In‐series compliance of gastrocnemius muscle in cat step cycle: do spindles signal origin‐to‐insertion length?

Abstract

SUMMARY1. It has been claimed that stretch in the non-contractile (extramysial) portion of muscles is substantial, and may produce large discrepancies between the origin-toinsertion muscle length and the internal length variations 'seen' by muscle spindle endings.2. In eight pentobarbitone-anaesthetized cats, we estimated stretch in the extramysial portion of medial gastrocnemius (MG) muscle with a method similar to the spindle null technique.3. Length variations of MG previously monitored in a normal step cycle were reproduced with a computer-controlled length servo. The responses of test MG spindle endings were monitored in dorsal root filaments. Distributed stimulation of ventral root filaments, rate-modulated by the step-cycle EMG envelope, served to reproduce step-cycle forces. The filaments were selected so as to have no fusimotor action on the test spindle.4. Spindle responses in active cycles were compared with those in passive cycles (stretch, but no distributed stimulation). In some cases concomitant tonic fusimotor stimulation was used to maintain spindle responsiveness throughout the cycle, both in active and passive trials. Generally, small discrepancies in spindle firing were seen. The passive trials were now repeated, with iterative adjustments of the length function, until the response matched the spindle firing profile in the active trial. The spindle 'saw' the same internal length change in the final passive trial as in the active trial. Any difference between the corresponding length profiles was attributed to extramysial displacement.5. Extramysial displacement estimated in this way was maximal at short mean muscle lengths, reaching about 05 mm in a typical step cycle (force rising from 0 to 10 N). At longer mean muscle lengths where muscle force rose from say 2 to 12 N in the cycle, extramysial displacement was in the range 02-0A4 mm.6. Except at very short lengths, the displacement was probably mainly tendinous. On this assumption, our results suggested that the stiffness of the MG tendinous J. ELEK AND OTHERS compartment was force related, and about double that of cat soleus muscle at any given force. Calculations indicated that though the stretch was small, the MG tendon would store and release enough strain energy per cycle to contribute significantly to the E3 phase of the step cycle. The discrepancies in spindle firing were generally quite subtle, so we reject the claim that extramysial stretch poses a serious difficulty for inferences about fusimotion from chronic spindle afferent recordings.

In vivo behaviour of human muscle tendon during walking

“…Our results, however, show that passive elastic mechanisms may operate in walking too. Other authors have reached similar conclusions using di¡erent in situ and in vivo methodologies (Elek et al 1990;Gri¤ths 1991;Prilutsky et al 1996).…”

Changes in pennation with joint angle and muscle torque: in vivo measurements in human brachialis muscle.

“…Slack muscles may need to shorten to less than optimum fibre length before the tendon becomes sufficiently stretched to sustain large tensions; yet, at these lengths the muscle fibres are less capable of generating force. It has already been established that intrafusal muscle fibres can become slack at physiological lengths, and that this influences the ability of muscle spindles to act as length transducers (Morgan, Prochazka & Proske, 1984;Elek, Prochazka, Hulliger & Vincent, 1990). Slackness in extrafusal muscle will also influence the length changes seen by muscle spindles and hence the proprioceptive and reflex roles ascribed to their afferent fibres (Jahnke et al 1989;Hagbarth et al 1994).…”

Ensemble firing of muscle afferents recorded during normal locomotion in cats

“…slightly so that spindles 'see' slightly less of the origin-toinsertion length (Hoffer et al 1989;Elek, Prochazka, Hulliger & Vincent, 1990;Griffiths, 1991). At the end of the stance phase, the tendon is unloaded and therefore shortens slightly, adding a small component of length to the spindles that is not registered by the length transducer.…”