After a period of eccentric exercise of elbow flexor muscles of one arm in young, adult human subjects, muscles became fatigued and damaged. Damage indicators were a fall in force, change in resting elbow angle and delayed onset of soreness. After the exercise, subjects were asked to match the forearm angle of one arm, whose position was set by the experimenter, with their other arm. Subjects matched the position of the unsupported reference arm, when this was unexercised, with a significantly more flexed position in their exercised indicator arm. Errors were in the opposite direction when the reference arm was exercised. The size of the errors correlated with the drop in force. Less consistent errors were observed when the reference arm was supported. A similar pattern of errors was seen after concentric exercise, which does not produce muscle damage. The data suggested that subjects were using as a position cue the perceived effort required to maintain a given forearm angle against the force of gravity. The fall in force from fatigue after exercise meant more effort was required to maintain a given position. That led to matching errors between the exercised and unexercised arms. It was concluded that while a role for muscle spindles in kinaesthesia cannot be excluded, detailed information about static limb position can be derived from the effort required to support the limb against the force of gravity.

Non-technical summary The sense of body ownership tells us that our body belongs to us, and other bodies do not. That our body belongs to us is fundamental to self-awareness. It is known that synchronous touch and vision can be used to induce an illusion of ownership over an artificial rubber hand. Like the skin receptors used for touch, sensory receptors in the muscles only provide information about events occurring to the body. Whether muscle receptors contribute to our sense of body ownership is not known. This study developed a technique to induce an illusion of ownership over a plastic finger using movement, which excites muscle receptors. This sense of ownership still occurred when the contribution of skin and joint receptors was removed using local anaesthetic. The results clearly show that muscle receptors can contribute to the sense of body ownership.Abstract The sense of body ownership, knowledge that parts of our body 'belong' to us, is presumably developed using sensory information. Cutaneous signals seem ideal for this and can modify the sense of ownership. For example, an illusion of ownership over an artificial rubber hand can be induced by synchronously stroking both the subject's hidden hand and a visible artificial hand. Like cutaneous signals, proprioceptive signals (e.g. from muscle receptors) exclusively signal events occurring in the body, but the influence of proprioceptors on the sense of body ownership is not known. We developed a technique to generate an illusion of ownership over an artificial plastic finger, using movement at the proximal interphalangeal joint as the stimulus. We then examined this illusion in 20 subjects when their index finger was intact and when the cutaneous and joint afferents from the finger had been blocked by local anaesthesia of the digital nerves. Subjects still experienced an illusion of ownership, induced by movement, over the plastic finger when the digital nerves were blocked. This shows that local cutaneous signals are not essential for the illusion and that inputs arising proximally, presumably from receptors in muscles which move the finger, can influence the sense of body ownership. Contrary to other studies, we found no evidence that voluntary movements induce stronger illusions of body ownership than those induced by passive movement. It seems that the congruence of sensory stimuli is more important to establish body ownership than the presence of multiple sensory signals.

Non-technical summary Even when the hand is stationary we know its position. This information is needed by the brain to plan movements. If the sensory input from a limb is removed through an accident, or an experiment with local anaesthesia, then a 'phantom' limb commonly develops. We used ischaemic anaesthesia of one arm to study the mechanisms which define the phantom hand. Surprisingly, if the wrist and fingers are held straight during anaesthesia, the perceived phantom hand becomes bent at the wrist and fingers, but if they are bent during anaesthesia, the final phantom is extended at the wrist and fingers. There is no 'default' posture for the phantom hand. Further, the hand appears to increase gradually in size as anaesthesia develops. The start of these perceptual changes occurs when input from large-diameter sensory nerve fibres is declining. These results provide new information about how the brain generates phantom limbs.Abstract Contorted 'phantom' limbs often form when sensory inputs are removed, but the neural mechanisms underlying their formation are poorly understood. We tracked the evolution of an experimental phantom hand during ischaemic anaesthesia of the arm. In the first study subjects showed the perceived posture of their hand and fingers using a model hand. Surprisingly, if the wrist and fingers were held straight before and during anaesthesia, the final phantom hand was bent at the wrist and fingers, but if the wrist and fingers were flexed before and during anaesthesia, the final phantom was extended at wrist and fingers. Hence, no 'default' posture existed for the phantom hand. The final perceived posture may depend on the initial and evolving sensory input during the block rather than the final sensory input (which should not differ for the two postures). In the second study subjects selected templates to indicate the perceived size of their hand. Perceived hand size increased by 34 ± 4% (mean ± 95% CI) during the block. Sensory changes were monitored. In all subjects, impairment of large-fibre cutaneous sensation began distally with von Frey thresholds increasing before cold detection thresholds (Aδ fibres) increased. Some C fibres subserving heat pain still conducted at the end of cuff inflation. These data suggest that changes in both perceived hand size and perceived position of the finger joints develop early when large-fibre cutaneous sensation is beginning to degrade. Hence it is unlikely that block of small-fibre afferents is critical for phantom formation in an ischaemic block.

Neuromuscular electrical stimulation (NMES) generates contractions by activation of motor axons (peripheral mechanism), but the afferent volley also contributes by recruiting spinal motoneurons synaptically (central mechanism), which recruits motoneurons according to Henneman's size principle. Thus, we hypothesized that contractions that develop due to a combination of peripheral and central mechanisms will fatigue less rapidly than when electrically evoked contractions are generated by the activation of motor axons alone. Plantar-flexion torque evoked by NMES over the triceps surae was compared in five able-bodied subjects before (Intact) and during (Blocked) a complete anesthetic block of the tibial and common peroneal nerves. In the Blocked condition, plantar-flexion torque could only develop from the direct activation of motor axons beneath the stimulating electrodes. NMES was delivered using three protocols: protocol A, constant 100 Hz for 30 s; protocol B, four 2-s bursts of 100 Hz alternating with 20-Hz stimulation; and protocol C, alternating 100 Hz bursts (1 s on, 1 s off) for 30 s. The percent change in evoked plantar flexion torque from the beginning to the end of the stimulation differed (P < 0.05) between Intact and Blocked conditions for all protocols (Intact: protocol A = +125%, B = +230%, C = +78%; Blocked: protocol A = -79%, B = -15%, C = -35%). These results corroborate previous evidence that NMES can evoke contractions via the recruitment of spinal motoneurons in addition to the direct recruitment of motor axons. We now show that NMES delivered for periods of up to 30 s generates plantar-flexion torque which decreases when only motor axons are recruited and increases when the central nervous system can contribute.

Non-technical summary If a weight is applied to a finger and the subject asked to produce the same force, the subject generates a force larger than the weight. That is, subjects overestimate the force applied by an external target when matching it. Details of this force overestimation are not well understood. We show that subjects overestimate small target weights, but not larger ones. Furthermore we show for the first time that the force overestimation consists of two components. The first component is a constant. The second component depends on the precise magnitude of the weight and is only present when subjects hold the target weight against gravity. We suggest that the two components are generated in different phases of the force-matching task, are due to different processes, and must have an influence on all proprioceptive judgements of force.Abstract To make accurate movements the brain must differentiate between forces it commands and forces imposed by the environment. This requires afferent information and signals related to central commands. If subjects match an externally generated target force with a self-generated force, they produce a force that is larger than the target. It has been proposed that this is due to simple attenuation of afferent force signals produced by the body's own actions, but the mechanisms are unclear. Four studies of forces applied to the index finger in 14 subjects investigated this force overestimation. We determined which sensory signals are involved, if handedness is important, if overestimation is present at high forces, and which muscle actions can generate it. Subjects overestimate an externally generated target force by 2-3 N when matching it with a voluntary force using a simple contraction or complex muscle synergy. This 'offset' occurs at low but not high forces. The effect occurs when only cutaneous inputs, or when only combined inputs from muscle and central command sources can signal force. We report a novel central factor that increases the gain, or gradient of the relationship, between the matching and target forces to ∼1.20. This increased gain is present only if the target force is received on an active finger and persists after the 'offset' is abolished. It may reflect processing of reactive forces during the target phase of the task. Overall, the previously described simple model of force attenuation cannot explain fully the overestimation of external forces.

Effect of eccentric exercise on position sense at the human forearm in different postures.

Blouin JS, Walsh LD, Nickolls P, Gandevia SC. High-frequency submaximal stimulation over muscle evokes centrally generated forces in human upper limb skeletal muscles.

A progressive decline in upper limb function is associated with ageing and disease. In this cross-sectional study we assessed the performance of 367 healthy individuals aged of 20 to 95 years across a battery of upper limb clinical tests, which we have termed the upper limb Physiological Profile Assessment (PPA). The upper limb PPA was designed to quantify the performance of the multiple physiological domains important for adequate function in the upper extremities. Included are tests of muscle strength, unilateral movement and dexterity, position sense, skin sensation, bimanual coordination, arm stability, along with a functional task. We report age and gender normative values for each test. Test-retest reliability ranged from good to excellent in all tests (intra-class correlation coefficients from 0.65 to 0.98) with the exception of position sense (0.31). Ten of the thirteen tests revealed differences in performance between males and females, twelve showed a decline in performance with increasing age, and eight discriminated between older people with and without upper limb functional impairment. Furthermore, most tests showed good external validity with respect to age, an upper limb functional test and self-reported function. This profiling approach provides a reference range for clinical groups with upper limb sensory and motor impairments and may assist in identifying undiagnosed deficits in the general population. Furthermore, the tests are sufficiently reliable to detect motor impairments in people with compromised upper limb function and evaluate the effectiveness of interventions.