Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC80
Energy expenditure during exposure to +Gz hypergravity wearing two different types of anti-G trousers
D. Siddons1,2, A. Stevenson2,3, D. Lythgoe2, J. Scott2
1. University of Leeds, Leeds, United Kingdom. 2. Human Sciences, QinetiQ Ltd., Farnborough, United Kingdom. 3. Centre of Human & Aerospace Physiological Sciences, King's College London, London, United Kingdom.
Fast-jet aircrew are exposed to inertial forces many times that of gravity (+Gz). The resultant head-level hypotension creates a risk of +Gz-induced loss of consciousness (G-LOC). In response, aircrew employ a physically demanding anti-G straining manoeuvre (AGSM) consisting of muscle contractions of the lower limbs and abdomen, and a Valsalva-like breathing manoeuvre. Anti-G trousers (AGT) provide protection against increased +Gz and should reduce the required effort of the AGSM. We evaluated the effect of two AGT types on oxygen uptake (VO2) associated with resisting the effects of increased +Gz. Six male, experienced, human centrifuge subjects completed discrete exposures to + 6, 7 and 8 Gz (1 G●s -1 onset rate, 15s at plateau G) and a 2 min simulated air combat manoeuvre (SACM) consisting of three +7.4 Gz peaks separated by +4 Gz nadirs (all 10s at plateau G). In two separate sessions, subjects wore a full-coverage (FC-AGT) or a partial-coverage anti-G trouser (PC-AGT), with inflating air bladders covering 95% and 30% of the body surface below the umbilicus respectively. Participants performed AGSM as required to maintain clear vision. Resting +Gz tolerance (RGT) was assessed by reference to each participant's subjective point of visual ‘grey-out’ prior to each exposure set. Respiratory mass spectrometry and a mixing box/tracer gas technique were used to measure VO2 and expired CO2 (VCO2). Heart rate (HR) was also measured. RGT improved in FC-AGT compared with PC-AGT (5.9 ± 1.2 vs 5.3 ± 0.6 +Gz, P < 0.05). ANOVA revealed a significant effect of AGT type and +Gz level on VO2, VCO2 and HR, with a significant AGT*+Gz level interaction observed for VO2 (P < 0.01) but not VCO2 (P = 0.09) or HR (P = 0.23). In PC-AGT, VO2, VCO2, and HR increased from baseline to peak at 1.4 ± 0.2 l●min-1, 1.2 ± 0.3 l●min-1 and 166 ± 11 bpm respectively at +8Gz. At +8Gz, FC-AGT significantly reduced the increase in VO2 (29%), VCO2 (26%) and HR (18%). There was a similar but less pronounced effect at +7Gz (VO2 [15%], VCO2 [15%] and HR [19%]). At +6Gz, only the increase in HR was significantly attenuated with FC-AGT (15%). During the SACM, VO2 (17%), VCO2 (25%) and HR (23%) were lower (P < 0.05) with FC-AGT. VO2 and HR were correlated with PC-AGT (Spearman’s, ρ=0.86, P < 0.05) but not FC-AGT (ρ=0.44, P = 0.07). Thus, FC-AGT improved RGT and attenuated the energy cost of resisting the effects of increased +Gz, notably above +6 Gz. The reduced energy cost in FC-AGT may delay the onset of fatigue in aircrew during repeated AGSMs and reduce the risk of G-LOC. Hence, FC-AGT might be more appropriate in aircraft capable of sustaining high (> +6 Gz) accelerations. The different relationships between VO2 and HR may reflect alternate mechanisms operating to increase HR; HR principally increased through baroreceptor activation in FC-AGT and through exercise in PC-AGT.
Where applicable, experiments conform with Society ethical requirements