Forces generated at the thumb interphalangeal joint during imposed sinusoidal movements

Abstract

SUMMARY1. A method is described for driving the interphalangeal joint of the thumb through repeated sinusoidal flexion-extension movements, while immobilizing other joints of the wrist and hand.2. The joint met the sinusoidal movement with a force that fluctuated in an approximately sinusoidal manner. This paper is concerned with the relationship between the position and force sinusoids.3. When the thumb was relaxed the forces were small, but when the flexor pollicis longus was tetanically stimulated there was a large force change in response to each cycle of the movement. In either case, the maximum resistance to extension occurred during the later part of the extension movement, some 20O45O in advance of maximum extension. A similar result was obtained when the subject exerted a maximal voluntary flexing force.4. The resistance to movement can conveniently be displayed as a stiffness vector, the amplitude of which is the force/displacement, and the phase is the angle by which the force sinusoid leads the position sinusoid.5. When the subject maintained a continuous, though sub-maximal, flexing effort the timing of the force fluctuations changed with changing frequency of movement in a characteristic way, and the stiffness vectors described a C-shaped or spiral path. With increasing frequency the stiffness vectors moved round this path in a clockwise direction.6. For descriptive purposes the resistance to movement can usefully be regarded as the vector sum of a reflex and a non-reflex component. Since the reflex pathway involves significant conduction delays, the reflex force can be expected to appear later in the cycle of a higher frequency movement, and give rise to a vector which moves round in a clockwise direction as the frequency increases. The non-reflex stiffness, however, changes much less with frequency.7. It is concluded that the size of the C-shaped or spiral vector path gives an indication of the strength of the reflex activity, while the position of the high frequency points gives an indication of the noni-reflex resistance to the movement.

Different types of tremor in the human thumb.

“…The forearm, hand and thumb were fixed in the way that has already been described (Brown et al 1982a), and the same force and position transducers were used. The wheel and crank that had been used for sinusoidal driving were, however, replaced either by a rigid link or by a compliant spring.…”

“…The rigid linkage was used in the investigation of isometric tremor, and its length was chosen so that the joint was in its mid-range, with an angle of about 135°between the dorsum of the proximal phalanx and the dorsum of the base of the distal phalanx. The subject was then instructed to maintain some pre-determined flexing force, and to assist him in this the output of the force transducer was displayed to him on a meter of long time constant (see Brown et al 1982a).…”

Electromyographic responses to imposed sinusoidal movement of the human thumb.

“…This result was to be expected, since the afferent discharge from the flexor pollicis longus would reach its maximum during lengthening. With increasing to the same cycle by cycle sinusoidal analysis that was used for the force records (Brown et al 1982a) and each phase and amplitude point is the average of all the cycles that fell within 05 Hz of the plotted value.…”

“…The previous paper (Brown, Rack & Ross, 1982a) described the forces which develop at the interphalangeal joint of the thumb in response to imposed sinusoidal movements. These forces were assumed to be the sum of forces which arise as a reflex response to the movement and forces that arise from other non-reflex mechanisms.…”

“…In either case, the timing of the e.m.g. and the timing of the reflex force (Brown, Rack & Ross, 1982a) changed together in ways that increased confidence in each of the measurements.5. The amplitude of the e.m.g.…”