Effects of prior instruction and anaesthesia on long‐latency responses to stretch in the long flexor of the human thumb.

SUMMARY1. 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. Subjects matched a reference weight (200 g) lifted by a digit on the right with a variable weight lifted by the left thumb; a concurrent weight was simultaneously lifted by a digit on the right (reference) side. The reference and concurrent weights were lifted either by the same muscle (digital portions of flexor digitorum profundus), or anatomically separate but functionally related muscles (flexor digitorum profundus and flexor pollicis longus). Anaesthesia of the radial nerve and/or posturing of the hand was used to eliminate any small forces generated by cocontraction of the extensor muscles of the digits.3. When the concurrent weight was equal to or greater than the reference weight, the perceived heaviness of the reference weight increased significantly from control trials (in which no concurrent weight was lifted). Although perceived heaviness of the reference weight increased progressively as the concurrent weight increased, reproducibility (expressed as the coefficient of variation) did not deteriorate when a weight was lifted concurrently. These findings were qualitatively similar when the reference and concurrent weights were lifted by two digital portions of flexor digitorum profundus or when the weights were lifted by flexion of the thumb and index finger. Also, anaesthesia of the digits which lifted the reference and concurrent weights did not alter the changes in perceived heaviness.4. Perceived heaviness of the reference weight lifted by flexor digitorum profundus did not change when subjects lifted the concurrent weight with a remote muscle group (ankle dorsiflexors).5. This study shows that signals of heaviness are systematically overestimated whenever more than one portion of the extrinsic muscles which flex the different digits are simultaneously active. Given that estimates of heaviness are biased by the central motor command, one explanation is that when the total motor drive MS 8902 S. L. KILBREATI AND S C GANDEVIA increases, the central nervous system is unable to partition precisely the destination of motor commands to functionally related 'muscles'.

Section: Discussioncontrasting

confidence: 39%

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

Kilbreath

Gandevia

1991

“…Edin, Westling & Johansson (1992) have recently shown that humans can control two finger tip forces independently when they contact surfaces with different frictional properties. The cutaneous reflex organization of FDP and FPL has not been investigated in detail in human subjects, although an important facilitation has been identified (Gandevia & McCloskey, 1977;Marsden, Merton & Morton, 1977;Loo & McCloskey, 1985).…”

Section: Discussionmentioning

confidence: 99%

Limited independent flexion of the thumb and fingers in human subjects.

Kilbreath

Gandevia

1994

244

220

1. We investigated whether human subjects can activate selectively flexor pollicis longus (FPL) and digital portions of flexor digitorum profundus (FDP). These muscles were selected because they are the only flexors of the distal phalanges. 2. Electromyographic activity (EMG) was recorded with intramuscular electrodes from one digital component of the deep flexors (‘test’) while subjects lifted weights by flexing the distal interphalangeal joint of the other digits in turn (‘lifting’ digits). Only recording sites at which single motor units were recruited selectively at low forces were used. The weights lifted represented 2.5‐50% of the maximal voluntary contraction (MVC). We measured the lowest weight lifted which produced phasic and tonic coactivation in the ‘test’ muscle. 3. The extent of coactivation varied with the ‘distance’ between the test and lifting digits although no significant difference occurred in the pattern of coactivation thresholds among the digital flexors. The extent of coactivation increased when angular displacement or velocity at the distal interphalangeal joint of the lifting digit increased but was not critically dependent on restraint of the hand. 4. Because mechanical ‘connections’ could interfere with the ability to move a distal phalanx independently, the arms of nine cadavers were studied. The separation of tendons between the thumb (FPL) and the index portion of FDP, and between the index and middle portions of FDP, usually extended more proximally in the forearm than separation between the tendons to the middle and ring fingers and between the ring and little fingers. Direct intertendinous links were also noted. 5. It is not possible to direct a sufficiently focal motor command to flex selectively the distal joint of the fingers and thumb when forces exceeding 2.5% MVC are generated. For the middle, ring and little fingers in particular, movement of adjacent digits may also involve ‘in‐series’ mechanical links between adjacent components of FDP.

“…and the change in stiffness and the two could be dissociated if required. The change in stiffness must therefore represent presetting of the reflex response to stretch, consistent with evidence for presetting of responses to rapid stretch (Hammond, 1956(Hammond, , 1960Tatton & Lee, 1975;Evarts & Granit, 1976;Iles, 1977;Colebatch, Gandevia, McCloskey & Potter, 1979;Kanosue, Akazawa & Fujii, 1983;Loo & McCloskey, 1985), although others have failed to demonstrate this (Marsden et al 1976;Crago, Houk & Hasan, 1976).…”

Section: Methodsmentioning

confidence: 99%

Maintenance of constant arm position or force: reflex and volitional components in man.

Colebatch

McCloskey

1987

Self Cite

SUMMARY1. Normal subjects, with closed eyes, attempted to keep constant either the force exerted at the wrist or the position of the wrist against an elastic load. The load was attached to the wrist 275 mm from the axis of rotation of the elbow joint. During recording, the far end of the elastic load was displaced slowly enough that it was not immediately perceived but far enough for perception to occur before its completion.2. The over-all relation between wrist force and position for the two conditions was approximately linear and could be described in terms of effective stiffness. The effective stiffness for the constant-position task averaged 2-8 N/mm (210 N m/rad), while for the constant-force task the mean effective stiffness was -0-028 N/mm (-2 1 N m/rad), indicative of slight over-compensation.3. Averaging the performance at the onset of the imposed disturbance indicated that the subjects' behaviour consisted of two parts: an initial, small-range response followed by a second phase over the remainder of the displacement. The transition corresponded to the subjects' threshold for detection of the disturbance.4. The stiffness measured for the response prior to perception was taken as a measure which included the tonic stretch reflex. The stiffness was altered appropriately for the two tasks, being lower when the subjects tried to maintain the force exerted constant (average 1 1 N/mm, 83 N m/rad) than when they attempted to keep the position constant (average 2-3 N/mm, 170 N m/rad). A small degree of co-contraction occurred but could be dissociated from the stiffness changes.5. Scaling the results allowed comparison of the initial stiffness with values for the decerebrate cat. When analysed in this way, the values recorded in man during the constant-position task were similar to those reported for short-range stiffness in the decerebrate cat.6. The thresholds for detection of the disturbance were much lower than those reported for subjects with relaxed muscles.7. The stretch reflex in man has a direct role in compensating for small disturbances during motor tasks. It may also function to improve detection ofapplied disturbances by magnifying the corresponding force change. Once the stimulus is perceived and voluntary intervention is possible, a greater contrast is seen between the subjects' performance of the two tasks.