Postural after‐contractions in man attributed to muscle spindle thixotropy

Abstract

It is an old observation that non‐volitional arm abduction movements accompanied by a sensation of arm lightness often occur as an after‐effect following forceful voluntary arm abductor contractions against a restraint. In the present study we have tested the hypothesis that such non‐volitional, so‐called ‘postural after‐contractions’ are tonic reflex responses to an enhanced resting discharge in primary muscle spindle afferents which in turn is a consequence of thixotropy‐dependent enhanced stiffness of intrafusal muscle fibres. Results obtained in ten volunteers show that the arm abductor after‐contraction phenomenon in man is most readily evoked by a type of conditioning procedure which in various respects mimics the procedure proven in animal experiments to be particularly effective in producing thixotropy‐dependent excitation of primary spindle endings. It is also shown that changes in arm abductor intramuscular temperature affect the strength of the after‐contractions in a direction predicted by the thixotropy hypothesis. Attention is drawn to several similarities between the after‐contraction phenomenon with accompanying sensory illusions and the tonic reflex responses and illusions that can be induced when primary spindle endings are excited by muscle vibration. The results support our hypothesis that postural after‐contractions are induced by activity in primary muscle spindle afferents as a consequence of thixotropic properties of intrafusal muscle fibres. Central excitability changes following the conditioning voluntary effort may contribute to the phenomenon.

Muscle history, fusimotor activity and the human stretch reflex

“…First, in confirmation of what was already known, the size of both the stretch reflex and the H reflex are dependent on the previous form of muscle conditioning (for a review see Proske et al 1993; see also Hagbarth et al 1995;Hagbarth & Nordin, 1998). Given that both reflexes are used routinely as diagnostic tools and both are notoriously variable in size, it is important to remember that their size may be halved or doubled depending on what has happened to the muscle beforehand.…”

Using voluntary motor commands to inhibit involuntary arm movements

“…In the periphery, the sustained voluntary contraction used for the induction generates a barrage of asymmetric proprioceptive inputs from antagonist and agonist muscles that may contribute to the involuntary movement [19,22]. Furthermore, such prolonged sensory activations or alterations in 'muscle history' are also associated with after-effects such as in inaccurate conscious perception of limb position [23].…”

The Activation of Muscle Spindles Enhances the Thixotropic Behavior of Rib Cage Respiratory Muscles.

“…Evidence suggests that voluntary muscle contraction at a muscle length is necessary for the development of muscle thixotropy [3,8]. Voluntary muscle contraction activates ␥ motoneurons via ␣-␥ linkage to maintain continuous feedback from muscle spindles.…”