Oxidative stress, antioxidant defenses and nitric oxide production following hyperoxic exposures.

Little data exist on the dose-response relationship between the partial pressure of inspired oxygen (PiO2) and the cellular oxidative stress response in humans. The objective of this study was to determine the effects of PiO2 on lipid peroxidation, antioxidant enzyme activity and nitric oxide (NO) production. Twelve healthy male divers breathed 100% O2 in a hyperbaric chamber for two hours at 1 (101 kPa), 1.5 (152 kPa) and 2 (203 kPa) atmospheres absolute (atm abs). Venous blood was collected pre-, within 15 minutes post-, one and two hours post-hyperoxic exposures to determine changes in plasma and erythrocyte lipid peroxidation (thiobarbituric acid reactive substances-TBARS), antioxidant enzyme activity (superoxide dismutase-SOD), catalase-CAT, glutathione peroxidase-GPx), and plasma NO production (L-arginine [L-Arg], asymmetric dimethylarginine-ADMA, and nitrites). There were minor changes in TBARS and mixed responses in plasma and erythrocyte CAT and GPx activity. Plasma L-Arg increased following 1 and 1.5 atm abs exposures, yet ADMA and nitrites were unchanged. Only erythrocyte CAT and plasma GPx activity, and plasma L-Arg/ADMA demonstrated a significant PiO2 dose-dependent relationship. Two-hour hyperoxic exposures at 1-2 atm abs of O2 results in mixed oxidant-antioxidant responses and unaltered NO production. Moreover, there does not appear to be a strong systemic dose-dependent oxidative stress response at these hyperoxic exposures.

Recreational scuba divers' aversion to low-frequency underwater sound.

Increasing use of active low-frequency sonar by submarines and ships raises the risk of accidental exposure of recreational divers to low-frequency underwater sound (LFS). This study aimed to characterize the subjective responses of recreational scuba divers to LFS to ascertain the extent to which LFS may impact their enjoyment, comfort, or time spent underwater. Seventeen male and nine female recreational scuba divers participated. Diving was conducted in an acoustically transparent tank located within a larger anechoic pool. Subjects wore scuba gear and were positioned I m below the surface in a prone position. The sound transducer was located 4 m directly below the diver's head. Sound exposures consisted of three signal types (pure tone, 30 Hz hyperbolic sweep up, and 30 Hz hyperbolic sweep down) each presented at six center frequencies from 100 to 500 Hz and six sound pressure levels(SPL) ranging from 130 to 157 dB re 1 microPa. The duration of each sound exposure was 7 s. Subjects responded via an underwater console to rate aversion to LFS on a category-ratio scale, and to indicate the presence or absence of vibration of any body part. Aversion to LFS and the percent incidence of vibration increased as the SPL increased. The percent incidence of vibration decreased linearly with increasing frequency. At the highest SPL the probability that an aversion rating would exceed Very Severe (7 on the category-ratio scale) was predicted to be 19%. There was no significant difference in aversion among signal types. The 100 Hz frequency was the most aversive frequency (P < 0.05). A plot of aversion vs. frequency showed a U-shaped function with minimum aversion at 250 Hz. In conclusion, diver aversion to LFS is dependent upon SPL and center frequency. The highest aversion rating was given for 100 Hz, this frequency corresponded with the greatest probability of detecting vibration. Factors other than vibration seem to account for aversion to the highest frequencies. Our data suggest that LFS exposures up to 145 dB re 1 microPa at frequencies between 100 and 500 Hz will have minimal impact on the recreational diver.

Aerobic performance of Special Operations Forces personnel after a prolonged submarine deployment.

The US Navy's Sea, Air and Land Special Operations Forces personnel (SEALs) perform a physically demanding job that requires them to maintain fitness levels equivalent to elite athletes. As some missions require SEALs to be deployed aboard submarines for extended periods of time, the prolonged confinement could lead to deconditioning and impaired mission-related performance. The objective of this field study was to quantify changes in aerobic performance of SEAL personnel following a 33-day submarine deployment. Two age-matched groups of SEALs, a non-deployed SEAL team (NDST, n = 9) and a deployed SEAL team (DST, n = 10), performed two 12-min runs for distance (Cooper tests) 5 days apart pre-deployment and one Cooper test post-deployment. Subjects wore a Polar Vantage NVTM heart rate (HR) monitor during the tests to record exercise and recovery HR. Variables calculated from the HR profiles included mean exercise heart rate (HRmean), maximum exercise heart rate (HRmax), the initial slope of the HR recovery curve (HRrecslope) and HR recovery time (HRrectime). The second pre-deployment test (which was used in the comparison with the post-deployment test) showed a 2% mean increase in the distance achieved compared with the first (n = 18, p < 0.05) with no difference in HRmean, HRmax, HRrecslope and HRrectime. The test-retest correlation coefficient and 95% limits of agreement for the Cooper tests were 0.79 (p < 0.001) and -68.6 +/- 267.5 m, respectively. For the NDST there were no changes in any of the HR measures or the distance run between the pre- and post-deployment tests. When individual running performances were expressed as a percentage change in the distance run between the pre- and post-deployment tests, the DST performed significantly worse than the NDST (p < 0.01). The DST showed a 7% mean decrement in the distance run following deployment (p < 0.01). The decrement in performance of the DST was not associated with any changes in HRmean or HRmax; however; there was a 17% decrease in the HRrecslope, (p < 0.05) and a 47% increase in HRrectime following the deployment (p < 0.05). In conclusion, prolonged confinement aboard a submarine compromises the aerobic performance of SEAL personnel. The resulting deconditioning could influence mission success.

Physiologic and perceptual responses to hypercarbia during warm- and cold-water immersion.

Thermoregulatory, respiratory, and perceptual responses to acute CO2 exposure during light exercise (75 W) were assessed in 12 U.S. Navy divers clad only in swim trunks while immersed to the neck in water at 18 degrees and 34 degrees C. The CO2 exposures consisted of a linear 10-min ramp increase in the inspired fraction of CO2 (FICO2) from 0 to 6% followed by 5 min of breathing 6% CO2. The ability to detect and rate the severity of hypercarbia, as well as subjective changes in thermal comfort, were assessed by comparing subjective ratings given during the CO2 exposures with those given during immersion trials where the FICO2 was maintained at 0%. Hypercarbia was recognized earlier and, at a given PETCO2, was perceived to be greater during cold- than during warm-water immersions (P < 0.01). The CO2 exposures did not affect the thermal balance of divers as assessed by changes in heat flux and rectal temperature. However, increased feelings of warmth were reported during both the cold and warm immersions when breathing raised concentrations of CO2 (P < 0.01). During the cold immersions, acute exposure to 6% CO2 significantly decreased forearm blood flow (P < 0.05), but did not affect finger blood flow. It is concluded that PETCO2 levels can reach unacceptably high concentrations (> 60 mmHg) before hypercarbia can be reliably detected by working divers. Furthermore, the ability to detect hypercarbia is poorer when immersed in warm water than when in cold water.

Diver respiratory responses to a tunable closed-circuit breathing apparatus.

Respiratory impedance of closed-circuit underwater breathing apparatus (UBA) is comprised of resistive, elastic, and inertial elements in series. Impedance is at a minimum when a UBA operates at its resonant frequency (f(n)). This study investigated the respiratory responses of 12 male U.S. Navy divers to changes in the f(n) of a simulated closed-circuit UBA. Respiratory effort, breathing comfort and ventilatory parameters were assessed during open- and closed-circuit breathing at rest and while exercising at 75 W on a bicycle ergometer in the dry at 1 atm abs. During closed-circuit breathing, the f(n) of the system was adjusted to different frequencies between 0.2 Hz and 0.4 Hz (12 and 24 breaths/min) by varying UBA inertance. When the simulated UBA was switched from open to closed-circuit breathing the subjects changed their breathing frequency in a direction toward the f(n) of the system and attempted to maintain minute ventilation constant by adjusting tidal volume. Results suggest that when divers breathe on a closed-circuit system with different f(n)'s they attempt to improve breathing comfort and reduce respiratory effort by adopting a breathing pattern that reduces their peak-to-peak mouth pressures.

Effects of N2O narcosis on breathing and effort sensations during exercise and inspiratory resistive loading.

The influence of nitrous oxide (N2O) narcosis on the response to exercise and inspiratory resistive loading was studied in thirteen male US Navy divers. Each diver performed an incremental bicycle exercise test at 1 ATA to volitional exhaustion while breathing a 23% N2O gas mixture and a nonnarcotic gas of the same PO2, density and viscosity. The same gas mixtures were used during four subsequent 30-min steady-state submaximal exercise trials in which the subjects breathed the mixtures both with and without an inspiratory resistance (5.5 vs. 1.1 cmH2O.s.l-1 at 1 l/s). Throughout each test, subjective ratings of respiratory effort (RE), leg exertion, and narcosis were obtained with a category-ration scale. The level of narcosis was rated between slight and moderate for the N2O mixture but showed great individual variation. Perceived leg exertion and the time to exhaustion were not significantly different with the two breathing mixtures. Heart rate was unaffected by the gas mixture and inspiratory resistance at rest and during steady-state exercise but was significantly lower with the N2O mixture during incremental exercise (P < 0.05). Despite significant increases in inspiratory occlusion pressure (13%; P < 0.05), esophageal pressure (12%; P < 0.001), expired minute ventilation (4%; P < 0.01), and the work rate of N2O mixture, RE during both steady-state and incremental exercise was 25% lower with the narcotic gas than with the nonnarcotic mixture (P < 0.05). We conclude that the narcotic-mediated changes in ventilation, heart rate, and RE induced by 23% N2o are not sufficient magnitude to influence exercise tolerance at surface pressure. Furthermore, the load-compensating respiratory reflexes responsible for maintaining ventilation during resistive breathing are not depressed by N2O narcosis.

The influence of task resistance on the characteristics of maximal one- and two-handed lifting exertions in men and women.

Dynamic lifting strength was assessed at lift velocities ranging from 0 to 1 m.s-1 using a hydrodynamometer in which the viscous resistance to motion could be preset. Nine men and nine women performed one- and two-handed dynamic exertions against low, medium, and high resistances. The subjects were required to lift as forcefully and as fast as possible from a height of 400 mm to just above head height. The instantaneous forces recorded at knee, knuckle, hip, shoulder, elbow, and head heights were compared with those obtained during maximal one- and two-handed isometric lifts performed at the same heights. The position of peak force relative to stature was lower for one-handed lifts (35.9% of stature) than for two-handed lifts (38.4% of stature) (P < 0.005), but was unaffected by lift resistance (P > 0.05) and subject sex (P > 0.05). Lifting forces during two-handed exertions were on average 1.5 times greater than for one-handed exertions (P < 0.0001). However, the difference in the strength of one- and two-handed lifts was highly dependent on hand height (P < 0.0001) and task resistance (P < 0.005), and was greater for the men than for the women (P < 0.001). When static lifting strength was measured at heights relative to stature and normalized by dividing by body weight, there was no significant difference in performance between the men and the women [mean normalized female:male: (f:m) strength ratio = 0.76, P > 0.05]. Under dynamic conditions the sex difference in lifting performance was pronounced (mean normalized f:m strength ratio = 0.68, P < 0.01), especially for power output (mean f:m power.unit body weight-1 ratio = 0.52, P < 0.005). The proportion of generality (r2 x 100) between dynamic and static lifting strength was found to range between 63% and 87%. These data suggest that the dynamic and static tests of lifting strength measured a common, intrinsic ability to produce maximal lifting forces. The finding that sex differences in strength are dependent on whether the exertion is static or dynamic has important implications for biomechanical models of human strength.

System for the triaxial measurement of manual force exertions.

A triaxial force measurement system has been developed to allow the measurement of whole-body manual force exertions in the three axes of the Cartesian system. The design of the system precludes the exertion of torques, thus enabling a description of the linear forces that an individual can exert in three dimensions. Forces are measured independently for each hand, allowing the examination of triaxial forces in both one- and two-handed whole-body exertions, and the analysis of the forces in the closed chain of the upper limbs in two-handed exertions. The hardware and computer software that constitute the triaxial force measurement system are described, as are the method of calibration used and the accuracy of the system. Asymmetric postures and exertions are seen as risk factors leading to musculoskeletal disorders. Measurement and descriptions of manual strength involving asymmetric postures and lateral components of exertion will enable risk analysis of such tasks to be undertaken.

Protocol for the omnidirectional assessment of manual strength.

This study presents a protocol for assessment of manual strength over all directions in three dimensions. The reliability and validity of the protocol was tested on four male subjects. They performed one-handed maximal exertions overthe entire sphere of exertion at hand heights of 1.0 and 1.75 m and at a horizontal foot distance of 0.5 m from the centre of force application. Testing sessions consisted of five 2 min data collection periods interspersed with rest pauses of the subject's own chosen duration. At least four testing sessions performed on separate days were required for a subject to fulfil the criteria for completion of the sphere of exertion. The results showed the methodology to be reliable. Repeat trials at the 1.0 m hand height revealed a high reliability coefficient for the three-dimensional forces (r = 0.96). The protocol was also suitable for assessment of three-dimensional manual strength at the higher hand height of 1.75 m. The distinctive patterns of forces over the sphere of exertion observed at the 1.0 and 1.75 m heights illustrated the importance of hand height on manual strength in three dimensions. The methodology presented in this paper provides a good basis on which to develop future clinical or scientific studies investigating human strength capabilities over all directions.

Human strength capabilities during one-handed maximum voluntary exertions in the fore and aft plane.

Maximal static strengths were determined for one-handed exertions in all directions in the fore and aft plane. Data from 12 males and 10 females (mean age 30.7 yrs, standard deviation (SD) = 8.9 yrs, n = 22) were obtained with handle heights of 1.0 and 1.75 m. Twelve of the subjects also performed two-handed exertions at the same handle heights. The ratio of mean strengths of females to that of males ranged from 0.50 to 0.83 (for absolute forces) and from 0.63 to 1.00 for forces normalized to body weight. The ratios of one-handed to two-handed strengths ranged from 0.64 to 1.04. Two-handed strengths commonly exceeded one-handed strengths at the lower handle height, but showed fewer significant strength differences (p less than 0.05) according to direction at 1.75 m. Both female/male and one-handed/two-handed strength ratios were found to be dependent on direction of exertion and handle height. The observed strength dependencies upon number of hands (one or two-handed), direction of exertion, handle height and sex are discussed. The strength data have implications for use in biomechanical models and task analysis.

Effects of CO2 and N2 partial pressures on cognitive and psychomotor performance.

This study examined N2 and CO2 components of narcosis by comparing the effects of three levels of PETCO2 [low = 29 mmHg (SD = 4 mmHg), medium = 47 mmHg (SD = 1 mmHg), high = 57 mmHg (SD = 2 mmHg)] at 1 and 6 atm abs in 12 male volunteers. Cognitive and psychomotor performances were examined using a variety of tasks, including a modified Stroop test, an arithmetic test, number comparison, a figure copying test, and the Purdue pegboard test. Performance on all tasks demonstrated significant (P less than 0.05) decrements at 6 atm abs. High CO2 tensions significantly impaired cognitive and psychomotor performance at 1 atm abs and caused further decrements at 6 atm abs (P less than 0.05). However, no significant N2-CO2 interaction (P greater than 0.05) or global threshold for the onset of CO2 narcosis was indicated by the test scores. The pattern of intratest results were different for N2 and CO2. At high PETCO2, performance deficits were due to a slowing of performance rather than a disruption of the accuracy of processing. Nitrogen narcosis, conversely, produced significant impairment through both decreases in the speed and accuracy of processing on the majority of performance tests. It was concluded that within the PETCO2 ranges studied: a) PETCO2 and PIN2 are additive in their effects on impaired cognitive and psychomotor performance at depth; b) high PETCO2 and PIN2 induce distinctly different strategic responses on the speed accuracy trade-off function of the performance tests; c) decrements in cognitive and psychomotor performance under high PETCO2 do not conform to the predicted narcotic potency of CO2 according to the lipid solubility theory of narcosis.