Human motor control consequences of thixotropic changes in muscular short‐range stiffness

Abstract

The primary aim of the present study was to explore whether in healthy subjects the muscle contractions required for unrestrained voluntary wrist dorsiflexions are adjusted in strength to thixotropy‐dependent variations in the short‐range stiffness encountered in measurements of passive torque resistance to imposed wrist dorsiflexions. After a period of rest, only the first movement in a series of passive wrist dorsiflexions of moderate amplitude exhibited clear signs of short‐range stiffness in the torque response. During analogous types of voluntary movements, the extensor EMG during the first movement after rest showed a steep initial rise of activity, which apparently served to compensate for the short‐range stiffness. The passive torque resistance to minute repetitive wrist dorsiflexions (within the range of short‐range stiffness) was markedly reduced after various types of mechanical agitation. During analogous low‐amplitude voluntary wrist dorsiflexions the extensor EMG signals were weaker after than before agitation. Mechanical agitation also led to enhancement of passive dorsiflexion movements induced by weak constant torque pulses. In an analogous way, the movement‐generating capacity of weak voluntary extensor activations (as determined by EMG recordings) was greatly enhanced by mechanical agitation. The signals from a force transducer probe pressed against the wrist flexor tendons ‐ during passive wrist dorsiflexions ‐ revealed short‐range stiffness responses which highly resembled those observed in the torque measurements, suggesting that the latter to a large extent emanated from the stretched, relaxed flexor muscles. During repetitive stereotyped voluntary wrist dorsiflexions, a close correspondence was observed between the degree of short‐range stiffness as sensed by the wrist flexor tension transducer and the strength of the initial extensor activation required for movement generation. The results provide evidence that the central nervous system in its control of voluntary movements takes account of and compensates for the history‐dependent degree of inherent short‐range stiffness of the muscles antagonistic to the prime movers.

The passive stiffness of the wrist and forearm

“…Prior measurements of passive wrist stiffness in flexion and/or extension range from Ͻ0.15 Nm/rad (Lehman and Calhoun 1990) and 0.32-0.7 Nm/rad (Gielen and Houk 1984) to 1 Nm/rad (Axelson and Hagbarth 2001), 2.2 Nm/rad (Leger and Milner 2000), and 3 Nm/rad (De Serres and Milner 1991). The measurements presented in this report fall in the middle of the range of measurements from prior studies.…”

“…This stiffness is mostly due to muscle stretch (Axelson and Hagbarth 2001) and is fundamentally different from the stiffness encountered at the limits of the wrist's range of motion, which is much larger and mostly due to ligament stretch. A recent cadaver study measured passive wrist stiffness in FE and RUD, as in this study, but at the limits of the wrist's range of motion (Crisco et al 2011).…”

“…This is to be expected because the stiffness measured in this study is mostly due to muscle stretch (Axelson and Hagbarth 2001) and increases with muscle cross-sectional area (like springs in parallel), and because the amount of muscle varies widely between subjects. This is similar to the large variability in inertia seen between subjects of different sizes.…”

“…Several studies have measured wrist stiffness in flexion and/or extension (Axelson and Hagbarth 2001;Cornu et al 2003;De Serres and Milner 1991;Gielen and Houk 1984;Halaki et al 2006;Lakie et al 1984;Lehman and Calhoun 1990;Leger and Milner 2000;Milner and Cloutier 1993;Sinkjaer and Hayashi 1989) or radial-ulnar deviation (Rijnveld and Krebs 2007). Although these 1-degree of freedom (DOF) measurements inform us of the dynamics the neuromuscular system must overcome to rotate in pure flexion-extension (FE) or pure radial-ulnar deviation (RUD), the wrist rarely rotates in pure FE or RUD.…”

A dominant role for mechanical resonance in physiological finger tremor revealed by selective minimization of voluntary drive and movement

“…During low levels of activation, the muscle will be very stiff, and the tendon and finger may resonate as a unit at high frequency. When activation is large, the muscle stiffness starts to drop due to its thixotropic nature (Axelson and Hagbarth 2001;Lakie and Robson 1988;Proske et al 1993). Consequently, more of the muscle resonates with the tendon and finger, causing a substantially lower resonant frequency.…”