Changes in Posture Following a Single Session of Long-Duration Water Immersion

Abstract

Transitioning between different sensory environments is known to affect sensorimotor function and postural control. Water immersion presents a novel environmental stimulus common to many professional and recreational pursuits, but is not well-studied with regard to its sensorimotor effects upon transitioning back to land. The authors investigated the effects of long-duration water immersion on terrestrial postural control outcomes in veteran divers. Eleven healthy men completed a 6-hour thermoneutral pool dive (4.57 m) breathing diver air. Center of pressure was observed before and 15 minutes after the dive under 4 conditions: (1) eyes open/stable surface (Open-Stable); (2) eyes open/foam surface (Open-Foam); (3) eyes closed/stable surface (Closed-Stable); and (4) eyes closed/foam surface (Closed-Foam). Postdive decreases in postural sway were observed in all testing conditions except for Open-Stable. The specific pattern of center of pressure changes in the postdive window is consistent with (1) a stiffening/overregulation of the ankle strategy during Open-Foam, Closed-Stable, and Closed-Foam or (2) acute upweighting of vestibular input along with downweighting of somatosensory, proprioceptive, and visual inputs. Thus, our findings suggest that postimmersion decreases in postural sway may have been driven by changes in weighting of sensory inputs and associated changes in balance strategy following adaptation to the aquatic environment.

Vection underwater illustrates the limitations of neutral buoyancy as a microgravity analog

“…Neutral buoyancy impacts other aspects of human perception and performance which may or may not match either Earth-normal behavior or behavior experienced in the environment found in other space analogs including human centrifugation 5 , microgravity aircraft flight 6 – 10 or long-duration bed rest 11 . For example, Glass et al 12 found that participants swayed less following long-duration water immersion compared to performance prior to exposure. They noted that proprioception 13 , somatosensation and vision 14 , possibly because human eyes evolved for viewing in air, are less informative of postural changes underwater and hypothesized that the sway changes may reflect a downweighting of these cues and a concomitant upweighting of vestibular input.…”

Neutral buoyancy and the static perception of upright

“…Neutral buoyancy may impact human perception and performance in a way that may or may not match either Earth-normal behaviour, behaviour in microgravity, or the behaviour found in space analogues such as human centrifugation 2 , microgravity aircraft flight 3 – 6 or long-duration bed rest 7 . Neutral buoyancy also introduces its own complications including distortions in visual perception due to light refraction at the glass-water interface between the participant’s goggles and the water column, and the effects of water pressure on the ears, an effect that is known to contribute to diver disorientation (see 8 and 9 for reviews). Does neutral buoyancy interfere with perceptual systems?…”

“…In space, both somatosensory and vestibular cues to the direction of gravity are compromised but in neutral buoyancy the vestibular cue remains unaffected while only the somatosensory cue, normally provided by pressure at the support surface, is disabled 10 , 11 , but see 8 for the potential of low-level connections between the vestibular and proprioceptive systems which argues for a low-level interaction between the two systems. Here we consider whether lack of the somatosensory cue affects the perception of self-orientation under neutral buoyancy conditions.…”

“…Several studies that have explored the effect of water immersion on balance and the perception of self-orientation. For example, Glass and colleagues 8 investigated the impact of long-term immersion on balance upon return to a non-buoyant environment, and others have compared the perception of self-orientation under neutral buoyancy conditions with responses under terrestrial, Earth-normal conditions 11 , 12 , and 13 . Traditionally, self-orientation experiments have evaluated this perception using the luminous line (LL) test to measure the subjective visual vertical (SVV).…”