Characteristic changes of motor unit activity in hip joint flexor muscles during voluntary isometric contraction during water immersion

The rubber hand illusion in microgravity and water immersion

Our body has evolved in terrestrial gravity and altered gravitational conditions may affect the sense of body ownership (SBO). By means of the rubber hand illusion (RHI), we investigated the SBO during water immersion and parabolic flights, where unconventional gravity is experienced. Our results show that unconventional gravity conditions remodulate the relative weights of visual, proprioceptive, and vestibular inputs favoring vision, thus inducing an increased RHI susceptibility.

Cellular Responses of Human Postural Muscle to Dry Immersion

Support withdrawal has been currently considered as one of the main factors involved in regulation of the human locomotor system. For last decades, several authors, including the authors of the present paper, have revealed afferent mechanisms of support perception and introduced the concept of the support afferentation system. The so-called "dry immersion" model which was developed in Russia allows for suspension of subjects in water providing the simulation of the mechanical support withdrawal. The present review is a summary of data allowing to appreciate the value of the "dry" immersion model for the purposes of studying cellular responses of human postural muscle to gravitational unloading. These studies corroborated our hypothesis that the removal of support afferentation inactivates the slow motor unit pool which leads to selective inactivation, and subsequent atony and atrophy, of muscle fibers expressing the slow isoform of myosin heavy chain (which constitutes the majority of soleus muscle fibers). Fibers that have lost a significant part of cytoskeletal molecules are incapable of effective actomyosin motor mobilization which leads to lower calcium sensitivity and lower range of maximal tension in permeabilized fibers. Support withdrawal also leads to lower efficiency of protective mechanisms (nitric oxide synthase) and decreased activity of AMP-activated protein kinase. Thus, "dry" immersion studies have already contributed considerably to the gravitational physiology of skeletal muscle.

Cardiorespiratory responses to aquatic treadmill walking in patients with rheumatoid arthritis

BACKGROUND AND PURPOSE

Hydrotherapy is popular with patients with rheumatoid arthritis (RA). Its efficacy as an aerobic conditioning aid is equivocal. Patients with RA have reduced muscle strength and may be unable to achieve a walking speed commensurate with an aerobic training effect because the resistance to movement increases with speed in water. The physiological effects of immersion may alter the heart rate-oxygen consumption relationship (HR-VO2) with the effect of rendering land-based exercise prescriptions inaccurate. The primary purpose of the present study was to compare the relationships between heart rate (HR), and ratings of perceived exertion (RPE), with speed during land and water treadmill walking in patients with RA.

METHOD

The study design used a two-factor within-subjects model. Fifteen females with RA (47+/-8 SD years) completed three consecutive bouts of walking for five minutes at 2.5, 3.5 and 4.5 km/h(-1) on land and water treadmills. Expired gas, collected via open-circuit spirometry, HR and RPE were measured.

RESULTS

HR and RPE increased on land and in water as speed increased. Below 3.5 km/h(-1) VO2 was significantly lower in water than on land (p<0.01). HR was lower (p<0.001), unchanged and higher (p<0.001) at 2.5, 3.5 and 4.5 km/h(-1) in water than on land. RPE was significantly higher in water than on land (p<0.05). VO2 was approximately 60% of the predicted VO2max during the fast walking speed in water. For a given VO2, HR was approximately nine beats/min(-1) and RPE 1-2 points on the 6-20 Borg scale, higher in water than on land.

CONCLUSIONS

The study showed that the metabolic demand of walking at 4.5 km/h(-1) was sufficient to stimulate an increase in aerobic capacity. The use of land-based prescriptive norms would underestimate the metabolic cost in water. Therefore, in water HR should be increased by approximately 9 beats/min(-1) to achieve similar energy demands to land treadmill walking.