Recovery of Neuromechanical Properties of the Human Plantarflexors after Long-term Spaceflight

Exposure to μG is known to induce a decrease in muscle strength in animals as well as in humans, principally in postural muscles (Edgerton & Roy, 1996). This is due to atrophy and induces mechanical adaptations affecting contractile (Thomason & Booth, 1990) and elastic properties (Canon & Goubel, 1995). Recently, changes in contractile and elastic properties of the human plantarflexors immediately after spaceflight (3-6 months) were reported (Lambertz et al., 2001). In terms of neurophysiological adaptations, changes in neural drive are usually considered to be present after exposure to μG (Koryak, 1998; Antonutto et al., 1999). Moreover, changes in elastic properties should account for lower T reflexes despite an hypothesized increase in synaptic efficiency (Anderson et al., 1999). The aim of the present study was to investigate changes in neuromechanical properties of the human plantarflexors when they return to a 1G environment just after long-term spaceflight and a few days after landing. For this purpose, a special ankle ergometer was designed to investigate both muscular contractile and elastic properties, as well as reflex excitability during a given experimental session. The determination of the muscular mechanical properties was made using isometric contractions, isokinetic movements and quick-release tests, respectively. Reflex excitability of pre-activated muscles was tested by sinusoidally evoked stretch reflexes at different frequencies.