Damping of the wrist joint during voluntary movement

Abstract

Damping characteristics of the musculoskeletal system were investigated during rapid voluntary wrist flexion movements. Oscillations about the final position were induced by introducing a load with the characteristics of negative damping, which artificially reduced the damping of the wrist. Subjects responded to increases in the negatively damped load by stronger cocontraction of wrist flexor and extensor muscles during the stabilization phase of the movement. However, their ability to counteract the effects of the negatively damped load diminished as the negative damping increased. Consequently, the number and frequency of oscillations increased. The oscillations were accompanied by phase-locked muscle activity superimposed on underlying tonic muscle activation. The wrist stiffness and damping coefficient increased with the increased cocontraction that accompanied more negatively damped loads, although changes in the damping coefficient were less systematic than the stiffness. Analysis of successive half-cycles of the oscillation revealed that the wrist stiffness and damping coefficient increased, despite decreasing muscle activation, as oscillation amplitude and velocity declined. This indicates that the inverse dependence of the damping coefficient on oscillation velocity contributes significantly to damping of joint motion. It is suggested that this property helps to offset a negative contribution to damping from the stretch reflex.

Systematic nonlinear relations between displacement amplitude and joint mechanics at the human wrist

“…With an increase in displacement amplitude from 0.4° to 6° RMS (0.2% to 3% of FCR muscle Insert Figure 6 here length), the stiffness decreased by 31%, the viscosity decreased by 73%, the damping ratio decreased by 71% and the resonant frequency decreased from 10.5 Hz to 7.3 Hz. The changes were consistent with previous findings that both joint stiffness and viscosity decrease with increasing amplitude (Kearney and Hunter, 1982;Milner and Cloutier, 1998;Nichols, 1985).…”

“…Milner and Cloutier (1993) determined wrist stiffness coefficients between 3.8 and 16 Nm.rad -1 and viscosity between 0 and 0.11 Nm.s.rad -1 at 15%MVC flexion for various oscillation amplitudes. In a later study, these workers reported damping coefficients at the wrist in the range of 0.06-0.075 Nm.s.rad -1 during voluntary movement (Milner and Cloutier, 1998). In the current study at 15%MVC, the wrist stiffness coefficients ranged between 2.63 and 16.44 Nm.rad -1 and damping coefficients ranged between 0.02 and 0.19 Nm.s.rad -1 .…”

“…In monitoring joint torque, however, the level of co-contraction of muscles operating across the joint was not controlled and could have varied with displacement amplitude. Co-contraction is a strategy utilised to increase joint stiffness (De Serres and Milner, 1991;Milner et al, 1995) and to a lesser degree, damping (Milner and Cloutier, 1998). Hence, unobserved changes in muscle cocontraction could have influenced joint mechanics in these previous studies.…”

“…This is followed by a sudden yield, after which the muscle stiffness remains constant at a lower level (Rack and Westbury, 1974). Studies of intact joint mechanics (with stretch reflex input) have reported that stiffness decreases as displacement amplitude increases, both in humans (Kearney and Hunter, 1982;Milner and Cloutier, 1998;Sinkjaer et al, 1988) and decerebrate cats (Nichols, 1985). Kearney and Hunter (1982) found a decrease in the stiffness of the ankle with increasing displacement up to 14° peak to peak amplitude, while Sinkjaer et al (1988) found that ankle stiffness decreased only between 1° and 2° of displacement and did not change from 2° to 7°.…”

Optimal Control of Redundant Muscles in Step-Tracking Wrist Movements

“…The measured moment of inertia of the wrist and manipulandum during stabilization of a wrist targeted movement ranges from 0.00322 to 0.00541 kgm 2 with an average of 0.00469 kgm 2 (Grey 1997). A similar value was also reported in Milner and Cloutier (1998). Although the viscosity and stiffness of the wrist still remain to be fully measured, the natural viscosity of the relaxed wrist was estimated to be 0.02-0.03 Nms/rad (Gielen and Houk 1984), consistent with the average viscosity of 0.03 Nms/rad estimated in the above stabilization study (Grey 1997).…”