Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, C47
Muscle movement while maintaining posture at the human wrist
J. Chew1, I. Loram2, R. Fitzpatrick1
1. Prince of Wales Medical Research Institute, Sydney, New South Wales, Australia. 2. Manchester Metropolitan University, Manchester, United Kingdom.
The ability to control the static posture of a joint depends on the elastic stiffness of the load being held (Chew et al., 2008). Here we investigate at the wrist the muscle and tendon contributions to this postural load-dependence. The Human Research Ethics Committee of the University of New South Wales approved the studies. Subjects (N=6) used the wrist flexor muscles to hold as still as possible three loads of different elastic stiffness (Chew et al., 2008). One had positive-stiffness (i.e. spring like; 0.1 Nm.deg-1) so that more force was required as the wrist flexed, one had negative stiffness (inverted-pendulum like; -0.1 Nm.deg-1) and the third had zero stiffness (isotonic). Each load was held with the wrist in the same posture and at this position each load required the same wrist torque (0.70 Nm) for static balance. Flexor carpi radialis were observed by ultrasound (Acuson 128XP, L7384 linear array), which provided a 38 mm parasagittal view of the full depth of the muscle with a temporal resolution of 25 Hz. Spatial cross correlation of image luminance was used to measure longitudinal tissue velocity of the proximal and distal tendon plates. Using measured changes in wrist angle and the moment arm of the wrist, changes in length of the tendon and contractile portions of the muscle were determined. For the positive- and zero-stiffness loads, the ranges of wrist movement over 30 s were approximately one degree whereas for the negative-stiffness load it was about five degrees. The relationship between contractile length and wrist angle at frequencies below 2 Hz varied with load stiffness. For the positive- and zero-stiffness loads, the muscle was shorter at more flexed wrist angles, with a higher gain for the positive-stiffness relationship. With the negative-stiffness load, the contractile length was longer at more flexed wrist positions. At ~ 2 Hz, a resonant peak in torque and wrist movement is reflected in the tendon length but not in the contractile portion. These load-stiffness effects are not apparent at higher frequencies (> 4 Hz) where the inertial properties of the load dominate and torque and angle are antiphase. A 10 Hz “tremor” oscillation in the EMG, contractile length and tendon length was minimal or absent in wrist angle because the tendon lengthened and shortened antiphase with the contractile portion. We conclude that there is no simple relationship between muscle length and joint angle. Rather, it is a dynamic function of frequency strongly dependent on the elastic stiffness of the load. At the point of resonance, the load and joint oscillate on the end of a compliant tendon without significant transfer of the movement to the contractile portion, thus constituting an effective proprioceptive and control blindspot. This muscle-tendon behaviour is a major determinant of the limits of human postural control.
Where applicable, experiments conform with Society ethical requirements