0.58°). A low but statistically significant association (r = -0.38, P = .006) was found between weekly duration of exercise and proprioceptive threshold as measured by TDPM.

CONCLUSION

These results suggest that perceptual thresholds of passive movement may be enhanced, depending on activity level and associated postural challenge, and that higher level and increased amount of exercise may promote enhanced neurosensory processing in these individuals. Consequently, high-intensity skilled training may deserve further emphasis in orthopaedic rehabilitation.

Cutaneous mechanisms of isometric ankle force control

The sense of force is critical in the control of movement and posture. Multiple factors influence our perception of exerted force, including inputs from cutaneous afferents, muscle afferents and central commands. Here, we studied the influence of cutaneous feedback on the control of ankle force output. We used repetitive electrical stimulation of the superficial peroneal (foot dorsum) and medial plantar nerves (foot sole) to disrupt cutaneous afferent input in 8 healthy subjects. We measured the effects of repetitive nerve stimulation on (1) tactile thresholds, (2) performance in an ankle force-matching and (3) an ankle position-matching task. Additional force-matching experiments were done to compare the effects of transient versus continuous stimulation in 6 subjects and to determine the effects of foot anesthesia using lidocaine in another 6 subjects. The results showed that stimulation decreased cutaneous sensory function as evidenced by increased touch threshold. Absolute dorsiflexion force error increased without visual feedback during peroneal nerve stimulation. This was not a general effect of stimulation because force error did not increase during plantar nerve stimulation. The effects of transient stimulation on force error were greater when compared to continuous stimulation and lidocaine injection. Position-matching performance was unaffected by peroneal nerve or plantar nerve stimulation. Our results show that cutaneous feedback plays a role in the control of force output at the ankle joint. Understanding how the nervous system normally uses cutaneous feedback in motor control will help us identify which functional aspects are impaired in aging and neurological diseases.

The proprioceptive senses: their roles in signaling body shape, body position and movement, and muscle force

This is a review of the proprioceptive senses generated as a result of our own actions. They include the senses of position and movement of our limbs and trunk, the sense of effort, the sense of force, and the sense of heaviness. Receptors involved in proprioception are located in skin, muscles, and joints. Information about limb position and movement is not generated by individual receptors, but by populations of afferents. Afferent signals generated during a movement are processed to code for endpoint position of a limb. The afferent input is referred to a central body map to determine the location of the limbs in space. Experimental phantom limbs, produced by blocking peripheral nerves, have shown that motor areas in the brain are able to generate conscious sensations of limb displacement and movement in the absence of any sensory input. In the normal limb tendon organs and possibly also muscle spindles contribute to the senses of force and heaviness. Exercise can disturb proprioception, and this has implications for musculoskeletal injuries. Proprioceptive senses, particularly of limb position and movement, deteriorate with age and are associated with an increased risk of falls in the elderly. The more recent information available on proprioception has given a better understanding of the mechanisms underlying these senses as well as providing new insight into a range of clinical conditions.

Coding of digit displacement by cell spiking and network oscillations in the monkey sensorimotor cortex

β-Band oscillations occur in motor and somatosensory cortices and muscle activity. Oscillations appear most strongly after movements, suggesting that they may represent or probe the limb's final sensory state. We tested this idea by training two macaque monkeys to perform a finger flexion to one of four displacements, which was then held for 2 s without visual feedback of absolute displacement. Local field potential (LFP) and single unit spiking were recorded from the rostral and caudal primary motor cortex and parietal areas 3a, 3b, 2, and 5. Information theoretic analysis determined how well unit firing rate or the power of LFP oscillations coded finger displacement. All areas encoded significant information about finger displacement after the movement into target, both in β-band (∼20 Hz) oscillatory activity and unit firing rate. On average, the information carried by unit firing was greater (0.07 bits) and peaked earlier (0.73 s after peak velocity) than that by LFP β-oscillations (0.05 bits and 0.95 s). However, there was considerable heterogeneity among units: some cells did not encode maximal information until midway through the holding phase. In 30% of cells, information in rate lagged information in LFP oscillations recorded at the same site. Finger displacement may be represented in the cortex in multiple ways. Coding the digit configuration immediately after a movement probably relies on nonoscillatory feedback, or efference copy. With increasing delay after movement cessation, oscillatory processing may also play a part.

Population coding of forelimb joint kinematics by peripheral afferents in monkeys

Various peripheral receptors provide information concerning position and movement to the central nervous system to achieve complex and dexterous movements of forelimbs in primates. The response properties of single afferent receptors to movements at a single joint have been examined in detail, but the population coding of peripheral afferents remains poorly defined. In this study, we obtained multichannel recordings from dorsal root ganglion (DRG) neurons in cervical segments of monkeys. We applied the sparse linear regression (SLiR) algorithm to the recordings, which selects useful input signals to reconstruct movement kinematics. Multichannel recordings of peripheral afferents were performed by inserting multi-electrode arrays into the DRGs of lower cervical segments in two anesthetized monkeys. A total of 112 and 92 units were responsive to the passive joint movements or the skin stimulation with a painting brush in Monkey 1 and Monkey 2, respectively. Using the SLiR algorithm, we reconstructed the temporal changes of joint angle, angular velocity, and acceleration at the elbow, wrist, and finger joints from temporal firing patterns of the DRG neurons. By automatically selecting a subset of recorded units, the SLiR achieved superior generalization performance compared with a regularized linear regression algorithm. The SLiR selected not only putative muscle units that were responsive to only the passive movements, but also a number of putative cutaneous units responsive to the skin stimulation. These results suggested that an ensemble of peripheral primary afferents that contains both putative muscle and cutaneous units encode forelimb joint kinematics of non-human primates.

Robotic assessment of sensorimotor deficits after traumatic brain injury

BACKGROUND AND PURPOSE

Robotic technology is commonly used to quantify aspects of typical sensorimotor function. We evaluated the feasibility of using robotic technology to assess visuomotor and position sense impairments following traumatic brain injury (TBI). We present results of robotic sensorimotor function testing in 12 subjects with TBI, who had a range of initial severities (9 severe, 2 moderate, 1 mild), and contrast these results with those of clinical tests. We also compared these with robotic test outcomes in persons without disability.

METHODS

For each subject with TBI, a review of the initial injury and neuroradiologic findings was conducted. Following this, each subject completed a number of standardized clinical measures (Fugl-Meyer Assessment, Purdue Peg Board, Montreal Cognitive Assessment, Rancho Los Amigos Scale), followed by two robotic tasks. A visually guided reaching task was performed to assess visuomotor control of the upper limb. An arm position-matching task was used to assess position sense. Robotic task performance in the subjects with TBI was compared with findings in a cohort of 170 person without disabilities.

RESULTS

Subjects with TBI demonstrated a broad range of sensory and motor deficits on robotic testing. Notably, several subjects with TBI displayed significant deficits in one or both of the robotic tasks, despite normal scores on traditional clinical motor and cognitive assessment measures.

DISCUSSION AND CONCLUSIONS

The findings demonstrate the potential of robotic assessments for identifying deficits in visuomotor control and position sense following TBI. Improved identification of neurologic impairments following TBI may ultimately enhance rehabilitation.

The independence of deficits in position sense and visually guided reaching following stroke

Background

Several studies have found correlations between proprioception and visuomotor function during stroke recovery, however two more recent studies have found no correlation. Unfortunately, most of the studies to date have been conducted with clinical assessments of sensation that are observer-based and have poor reliability. We have recently developed new tests to assess position sense and motor function using robotic technology. The present study was conducted to reassess the relationship between position sense and upper limb movement following stroke.

Methods

We assessed position sense and motor performance of 100 inpatient stroke rehabilitation subjects and 231 non-disabled controls. All subjects completed quantitative assessments of position sense (arm-position matching task) and motor performance (visually-guided reaching task) using the KINARM robotic device. Subjects also completed clinical assessments including handedness, vision, Purdue Pegboard, Chedoke-McMaster Stroke Assessment-Impairment Inventory and Functional Independence Measure (FIM). Neuroimaging documented lesion localization. Fisher’s exact probability tests were used to determine the relationship between performances on the arm-position matching and visually-guided reaching task. Pearson’s correlations were conducted to determine the relationship between robotically measured parameters and clinical assessments.

Results

Performance by individual subjects on the matching and reaching tasks was statistically independent (Fisher’s test, P<0.01). However, performance on the matching and reaching tasks both exhibited relationships with abilities in daily activities as measured by the FIM. Performance on the reaching task also displayed strong relationships with other clinical measures of motor impairment.

Conclusions

Our data support the concept that position sense deficits are functionally relevant and point to the importance of assessing proprioceptive and motor impairments independently when planning treatment strategies.

Presbypropria: the effects of physiological ageing on proprioceptive control

Several changes in the human sensory systems, like presbycusis or presbyopia, are well-known to occur with physiological ageing. A similar change is likely to occur in proprioception, too, but there are strong and unexplained discrepancies in the literature. It was proposed that assessment of the attentional cost of proprioceptive control could provide information able to unify these previous studies. To this aim, 15 young adults and 15 older adults performed a position matching task in single and dual-task paradigms with different difficulty levels of the secondary task (congruent and incongruent Stroop-type tasks) to assess presumed age-related deficits in proprioceptive control. Results showed that proprioceptive control was as accurate and as consistent in older as in young adults for a single proprioceptive task. However, performing a secondary cognitive task and increasing the difficulty of this secondary task evidenced both a decreased matching performance and/or an increased attentional cost of proprioceptive control in older adults as compared to young ones. These results advocated for an impaired proprioception in physiological ageing.

Quantifying lower limb joint position sense using a robotic exoskeleton: a pilot study

Clinicians and scientists often focus on tracking the recovery of motor skills after spinal cord injury (SCI), but less attention is paid to the recovery of sensory skills. Measures of sensory function are imperative for evaluating the efficacy of treatments and therapies. Proprioception is one sensory modality that provides information about static position and movement sense. Because of its critical contribution to motor control, proprioception should be measured during the course of recovery after neurological injury. Current clinical methods to test proprioception are limited to crude, manual tests of movement and position sense. The purpose of this study was to develop a quantitative assessment tool to measure joint position sense in the legs. We used the Lokomat, a robotic exoskeleton, and custom software to assess joint position sense in the hip and knee in 9 able-bodied (AB) subjects and 1 person with incomplete SCI. We used two different test paradigms. Both required the subject to move the leg to a target angle, but the presentation of the target was either a remembered or visual target angle. We found that AB subjects had more accurate position sense in the remembered task than in the visual task, and that they tended to have greater accuracy at the hip than at the knee. Position sense of the subject with SCI was comparable to those of the AB subjects. We show that using the Lokomat to assess joint position sense may be an effective clinical measurement tool.

Shoulder position sense during passive matching and active positioning tasks in individuals with anterior shoulder instability

BACKGROUND

Altered neuromuscular control due to compromised joint position sense may contribute to recurrent shoulder instability.

OBJECTIVE

The purpose of the present study was to examine whether individuals with anterior shoulder instability exhibit larger shoulder position sense errors than those with healthy shoulders in both passive matching and active positioning.

DESIGN

This was a between-groups study with repeated measures.

METHODS

Ten people with anterior shoulder instability and 15 people with healthy shoulders participated in the study. Shoulder position sense was examined with 3 different protocols (passive motion to remembered shoulder rotation angles and active shoulder abduction and rotation to verbally specified positions) in positions of both mid-range and end-range of motion.

RESULTS

Participants with unstable shoulders exhibited significantly larger errors (by 1.8° on average) in perception of shoulder position compared with those with healthy shoulders during passive matching. During active positioning, participants with unstable shoulders were able to voluntarily move the shoulder to verbally specified angles as accurately as those with healthy shoulders in both abduction (0.85° difference) and rotation (0.99° difference) tasks.

CONCLUSIONS

Results of this study indicate that people with unstable shoulders can perceive shoulder angles as accurately as people with healthy shoulders in activities with voluntary arm movements. Compared with passive matching, better information from muscle spindles and other sources during voluntary arm movements may compensate for the potential joint position sense deficits after the injury. Therefore, individuals with an unstable shoulder may have adequate neuromuscular control to engage proper protective mechanisms to stabilize the shoulder joint during functional activities.

Constraints for control of the human hand

More than 30 muscles drive the hand to perform a multitude of essential dextrous tasks. Here we consider new views on the evolution of hand structure and on peripheral and central constraints for independent control of the digits of the hand. The human hand is widely assumed to have evolved from hands like those of African apes, yet recent studies have shown that our hands and those of the earliest hominids are very similar and unlike those of living apes. Understanding the limits of hand function may come from investigation of our last common ancestor with the great apes, rather than the great apes themselves. In the periphery, movement across the full range of joint space can be limited by mechanical linkages among the extrinsic muscles. Further, peripheral limits occur when the hand adopts some positions in which the contraction of muscles fails to move the joints on which they usually act; there is muscle 'disengagement' and functional paralysis for some actions. Surprisingly, the central nervous system drives the hand seamlessly through this landscape of mechanical limits. Central constraints on control of the individual digits include the spillover of neural drive to neighbouring muscles and their 'compartments', and the inability to make maximal muscle forces when multiple digits contract strongly which produces a force deficit. The pattern of these latter constraints correlates with amounts of daily use of each digit and favours enslaved extension to lift fingers from an object but selective flexion of fingers to contact it.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.