Stable human standing with lower-limb muscle afferents providing the only sensory input.

1. This study investigated the sources of sensory information upon which normal subjects' ability to stand depends. 2. An 'equivalent body' was used to simulate the physical properties of each subject's body during standing. The modulation of ankle torque required to support the equivalent body in an upright position was similar to that required to support the subject's own body when standing. However, when balancing the equivalent body, vestibular inputs were excluded from directing the appropriate changes in ankle torque. Thus, stability of stance could be studied with (normal stance) and without (balancing equivalent body) modulation by vestibular inputs. Vision could be excluded by closing the eyes. Sensory input from the feet and ankles could be removed by local anaesthesia from prolonged ischaemia, induced by occluding blood flow with inflated pneumatic cuffs just above the ankles. With vestibular, visual and peripheral sensory inputs negated, standing could rely only upon remaining sensory inputs, notably those from sensory receptors in the leg muscles. 3. Unlike the human body, the equivalent body used to negate vestibular inputs is not segmented. Therefore, the effects on stability of having a segmented body were determined by splinting subjects during standing so that only ankle movement was possible. This was done in the presence and absence of visual stabilization. 4. For each experimental task, either standing or balancing the equivalent body, sway was recorded while posture was unperturbed. Root mean square values of sway amplitude and power spectra were used to compare conditions. 5. Every subject could balance the equivalent body in a stable way when the eyes were closed, and when the feet were anaesthetized.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Long-latency (40-80 ms) electromyographic (e.m.g.) responses of the contracting flexor pollicis longus to stretches applied at the thumb-tip, were studied in normal human subjects. Stretches were applied during four classes of contraction: (i) isometric 'hold', in which the subject held a steady isometric contraction; (ii) isometric tracking, in which the subject tracked a steadily rising force target; (iii) isotonic tracking, in which the subject flexed against a constant torque to track a position target; (iv) weight-lifting, in which the subject lifted a weight hung at one end of a lever by pressing the thumb-tip on the other end of the lever. The effects on the responses of prior instructions to 'resist' or to 'let go', and of local anaesthesia of the thumb, were studied. The ability to modify the size of the long-latency e.m.g. response in accordance with prior instruction was variable. All subjects tested could do so during isometric holding contractions, but many could not do so during the other forms of contraction. Local anaesthesia of the thumb significantly reduced the long-latency e.m.g. response in only some subjects, and abolished it in none. The reduction was most reliably seen for isometric force tracking contractions. During thumb anaesthesia in different subjects, there was a significant correlation between the proportional increase in apparent heaviness of an object lifted by thumb flexion and the proportional reduction in the size of the long-latency e.m.g. response to muscle stretch.

Changes in motor commands, as shown by changes in perceived heaviness, during partial curarization and peripheral anaesthesia in man.

1. The centrally generated ;effort' or direct voluntary command to motoneurones required to lift a weight was studied using a simple weight-matching task when the muscles lifting a reference weight were weakened. This centrally generated input to motoneurones was increased when the lifting muscles were partially paralysed with curare or decamethonium as judged by the increased perceived heaviness of a reference weight lifted by the weakened muscles.2. If subjects were asked simply to make matching isometric contractions when the lifting muscles were weakened the isometric tension produced by a weakened muscle was over-estimated.3. When subjects matched weights by flexing the distal joint of the thumb the perceived heaviness of a reference weight during a control partial curarization was compared with its perceived heaviness during a similar partial curarization when the thumb was also anaesthetized. At any level of maximal strength during curarization the perceived heaviness (which reflects the motor command to lifting motoneurones) was increased when the thumb was anaesthetized.4. This increased voluntary command to lifting motoneurones may be required because automatic reflex assistance provided by apparent servo action from the long flexor of the thumb is suppressed by anaesthesia of the thumb (Marsden, Merton & Morton, 1971, 1973, 1976a; Dyhre-Poulsen & Djørup, 1976).

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

1. An anatomical peculiarity allows the hand to be positioned so that the terminal phalanx of the middle finger cannot be moved by voluntary effort. When positioned in this way only joint and cutaneous mechanisms subserve position sense. By altering the position of the hand the muscles are again engaged and able to move the finger. Moving the joint then also excites muscular afferents. 2. The position sense of twelve subjects was assessed with and without engagement of the muscles at the joint. Three tests were used in which either angular displacement, angular velocity or duration of displacement were held constant. 3. When muscular attachment was restored, performance in all tests was greatly enhanced. As engagement of the muscles caused little change in the 'stiffness' of the joint, it is unlikely that the improved performance resulted from increased discharges from the joint receptors. Cutaneous mechanisms are unlikely to mediate this improvement as they are likely to have been unaffected by engagement of muscles. It is concluded that intramuscular receptors are partly responsible for normal position sense. 4. In seven of the twelve subjects the test finger was anaesthetized to isolate the contribution of intramuscular receptors. This muscle sense was variable. In some subjects it provided accurate kinaesthetic information but in others the information was crude. If with the test finger anaesthetized subjects exerted voluntary tension with the muscles that move the joint the muscle sense was improved.