Disuse of the musculo-skeletal system in space and on earth.

Muscle mass and strength are well known to decline in response to actual and simulated microgravity exposure. However, despite the considerable knowledge gained on the physiological changes induced by spaceflight, the mechanisms of muscle atrophy and the effectiveness of in-flight countermeasures still need to be fully elucidated. The present review examines the effects and mechanisms of actual and simulated microgravity on single fibre and whole muscle structural and functional properties, protein metabolism, tendon mechanical properties, neural drive and reflex excitability. The effects of inflight countermeasures are also discussed in the light of recent advances in resistive loading techniques, in combined physical, pharmacological and nutritional interventions as well as in the development of artificial gravity systems. Emphasis has been given to the pioneering work of Pietro Enrico di Prampero in the development of artificial gravity systems and in the progress of knowledge on the limits of human muscular performance in space.

Knee angle-dependent oxygen consumption during isometric contractions of the knee extensors determined with near-infrared spectroscopy.

Fatigue resistance of knee extensor muscles is higher during voluntary isometric contractions at short compared with longer muscle lengths. In the present study we hypothesized that this would be due to lower energy consumption at short muscle lengths. Ten healthy male subjects performed isometric contractions with the knee extensor muscles at a 30, 60, and 90 degrees knee angle (full extension = 0 degrees ). At each angle, muscle oxygen consumption (m.VO2) of the rectus femoris, vastus lateralis, and vastus medialis muscle was obtained with near-infrared spectroscopy. m.VO2 was measured during maximal isometric contractions and during contractions at 10, 30, and 50% of maximal torque capacity. During all contractions, blood flow to the muscle was occluded with a pressure cuff (450 mmHg). m.VO2 significantly (P < 0.05) increased with torque and at all torque levels, and for each of the three muscles. m.VO2 was significantly lower at 30 degrees compared with 60 degrees and 90 degrees and m.VO2 was similar (P > 0.05) at 60 degrees and 90 degrees . Across all torque levels, average (+/- SD) m.VO2 at the 30 degrees angle for vastus medialis, rectus femoris, and vastus lateralis, respectively, was 70.0 +/- 10.4, 72.2 +/- 12.7, and 75.9 +/- 8.0% of the average m.VO2 obtained for each torque at 60 and 90 degrees . In conclusion, oxygen consumption of the knee extensors was significantly lower during isometric contractions at the 30 degrees than at the 60 degrees and 90 degrees knee angle, which probably contributes to the previously reported longer duration of sustained isometric contractions at relatively short muscle lengths.

Early structural adaptations to unloading in the human calf muscles.

AIM: The present study investigated the influence of muscle architectural changes on muscle torque during 3-week unilateral lower limb suspension (ULLS). METHODS: Plantarflexion maximal voluntary contraction (MVC), soleus (SOL), gastrocnemius medialis (GM) and lateralis (GL) muscle volume (VOL), GL fascicle length (L(f)) and pennation angle (theta), physiological cross-sectional area (PCSA), and electromyographic (EMG) activity were assessed in eight healthy men (aged 19 +/- 0 years) after days 14 and 23 of ULLS. RESULTS: After 14 day of ULLS, MVC and SOL EMG decreased (P < 0.05) by 10% and 29%, respectively, but did not further decline between days 14 and 23. SOL, GM and GL muscle VOL decreased by 5%, 6% and 5%, respectively (P < 0.05), on day 14, and by 7% (SOL), 10% (GM) and 6% (GL) on day 23. In GL, theta and L(f) were reduced by 3% (P < 0.05) and 2% (NS), respectively, on day 14, and by 5% (P < 0.05) and 4% (P < 0.05), respectively, on day 23. Consequently, GL PCSA declined by 3% (P < 0.05) on day 14, but did not further decrease on day 23. Similarly, the 7% (P < 0.05) loss in GL force/PCSA observed on day 14 persisted until the end of the unloading period. CONCLUSION: These findings suggest that rapid muscle architecture remodelling occurs with lower limb unloading in humans, with changes occurring within 14 days of weight bearing removal. These adaptations, mitigating the decline in muscle PCSA, might protect from a larger loss of muscle force.

Soleus T reflex modulation in response to spinal and tendinous adaptations to unilateral lower limb suspension in humans.

AIM: To investigate the influence of tendinous and synaptic changes induced by unilateral lower limb suspension (ULLS) on the tendon tap reflex. METHODS: Eight young men underwent a 23-day period of ULLS. Muscle cross-sectional area (CSA), torque and electromyographic (EMG) activity of the plantar flexor muscles (normalized to the M wave), Achilles tendon-aponeurosis mechanical properties, soleus (SOL) H and T reflexes and associated peak twitch torques were measured at baseline, after 14 and 23 days of ULLS, and 1 week after resuming ambulatory activity. RESULTS: Significant decreases in muscle CSA (-9%), in maximal voluntary torque (-10%) and in the associated SOL EMG activity (-16%) were found after ULLS (P < 0.05). In addition to a 36% (P < 0.01) decrease in tendon-aponeurosis stiffness, normalized H reflex increased by 35% (P < 0.05). An increase in the slope (28%, P < 0.05) and intercept (85%, P < 0.05) of the T reflex recruitment curve pointed to an increase in the gain and to a decrease in the sensitivity of this reflex, possibly resulting from the decrease in the tendon-aponeurosis stiffness at low forces. Following ULLS, changes in tendinous stiffness correlated with changes in neuromuscular efficiency (peak twitch torque to reflex ratio) at higher tendon tap forces. CONCLUSION: These findings point out the dual and antagonistic influences of spinal and tendinous adaptations upon the tendon tap reflex in humans under conditions of chronic unloading. These observations have potential implications for the sensitivity of the short-latency Ia stretch response involved in rapid compensatory contractions to unexpected postural perturbations.