Independent control of the digits: changes in perceived heaviness over a wide range of force.

Perceived heaviness of a weight lifted by flexion of the distal joint of one digit increases when an adjacent digit concurrently lifts a weight. The present study confirmed this finding for relatively low weights (representing 3-5% maximal voluntary force) and a method was adapted to show that this effect occurs for much larger weights (20-25% maximal force). Thus, the increase in perceived heaviness is likely to operate over a wide range of muscle force generated by the hand.

Independent digit control: failure to partition perceived heaviness of weights lifted by digits of the human hand.

1. A weight-matching task was used to investigate the ability to estimate heaviness when weight lifting was isolated to the extrinsic flexor muscles (or portions thereof) that act on the digits of the hand. 2.

The cortical drive to human respiratory muscles in the awake state assessed by premotor cerebral potentials.

1. We investigated the possibility of a cortical contribution to human respiration by recording from the scalp of awake subjects the premotor cerebral potentials that are known to precede voluntary limb movements. 2.

Recovery from fatigue of human diaphragm and limb muscles.

This study was designed to compare the recovery from fatigue of human inspiratory and limb muscles using repeated maximal static contractions. Series of 18 maximal contractions of 10 sec duration were performed with a duty cycle of 50% for maximal inspiratory efforts (against a shutter at FRC), and with duty cycles of 5%, 10%, 20% and 50% for the elbow flexors in repeated studies on 6 subjects. The peak inspiratory pressure at the end of the series declined to 86.

Muscle performance, voluntary activation, twitch properties and perceived effort in normal subjects and patients with the chronic fatigue syndrome.

The decrease in maximal force-generating capacity, the degree of central activation of the muscle, and the subjective perception of effort were measured during prolonged submaximal isometric exercise in 12 male patients suffering from the 'chronic fatigue syndrome' and 13 naive, healthy male subjects. Maximal voluntary isometric torque generated by the elbow flexors was measured before, and at 5 min intervals during an endurance sequence of 45 min of repetitive isometric contractions (6 s duration, 4 s rest interval) producing 30% of the initial maximal voluntary torque. Electrical stimuli were also delivered to the elbow flexors to measure the contractile force in the intervals between voluntary contractions.

Decline in spindle support to alpha-motoneurones during sustained voluntary contractions.

1. To address whether the muscle spindle support to alpha-motoneurones is maintained during prolonged isometric voluntary contractions, the discharge of eighteen muscle spindle afferents, originating in the dorsiflexors of the ankle or toes, was recorded from the common peroneal nerve in eight subjects. Isometric contractions were generally sustained for 1 min, usually below 30% of the maximal voluntary dorsiflexion force.

Activation of fusimotor neurones by motor cortical stimulation in human subjects.

1. Neural recordings were made from motor fascicles of the ulnar or radial nerves while the motor cortex was stimulated percutaneously using high-voltage electrical stimuli or transient magnetic pulses to determine whether human muscle spindle endings could be activated by such stimuli and, if so, whether this occurred before the recruitment of alpha-motoneurones. 2.

Concordance between colonic myoelectrical signals recorded with intramuscular electrodes in the human rectosigmoid in vivo.

The myoelectrical activity of the human rectosigmoid colon was studied simultaneously in six subjects at two sites using two pairs of fine wire bipolar electrodes. The electrodes were spaced 2-5 cm apart in the rectosigmoid after insertion into the smooth muscle layers under direct vision at sigmoidoscopy. The electrodes were implanted at positions between 8 and 25 cm from the anal verge in different subjects.

Cardiovascular responses to static exercise in man: central and reflex contributions.

1. To assess the contributions of muscle chemoreflexes and central signals of motor command to cardiovascular to static exercise, blood pressure and heart rate were measured during three separate conditions: (i) isometric handgrip contractions, (ii) entrapment of metabolites produced by these contractions within the contracting muscles (chemoreflex effect), and (iii) attempted contractions of acutely paralysed muscles at three levels of effort (command effect). 2.

Voluntary activation of human motor axons in the absence of muscle afferent feedback. The control of the deafferented hand.

The ability to activate human motoneurons supplying individual intrinsic muscles of the hand was examined during acute deafferentation of the muscles. Tungsten microelectrodes were inserted percutaneously into motor fascicles of the ulnar nerve of 5 subjects, which was then blocked distally with local anaesthetic. In 4 subjects unitary action potentials were recorded from 16 motor axons, which were identified with respect to their target muscles.

The distribution of muscle weakness in upper motoneuron lesions affecting the lower limb.

To determine the distribution of weakness in the lower limb after upper motoneuron lesions the strength of 8 muscle groups was measured. Four groups of patients were studied: 22 control subjects, 16 patients with unilateral leg paresis, 4 patients with severe unilateral paralysis and 5 patients with paraparesis. In the testing posture (seated), patients with cerebral upper motoneuron lesions showed no selective loss of power in flexors or extensors on the contralateral side.

Perceptual responses to microstimulation of single afferents innervating joints, muscles and skin of the human hand.

1. Microneurographic techniques were used to isolate single afferent axons within cutaneous and motor fascicles of the median and ulnar nerves at the wrist in thirteen subjects. Of the sixty-five identified afferents, eleven innervated the interphalangeal and metacarpophalangeal joints, sixteen innervated muscle spindles, three innervated Golgi tendon organs and thirty-five supplied the glabrous skin of the hand.

Activation of human respiratory muscles during different voluntary manoeuvres.

1. This study used three techniques (bilateral phrenic nerve stimulation, motor cortex stimulation and quantitative electromyography) to assess the degree of activation of the diaphragm, intercostal-accessory muscles and abdominal muscles during postural tasks and respiratory manoeuvres. They included maximal inspiratory and expulsive efforts.

Projection of thenar muscle afferents to frontal and parietal cortex of human subjects.

There is some controversy about the projection of muscle afferents from the human upper limb to cerebral cortex and about their contribution to somatosensory evoked potentials. In 8 normal volunteers, the somatosensory projections of muscle and cutaneous afferents from the hand were recorded at 21 scalp sites, using a non-cephalic reference. Low-threshold thenar muscle afferents were selectively activated by intramuscular microstimulation.

Reflex activation of muscle spindles in human pretibial muscles during standing.

1. Experiments were performed in standing subjects to determine whether low-threshold cutaneous and muscle afferents from mechanoreceptors in the human foot reflexly influence fusimotor neurons innervating pretibial flexor muscles. Recordings were made from 30 identified muscle-spindle afferents, four tendon-organ afferents, and one alpha-motor axon innervating the pretibial flexor muscles.

Behavior of human muscle receptors when reliant on proprioceptive feedback during standing.

1. This study investigated the muscle-spindle discharge from the pretibial flexor muscles of standing human subjects while they performed maneuvers that altered their reliance on proprioceptive feedback to control balance. Single-unit recordings were made from 100 identified muscle afferents, 81 from muscle-spindle endings and 19 from Golgi tendon organs.

Transcranial magnetic stimulation can influence the selection of motor programmes.

Transcranial magnetic stimulation is becoming increasingly popular to study the rapidly conducting output from the motor cortex. Little is known about the effects of such stimuli on other aspects of cortical function. In the study single magnetic stimuli, subthreshold for movement, produced significant preference for selection of one hand in a forced-choice task.

Accuracy of weight estimation for weights lifted by proximal and distal muscles of the human upper limb.

1. It is well established that tactile acuity is greater over digits than over the proximal parts of the upper limb and that the corticospinal projection is especially dense for distal muscles. To determine whether the acuity for judgements of forces exerted by distal muscles differed from that for proximal muscles, a weight-matching task was used with first dorsal interosseous, flexor pollicus longus and elbow flexors.

Conduction velocities of muscle and cutaneous afferents in the upper and lower limbs of human subjects.

In the cat and monkey the fastest axons in the peripheral nerve are group I afferents from muscle, but there are no definitive data on conduction velocity for these afferents in human subjects. Knowledge of the relative conduction velocities of muscle and cutaneous afferents is important for the interpretation of reflex studies, evoked potentials and other aspects of motor control. To rectify this deficiency, the conduction velocities of the fastest muscle and cutaneous afferents were determined for the median, ulnar and tibial nerves of normal subjects.

Projection of low-threshold afferents from human intercostal muscles to the cerebral cortex.

Low-threshold afferents from human limb muscles are known to project to the sensorimotor cortex and to contribute to proprioception. However, there are few data on the cortical projection of afferents from human respiratory muscles. The present study employed evoked-potential techniques to determine whether low-threshold muscle afferents from the chest wall project to cortical levels in conscious human subjects.