Position sense at the human elbow joint measured by arm matching or pointing.

Position sense at the human elbow joint has traditionally been measured in blindfolded subjects using a forearm matching task. Here we compare position errors in a matching task with errors generated when the subject uses a pointer to indicate the position of a hidden arm. Evidence from muscle vibration during forearm matching supports a role for muscle spindles in position sense. We have recently shown using vibration, as well as muscle conditioning, which takes advantage of muscle's thixotropic property, that position errors generated in a forearm pointing task were not consistent with a role by muscle spindles. In the present study we have used a form of muscle conditioning, where elbow muscles are co-contracted at the test angle, to further explore differences in position sense measured by matching and pointing. For fourteen subjects, in a matching task where the reference arm had elbow flexor and extensor muscles contracted at the test angle and the indicator arm had its flexors conditioned at 90°, matching errors lay in the direction of flexion by 6.2°. After the same conditioning of the reference arm and extension conditioning of the indicator at 0°, matching errors lay in the direction of extension (5.7°). These errors were consistent with predictions based on a role by muscle spindles in determining forearm matching outcomes. In the pointing task subjects moved a pointer to align it with the perceived position of the hidden arm. After conditioning of the reference arm as before, pointing errors all lay in a more extended direction than the actual position of the arm by 2.9°-7.3°, a distribution not consistent with a role by muscle spindles. We propose that in pointing muscle spindles do not play the major role in signalling limb position that they do in matching, but that other sources of sensory input should be given consideration, including afferents from skin and joint.

Muscle thixotropy as a tool in the study of proprioception.

When a muscle relaxes after a contraction, cross-bridges between actin and myosin in sarcomeres detach, but about 1% spontaneously form new, non-force-generating attachments. These bridges give muscle its thixotropic property. They remain in place for long periods if the muscle is left undisturbed and give the muscle a passive stiffness in response to a stretch. They are detached by stretch, but reform at the new length. If the muscle is then shortened, the presence of these bridges prevents muscle fibres from shortening and they fall slack. So, resting muscle can be in one of two states, where it presents in response to a stretch with a high stiffness, if no slack is present, or with a compliant response in the presence of slack. Intrafusal fibres of muscle spindles show thixotropic behaviour. For spindles, after a conditioning contraction, they are left stretch sensitive, with a high level of background discharge. Alternatively, if after the contraction the muscle is shortened, intrafusal fibres fall slack, leaving spindles with a low level of background activity and insensitivity to stretch. Muscle spindles are receptors involved in the senses of human limb position and movement. The technique of muscle conditioning can be used to help understand the contribution of muscle spindles to these senses and how the brain interprets signals arising in spindles. When, in a two-arm position-matching task, elbow muscles of the two arms are deliberately conditioned in opposite ways, the blindfolded subject makes large position errors of which they are unaware. The evidence suggests that the brain is concerned with the difference signal coming from the antagonists acting at the elbow and with the overall difference in signal from the two arms. Another way of measuring position sense is to use a single arm and indicate its perceived position with a pointer. Here, there is no access to a signal from the other limb, and position sense relies on referral to a central map of the body, the postural schema.

The fall in force after exercise disturbs position sense at the human forearm.

We reported previously that concentric or eccentric exercise can lead to errors in human limb position sense. Our data led us to conclude that the errors, post-exercise, were not due to an altered responsiveness of the proprioceptive afferents, and we proposed that they resulted from central changes in the processing of the afferent input. However, it remained uncertain what was responsible for triggering those changes, the volume of afferent traffic during the exercise or the developing fatigue. The afferent traffic hypothesis was tested by subjects carrying out a series of 250 lightly loaded concentric contractions of elbow flexors that produced little fatigue (6 %). This did not lead to significant position errors. In a second experiment, a series of fatiguing isometric contractions, which kept movements of the muscle to a minimum, led to a 24 % fall in force and significant position errors (3°, direction of extension). In the third experiment, at 24 h after eccentric exercise, when the short-term effects of fatigue and accumulated metabolites were gone, but force was still 28 % below control values, this was accompanied by significant position errors in the direction of extension, 3.2° in the relaxed arm and 3.3° in the self-supported arm. It is concluded that it is the fall in force accompanying exercise which is responsible for disturbing limb position sense. It is suggested that the exercise effects are generated in the brain, perhaps as a result of an alteration of the body map, triggered by the fall in force.

How much space do drivers provide when passing cyclists? Understanding the impact of motor vehicle and infrastructure characteristics on passing distance.

BACKGROUND: Understanding factors that influence the distance that drivers provide when passing cyclists is critical to reducing subjective risk and improving cycling participation. This study aimed to quantify passing distance and assess the impact of motor vehicle and road infrastructure characteristics on passing distance. METHODS: An on-road observational study was conducted in Victoria, Australia. Participants had a custom device installed on their bicycle and rode as per their usual cycling for one to two weeks. A hierarchical linear model was used to investigate the relationship between motor vehicle and infrastructure characteristics (location, presence of on-road marked bicycle lane and the presence of parked cars on the kerbside) and passing distance (defined as the lateral distance between the end of the bicycle handlebars and the passing motor vehicle). RESULTS: Sixty cyclists recorded 18,527 passing events over 422 trips. The median passing distance was 173 cm (Q1: 137 cm, Q3: 224 cm) and 1085 (5.9%) passing events were less than 100 cm. Relative to sedans, 4WDs had a reduced mean passing distance of 15 cm (Q1: 12 cm, Q3: 17 cm) and buses had a reduced mean passing distance of 28 cm (Q1: 16 cm, Q3: 40 cm). Relative to passing events that occurred on roads without a marked bicycle lane and without parked cars, passing events on roads with a bike lane with no parked cars had a reduced mean passing distance of 27 cm (Q1: 25 cm, Q3: 29 cm), and passing events on roads with a bike lane and parked cars had a mean lower passing distance of 40 cm (Q1: 37 cm, Q3: 43 cm). CONCLUSIONS: One in every 17 passing events was a close (<100 cm) passing event. We identified that on-road bicycle lanes and parked cars reduced passing distance. These data can be used to inform the selection and design of cycling-related infrastructure and road use with the aim of improving safety for cyclists.

Muscle damage produced by isometric contractions in human elbow flexors.

Isometric exercise is often prescribed during rehabilitation from injury to maintain muscle condition and prevent disuse atrophy. However, such exercise can lead to muscle soreness and damage. Here we investigate which parameters of isometric contractions are responsible for the damage. Bouts of 30 repetitions of maximum voluntary contractions of elbow flexors in 38 subjects were carried out and peak force, soreness, and tenderness were measured before the exercise, immediately afterwards, at 2 h, and at 24 h postexercise. When one arm was held near the optimum angle for force generation (90°), the force it produced was greater by 28% than by the other arm held at a longer length (155°). However, despite the smaller contraction forces of the muscle held at the longer length, after the exercise it exhibited a greater fall in force that persisted out to 24 h (20% fall) and more delayed soreness than the muscle exercised at 90° (7% fall at 24 h). The result indicates a length dependence of the damage process for isometric contractions at maximum effort. In four additional experiments, evidence was provided that the damage occurred during the plateau of the contraction and not the rising or relaxation phases. The damage had a prompt onset and was cumulative, continuing for the duration of the contraction. We interpret our findings in terms of the nonuniform lengthening of sarcomeres during the plateau of the contractions and conclude that muscle damage from isometric exercise is minimized if carried out at lengths below the optimum, using half-maximum or smaller contractions. NEW & NOTEWORTHY Isometric exercise, where muscle contracts while the limb is held fixed, is often possible for individuals rehabilitating from injury and can help maintain muscle condition. Such exercise has been reported to cause some muscle damage and soreness. We confirm this and show that to minimize damage, exercising muscles should be held at shorter than the optimum length for force and carried out at half-maximum effort or less.

Apparent motion perception in lower limb amputees with phantom sensations: "obstacle shunning" and "obstacle tolerance".

Phantom limbs are the phenomenal persistence of postural and sensorimotor features of an amputated limb. Although immaterial, their characteristics can be modulated by the presence of physical matter. For instance, the phantom may disappear when its phenomenal space is invaded by objects ("obstacle shunning"). Alternatively, "obstacle tolerance" occurs when the phantom is not limited by the law of impenetrability and co-exists with physical objects. Here we examined the link between this under-investigated aspect of phantom limbs and apparent motion perception. The illusion of apparent motion of human limbs involves the perception that a limb moves through or around an object, depending on the stimulus onset asynchrony (SOA) for the two images. Participants included 12 unilateral lower limb amputees matched for obstacle shunning (n = 6) and obstacle tolerance (n = 6) experiences, and 14 non-amputees. Using multilevel linear models, we replicated robust biases for short perceived trajectories for short SOA (moving through the object), and long trajectories (circumventing the object) for long SOAs in both groups. Importantly, however, amputees with obstacle shunning perceived leg stimuli to predominantly move through the object, whereas amputees with obstacle tolerance perceived leg stimuli to predominantly move around the object. That is, in people who experience obstacle shunning, apparent motion perception of lower limbs was not constrained to the laws of impenetrability (as the phantom disappears when invaded by objects), and legs can therefore move through physical objects. Amputees who experience obstacle tolerance, however, had stronger solidity constraints for lower limb apparent motion, perhaps because they must avoid co-location of the phantom with physical objects. Phantom limb experience does, therefore, appear to be modulated by intuitive physics, but not in the same way for everyone. This may have important implications for limb experience post-amputation (e.g., improving prosthesis embodiment when limb representation is constrained by the same limits as an intact limb).

Position Sense in Chronic Pain: Separating Peripheral and Central Mechanisms in Proprioception in Unilateral Limb Pain.

UNLABELLED: Awareness of limb position is derived primarily from muscle spindles and higher-order body representations. Although chronic pain appears to be associated with motor and proprioceptive disturbances, it is not clear if this is due to disturbances in position sense, muscle spindle function, or central representations of the body. This study examined position sense errors, as an indicator of spindle function, in participants with unilateral chronic limb pain. The sample included 15 individuals with upper limb pain, 15 with lower limb pain, and 15 sex- and age-matched pain-free control participants. A 2-limb forearm matching task in blindfolded participants, and a single-limb pointer task, with the reference limb hidden from view, was used to assess forearm position sense. Position sense was determined after muscle contraction or stretch, intended to induce a high or low spindle activity in the painful and nonpainful limbs, respectively. Unilateral upper and lower limb chronic pain groups produced position errors comparable with healthy control participants for position matching and pointer tasks. The results indicate that the painful and nonpainful limb are involved in limb-matching. Lateralized pain, whether in the arm or leg, does not influence forearm position sense. PERSPECTIVE: Painful and nonpainful limbs are involved in bilateral limb-matching. Muscle spindle function appears to be preserved in the presence of chronic pain.