Endurance Exercise Performance Is Reduced after 6-h Dives at 1.35 ATA When Breathing 100% Oxygen Compared with Air

ABSTRACT

Introduction

Long-duration dives on consecutive days reduces muscular performance, potentially affecting military personnel. However, a paucity of data exists on how breathing gases affect endurance performance. This study examined the influence of long-duration diving with different breathing gases on aerobic endurance and handgrip performance.

Methods

Twenty-three military divers completed a single 6-h dive (single dive [SD]) and five 6-h dives over consecutive days (dive week [DW]) with 30-min cycling intervals using air (AIR, n = 13) or 100% oxygen (OXY, n = 10). Before and after SD and DW, subjects completed a maximum handgrip strength test, a handgrip endurance test at 40% maximal strength, and a time to exhaustion run.

Results

Handgrip endurance decreased after DW in OXY (SD, 1.9 ± 0.0 vs 1.4 ± 0.3 min) compared with AIR (1.8 ± 0.0 vs 1.8 ± 0.2 min) (P < 0.001). Run time decreased after SD (Pre, 20.7 ± 10.4 min; Post, 16.6 ± 7.6 min; P = 0.039) and DW (Pre, 21.6 ± 9.0 min; Post, 11.2 ± 4.0 min; P < 0.001) in OXY and after overall diving in AIR (Pre, 26.5 ± 10.2 min; Post, 22.3 ± 7.5 min; P = 0.025). V̇O₂ decreased after diving only in AIR (Pre, 42.6 ± 3.4 mL·kg⁻¹⋅min⁻¹; Post, 40.4 ± 3.7 mL·kg⁻¹⋅min⁻¹; P = 0.010). There were no other significant effects.

Conclusions

Breathing 100% oxygen during long-duration dives on consecutive days may exacerbate decreases in aerobic endurance and impairs handgrip endurance compared with air. Additional research is needed to elucidate mechanisms of action and possible mitigation strategies.

The ability to perform strenuous physical activity after long-duration diving on consecutive days may be required by military personnel. However, previous data have shown that long-duration resting dives on five consecutive days reduce some aspects of muscular performance (1). Additionally, depending on the operational objectives, repetitive long-duration diving may involve various forms of physical activity and breathing gases different from the composition of normobaric air, such as 100% oxygen (O₂), which has been postulated to further exacerbate muscular decrements (2,3). Previous investigations on long-duration dives over consecutive days have focused primarily on muscular strength measures, whereas the effects on aerobic endurance performance are not well understood. Given that aerobic endurance is highly emphasized for optimal military operational performance (4), investigating changes in aerobic performance may enhance strategies for postdiving activities yielding to greater mission success.

Diving is known to alter multiple organ systems that may influence physically demanding activities after water egress. During immersion, there is an increase in hydrostatic pressure that causes fluid to shift toward the thorax, which increases central venous pressure, stroke volume, cardiac output, and tidal volume (V_T), while decreasing heart rate, systemic vascular resistance, and respiratory frequency (Rf) (5–7). Although some of these changes appear to be advantageous for aerobic exercise performance, postdiving alterations include vascular and cardiac dysfunction, along with differential augmentation and depression during physiological stressor tests (e.g., cold presser, head-up tilt) (8,9). Further, consecutive day long-duration diving, particularly with 100% O₂, causes pulmonary oxygen toxicity symptoms like inspiratory burning, coughing, chest tightness, and dyspnea, and reduced lung function (10). Moreover, some diving-induced changes may not return to predive values for up to 24 h after diving (1–3,8).

Breathing 100% O₂ for extended periods may also adversely affect physical performance, although data are scarce. Previous work from our laboratory found that during five 6-h resting dives to 4.6 m on consecutive days while breathing 100% O₂ (1.35 atmospheres absolute [ATA] O₂), there were reductions in maximum isometric handgrip, knee extension, and elbow flexion strength; decreases in isokinetic knee extension and elbow flexion strength, along with diminished hand grip endurance; and increased rate of fatigue in the knee extensors when measured at approximately 1-h postdiving (2,3). Further, some of the muscular decrements lasted for at least 24 h postdiving, suggesting a prolonged negative effect of consecutive, long-duration hyperoxic diving. However, we were not able to compare 100% O₂ to air in our previous studies. Under normobaric conditions, breathing 100% O₂ influences metabolic parameters, including reductions in the rate of whole body oxygen consumption and heart rate, and increases RER, which may attenuate aerobic endurance performance (11–13). Additionally, breathing 100% O₂ tends to alter minute ventilation (V̇_E), V_T, and breathing frequency, albeit there are conflicting reports on the directional changes of these values (13–15). However, in most studies, hyperoxic breathing is done acutely (~10–60 min), and values rapidly return to baseline upon breathing normal air. Because consecutive, long-duration diving induces prolonged effects, it is possible that breathing 100% O₂ for longer durations may also elongate the recovery of some metabolic and respiratory parameters.

The primary aim of this study was to examine the influence of long-duration, consecutive day diving and breathing gases (100% O₂ vs air) on aerobic endurance and handgrip performance. We hypothesized that five consecutive, long-duration shallow water dives would significantly diminish endurance performance compared with one dive and that breathing 100% O₂ during either diving scenario would significantly reduce performance compared with air. A secondary purpose was to examine breath-by-breath data obtained from a metabolic system during aerobic performance tests.

METHODS

Subjects

Twenty-three male, healthy, active, normotensive, nonsmoking military divers completed the study. Before participation, each subject underwent medical screening that included complete blood count, complete metabolic panel, lipid profile evaluation, urinalysis, physical examination by an undersea medical officer, seven-site skinfold body fat measurement (16), and a maximal oxygen uptake aerobic exercise test (V̇O_2max) (see Table 1). Subjects wore running shorts and T-shirts during laboratory visits for all exercise testing visits. Approval was obtained from the Institutional Review Board of the Navy Experimental Diving Unit. Each subject gave written informed consent before any participation, and all procedures conformed to the Declaration of Helsinki.

TABLE 1.

Subject characteristics.

	OXY Group (n = 10)	AIR Group (n = 13)
Age (yr)	30 ± 9	32 ± 5
Height (cm)	178 ± 8	182 ± 5
Weight (kg)	84 ± 11	86 ± 11
Body mass index (kg·m−2)	26 ± 3	26 ± 2
Body fat (%)	15 ± 5	16 ± 5
V̇O2max (mL·kg−1⋅min−1)	46 ± 5	46 ± 4

Values represent M ± SD. OXY, 100% oxygen; AIR, air.

As this study was part of larger multiseries project, other parameters such as lung function and autonomic function were analyzed but are not included in this publication.

Study design

The study was conducted in two phases: dives while breathing 100% O₂ (oxygen dives [OXY]) and dives while breathing air (air dives [AIR]). Each subject participated in only one of the two phases. To assess the influence of repeated diving, each phase consisted of a single 6-h dive (single dive [SD]) and five 6-h dives on consecutive days with 18-h surface intervals (dive week [DW]). Ten subjects completed the SD and the DW in OXY, and 13 subjects completed the SD and the DW in AIR. All dives were completed in a 4.6-m (15-ft) deep indoor pool filled with comfortably warm water (30.6 ± 1.7°C [87 ± 3°F]). Subjects wore running shorts and T-shirts for all dives. The SD and the DW were performed in random order and separated by at least 10 d to minimize any residual effects. Study design is illustrated in Figure 1.

FIGURE 1.

Study timeline. CMP, complete metabolic panel; CBC, complete blood count.

For all dives, subjects used an MK 20 full-face mask (AGA mask, Interspiro) configured for surface supplied breathing gas. Before each dive, subjects received a standardized meal containing 1.47 megajoule (MJ) (69% carbohydrate, 19% fat, 12% protein) then donned a condom catheter. Subjects in the OXY group breathed humidified 100% O₂ for a PO₂ of 137.8 kPa, and subjects in the AIR group breathed humidified air for a PO₂ of 29.4 kPa. They alternated between 30-min rest and exercise periods for the 6-h dive duration. Underwater cycle ergometers were configured to mimic the horizontal position of swimming. Cycling loads began at 50 W and were adjusted to elicit a target heart rate of 105 ± 5 bpm as measured by three-lead ECG. If the subject could not maintain a cadence of 60 rpm or if their heart rate increased above the target heart rate, the load was reduced. As a safety precaution, subjects were queried once every hour for symptoms of central nervous system or pulmonary oxygen toxicity during each dive. After 3 h of water immersion, subjects returned to the surface, stood on a platform with head and shoulders out of the water, and removed their breathing mask for 10 min to consume 2.81 MJ of energy (26% fat, 62% carbohydrate, 12% protein) and 500 mL of liquid, which included a supplementation shake (Ensure, Abbott Laboratories, Chicago, IL) and sports drink (Gatorade, Chicago, IL). Approximately 72–96 h before and 18 h after diving, subjects completed a handgrip testing and a time-to-exhaustion run at an estimated workload at 85% of their V̇O_2max, while equipped with a portable metabolic system.

Body Weight

On each dive day, subjects were weighed immediately before the dive (after their morning meal and urination) and immediately after the postdive urination.

V̇O_2max Run

Subjects completed one V̇O_2max test approximately 48–72 h before the first time-to-fatigue run. The modified Balke Protocol was used to assess V̇O_2max. The V̇O_2max value was used to calculate the subsequent time-to-exhaustion run workload. Heart rate and gas exchange parameters were measured using a chest strap (Polar, Bethpage, NY) and portable metabolic system (K4, COSMED, Rome, Italy), respectively. During the test, subjects were asked about their RPE on the Borg 6–20 RPE scale after each stage of the protocol (17). Criteria for a successful V̇O_2max test included a plateau in oxygen consumption with an increased workload, respiratory exchange quotient greater than 1.10, and RPE greater than 17. V̇O_2max was determined from a 10-breath average at plateau.

Maximum Handgrip Strength

Before the time-to-exhaustion run, subjects used a custom-built handgrip dynamometer to determine maximum voluntary contraction (MVC) handgrip strength. Subjects were in a supine position with their elbow at an approximately 90° angle. Thereafter, the dynamometer was adjusted to the subject’s left hand, and they were instructed to squeeze as hard as possible for 3 s. The highest value of three trials was used for analysis.

Handgrip Endurance

After approximately 30 min of rest after the handgrip MVC test, subjects placed the dynamometer in the left hand and maintained 40% of MVC until fatigue using a custom-made visual force feedback system. Handgrip endurance time was recorded from initial force production until the subject failed to maintain 80% of target strength output for 5 s. During the test, subjects were instructed to avoid the Valsalva maneuver as well as leg or abdominal muscle tension.

Time to Exhaustion Endurance Run

Approximately 72–96 h before and 18 h after both the SD and the DW, subjects ran on a treadmill at a calculated workload of approximately 85% of V̇O_{2 max} until voluntary exhaustion using the running V̇O₂ metabolic equation from the American College of Sports Medicine (18). Subjects wore a COSMED K4 portable metabolic system, which provides breath-by-breath data acquisition of Rf, V_T, V̇_E, relative oxygen consumption (V̇O₂), RER, and end-tidal carbon dioxide fraction (F_ECO₂). Subjects also wore a heart rate monitor (Polar) over their chests at the level of the xiphoid process.

The trial began with a warm-up that consisted of a 3-min walk at 3 mph, then a 1-min run at the calculated 85% V̇O_{2 max} workload speed and a 0% incline. Then subjects ran to voluntary exhaustion at the same speed with a 10% grade to equal approximately 85% of the workload of V̇O_2max. Only the data obtained from the 10% grade portion of the run were used for analysis.

Data Reduction

Data from the time to exhaustion run were recorded with OMNIA software (COSMED). Markers were placed at each phase of the run. Data were exported into an Excel sheet (Microsoft Office, Excel 2016, Redmond, WA) and then imported into Matlab (Matlab Version R2021B; The Mathworks, Inc., Natick, MA) where a custom script was used to extract values from the 10% incline portion, which were averaged for analysis. The run time at 10% incline was recorded manually with a stop watch.

Statistical Analysis

Data were tested for outliers using the robust regression and outlier removal method with Q set to 1% (19) and were tested for normality with the Shapiro–Wilk test. When an outlier was identified, investigators reviewed the data point and determined if removal was necessary. When normality was violated, data were log-transformed and normality was retested. If log transformation corrected for the normality violation, then transformed data were analyzed. If log-transformed data still violated normality, a nonparametric equivalent test was performed. However, if a nonparametric equivalent test was not available, then data were analyzed with a parametric test. Outlier removal and normality violations are specified in text. When sphericity was not assumed, ANOVA main effects and interactions were interpreted with the Greenhouse–Geisser correction.

An unpaired two-tailed t-test was used to test for baseline differences in subject characteristics between breathing gas groups. To assess differences in baseline values between conditions, a two-way ANOVA was performed with breathing gas and repeated diving prevalues. A three-way ANOVA was used to examine the interaction between breathing gas (OXY and AIR), repeated diving (SD and DW), and predive and postdive (pre/post). When a three-way interaction occurred, two follow-up two-way ANOVA were performed with the factors repeated diving and pre/post dive, grouped by gas. When a three-way interaction was not present, but there was a two-way interaction, data were consolidated by interaction factors, and a two-way ANOVA was performed.

Sidak-corrected multiple comparisons were used to analyze significant interactions and main effects. The alpha level was set at 0.05. Data are presented as mean (M) ± standard deviation (SD). All analyses were performed with GraphPad Prism version 9.3.0 for Windows (GraphPad Software, San Diego, CA).

RESULTS

Subject Characteristics

There were no significant baseline differences between the OXY and the AIR groups with regard to age, weight, body mass index, body fat, or V̇O_2max. Values for height did not pass the assumption of normality in the OXY group, and log transformation did not correct this. Nevertheless, there were no significant baseline differences in height. Anthropometric data are presented in Table 1.

Body Weight

One subject was removed from the OXY group and two from the AIR group because of equipment malfunction. Body weight decreased after diving (pre/post effect, (F_{1, 18} = 30.65, P < 0.001) but increased with repeated diving (repeated diving effect, F_{1, 18} = 5.744, P = 0.028). There were no other main effects or interactions. Body weight data are found in Table 2.

TABLE 2.

Body weight.

	OXY (n = 9)				AIR (n = 11)
	SD Pre	SD Post	DW Pre	DW Post	SD Pre	SD Post	DW Pre	DW Post
Body weight (kg)a,b	80.5 ± 8.1	79.7 ± 8.1	82.3 ± 9.6	79.8 ± 7.9	86.9 ± 11.5	85.8 ± 11.3	87.8 ± 12.0	86.0 ± 11.4

Values represent M ± SD.

^aPre/Post main effect.

^bRepeated diving main effect.

OXY, 100% oxygen; AIR, air.

Maximum Handgrip Strength

Overall, diving, repeated diving, or gas did not influence maximum handgrip strength. There was a gas–pre/post interaction (F_{1, 21} = 4.468, P = 0.047) but no other interactions or main effects. Data were consolidated into gas and pre/post factors, and after a follow-up two-way ANOVA, the gas–pre/post interaction remained (F_{1, 44} = 5.345, P = 0.026) with no main effects. However, the interaction was not significant with multiple comparisons testing (OXY pre vs post, P = 0.310, 95% confidence interval [CI] = −1.151 to 4.653; AIR pre vs post, P = 0.120, 95% CI = −4.649 to 0.442). M ± SD as well as individual values can be found in Figure 2.

FIGURE 2.

Maximum handgrip strength. OXY, 100% oxygen; AIR, air; pre, predive; post, postdive. Circles represent individual values; bars represent M ± SD.

Handgrip Endurance

Overall, handgrip endurance decreased during repeated diving and was particularly affected in OXY. However, there were no changes from pre- to post-SD for either OXY or AIR. There was a gas–repeated diving interaction (F_{1, 21} = 4.917, P = 0.038) as well as a repeated diving–pre/post interaction (F_{1, 21} = 13.20, P = 0.002) and a repeated diving effect (F_{1, 22} = 10.825, P = 0.003), but there were no other interactions or main effects. Therefore, data were consolidated into gas–repeated diving factors and repeated diving–pre/post factors. For the gas–repeated diving follow-up two-way ANOVA, the same interaction was present (gas–repeated diving interaction, F_{1, 44} = 7.861, P = 0.007) with a repeated diving main effect (F_{1, 44} = 17.305, P < 0.001) but no gas effect. On multiple comparisons analysis, there was a decrease in handgrip endurance time from SD to DW in OXY (OXY, P < 0.001; 95% CI = 0.260–0.780) but not AIR (AIR, P = 0.523; 95% CI = −0.127 to 0.329). For the repeated diving–pre/post two-way ANOVA, the same interaction was present (repeated diving–pre/post interaction, F_{1, 22} = 13.75, P = 0.001) with a repeated diving main effect (F_{1, 22} = 7.777, P = 0.011) but no pre/post effect. Multiple comparisons analysis revealed that handgrip endurance performance decreased during the DW (P = 0.023; 95% CI = 0.038–0.561) but not during the SD (P = 0.999, 95% CI = −0.160 to 0.166). M ± SD values, including individual values, are presented in Figure 3.

FIGURE 3.

Handgrip endurance time. A. Gas–repeated diving. OXY, 100% oxygen; AIR, air. * P < 0.05. Triangles represent individual values; bars represent M ± SDs. * P < 0.05. B. Repeated diving–pre/post. Pre, predive; post, postdive. Circles represent individual values; bars represent M ± SD. * P < 0.05.

Run Time to Exhaustion

Run time to exhaustion decreased after SD and DW in OXY and after diving in general (main dive effect) in AIR. Run time to exhaustion data (Fig. 4) failed the assumption of normality, which was corrected by log transformation. There was a three-way interaction (gas–repeated diving–pre/post interaction, F_{1, 21} = 10.98, P = 0.003). On follow-up two-way ANOVA by breathing gas, OXY failed the assumption of normality and was corrected by log transformation. OXY had a significant repeated diving–pre/post interaction (F_{1, 9} = 29.67, P < 0.001) and decreased run times after diving (pre/post effect, F_{1, 9} = 5.378, P = 0.046) and after DW (repeated diving effect, F_{1, 9} = 45.18, P < 0.001). Multiple comparisons testing revealed a significant decline after diving after both SD and DW (SD pre- vs postdive, P = 0.039, 95% CI = 0.005–0.182; DW pre- vs postdive, P < 0.001, 95% CI = 0.195–0.370). The AIR group no longer had a significant repeated diving–pre/post interaction, but the postdive run times remained shorter than the predive run times (pre/post effect, F_{1, 12} = 6.559, P = 0.025).

FIGURE 4.

Run time to exhaustion. A. OXY group run performance. OXY, 100% oxygen; pre, predive; post, postdive. B. AIR group run performance. AIR, air. Circles represent individual values; bars represent M ± SD. * P < 0.05.

Respiratory Parameters

One subject in AIR was identified as an outlier and removed from analysis.

Rf

There were no significant interactions or main effects between any of the three factors. M ± SD can be found in Table 3.

TABLE 3.

Respiratory parameters during time to exhaustion endurance run.

	OXY (n = 10)				AIR (n = 12)
	SD Pre	SD Post	DW Pre	DW Post	SD Pre	SD Post	DW Pre	DW Post
Rf (breaths per minute)	44.2 ± 7.8	44.7 ± 7.2	44.7 ± 7.7	44.1 ± 10.0	40.7 ± 7.3	41.1 ± 7.8	40.1 ± 7.4	40.1 ± 7.0
VT (BTPS, L)	2.8 ± 0.6	2.6 ± 0.7	2.8 ± 0.5	2.8 ± 0.6	2.9 ± 0.4	2.9 ± 0.5	2.9 ± 0.5	2.9 ± 0.4
V̇E (BTPS, mL·min−1)	121 ± 16	113 ± 26	121 ± 16	120 ± 20	114 ± 16	115 ± 17	115 ± 17	114 ± 16

Values represent M ± SD.

OXY, 100% oxygen; AIR, air; BTPS, body temperature pressure saturated.

V_T

Overall, diving, diving, or gas did not affect V_T. There was a gas–repeated diving–pre/post interaction (F_{1, 20} = 5.416, P = 0.031). For the OXY follow-up two-way ANOVA, there was a repeated diving–pre/post interaction (F_{1, 9} = 10.58, P = 0.010) but no main effects. However, the interaction was not significant with multiple comparisons testing (SD pre- vs postdive, P = 0.244, 95% CI = −0.130 to 0.556; DW pre vs postdive, P = 0.860, 95% CI = −0.262 to 0.179). A follow-up two-way ANOVA for AIR (data did not pass the assumption of normality and were log-transformed) indicated there were no interactions or main effects. M ± SD are located in Table 3.

V̇_E

There were no significant interactions or main. M ± SD are presented in Table 3.

Metabolic Parameters

Five subjects were identified as outliers in AIR for V̇O₂, F_ECO₂, and RER and removed from analysis. Additionally, two subjects in OXY and one subject in AIR were identified as outliers for HR and removed from analysis.

V̇O_2.

V̇O₂ decreased in AIR after the DW

There was a gas–repeated diving–pre/post interaction (F_{1, 15} = 5.226, P = 0.037). For the follow-up two-way ANOVA for OXY, data did not pass the assumption of normality and were not corrected by log transformation. There was no interaction or main effects. The two-way ANOVA for AIR demonstrated a decrease after diving (pre/post effect, F_{1, 6} = 13.73, P = 0.010), which was only present in the dive week (DW pre- vs postdive, P = 0.004, 95% CI = 1.522–5.605), but no repeated diving–pre/post interaction or repeated diving effect. M ± SD are presented in Table 4.

TABLE 4.

Metabolic parameters during time to exhaustion endurance run.

	OXY (n = 10)				AIR (n = 7)
	SD Pre	SD Post	DW Pre	DW Post	SD Pre	SD Post	DW Pre	DW Post
V̇O2 (STPD, mL·kg−1⋅min−1)	42.2 ± 4.3	39 ± 6.5	41.5 ± 3.6	40.7 ± 2.9	41.8 ± 2.7	41.0 ± 4.5	43.4 ± 4.1	39.8 ± 2.8*
RER	1.10 ± 0.07	1.09 ± 0.06	1.09 ± 0.06	1.1 ± 0.06	1.03 ± 0.13	1.06 ± 0.14	1.01 ± 0.09	1.07 ± 0.07
FECO2 (%)	4.1 ± 0.36	3.99 ± 0.37	4.00 ± 0.33	4.05 ± 0.55	4.15 ± 0.68	4.16 ± 0.54	4.19 ± 0.46	4.11 ± 0.49
HR (beats per minute)a	175 ± 13	171 ± 12	172 ± 8	173 ± 10	175 ± 7	173 ± 8	173 ± 10	176 ± 8

Values represent M ± SD.

*Significantly decreased compared with DW Pre.

^aOXY (n = 8), AIR (n = 11).

OXY, 100% oxygen; AIR, air; V̇O₂, oxygen consumption; STPD, standard temperature pressure dry; HR, heart rate.

F_ECO₂

There were no significant interactions or main effects. M ± SD are presented in Table 4.

RER

There were no significant interactions or main effects. M ± SD are found in Table 4.

HR

Overall, diving, repeated diving, or gas did not affect HR. Data did not pass the assumption of normality, and log transformation did not correct the violation. There was a repeated diving–pre/post interaction (F_{1, 17} = 10.57, P = 0.005) but no other interactions or main effects. Data were consolidated into repeated diving and pre/post factors for a follow-up two-way ANOVA, which demonstrated the same interaction (repeated diving–pre/post interaction, F_{1, 18} = 10.91, P = 0.004) with no main effects. However, the interaction was not significant with multiple comparisons testing (SD pre- vs postdive, P = 0.077, 95% CI = −0.269 to 5.747; DW pre- vs postdive, P = 0.236, 95% CI = −4.875 to 1.006). HR data are found in Table 4.

DISCUSSION

This study demonstrates that breathing 100% O₂ at 1.35 ATA during repeated 6-h exercise dives adversely affects parameters of endurance performance compared with breathing air. Specifically, breathing 100% O₂ resulted in decrements to handgrip endurance after the DW compared with the SD, and time to exhaustion during aerobic exercise was reduced after SD and DW. Breathing air did not influence handgrip endurance, but there was a decrease in time to exhaustion after diving. Interestingly, V̇O₂ decreased after diving when breathing air; however, there were no other alterations to metabolic or respiratory parameters for either group. Additionally, there were no changes in maximum handgrip strength for either group. These findings suggest that undergoing 6-h dives with exercise negatively affects aerobic endurance performance; yet, breathing 100% oxygen incurs these deficits sooner, in addition to reducing muscular endurance.

Handgrip maximal strength performance

Neither breathing gas nor repeated diving had an effect on maximal handgrip strength. Although maximal handgrip force does not seem to change after a single 6-h water immersion to 4.5 m (9), previous data have demonstrated fluctuations during five consecutive resting dives at shallow depths, particularly on days 2 and 3, but ultimately return to baseline after the fifth dive (3). Furthermore, maximum handgrip strength was not affected in divers after a dry chamber dive to 1.6 ATA while breathing 100% O₂ for 3 h (20). However, increased diving depth may influence strength, as decreases in adductor pollicis MVC have been recorded while diving to 30.60 ATA, which is greater than 22 times the pressure in the present study (1.35 ATA) (21). Indeed, this decrement was transient and returned to values comparable with baseline controls by approximately nine ATA. Those authors suggest that the decline in MVC could be a result of breathing denser than normal gas, leading to interactions within the central nervous system between the respiratory afferent nerves and the cortical muscle drive to the skeletal muscles. Taken together, these data suggest that maximal handgrip strength is not significantly diminished in shallow dives that include moderate physical activity.

Endurance performance

Despite similar performances in handgrip strength, handgrip endurance was affected by repeated diving and breathing gas. Overall, breathing 100% O₂ during a single and five consecutive, long-duration dives negatively influenced handgrip endurance compared with breathing air. These findings are in accordance with previous work from our laboratory that found a similar DW (five consecutive 6-h hyperoxic water immersions) led to a decrement in handgrip endurance performance and lower body muscular endurance (50-repetition maximal isokinetic knee extension test) (2). Moreover, our previous work also found that oxygen availability and muscular excitation were not altered during the DW, indicating that these are not mechanisms of performance decreases (2). However, hyperoxia can increase reactive oxygen species (ROS) in skeletal muscle, which is a causal contributor to muscle fatigue because of changes in calcium kinetics (22,23). Previous data do suggest that a 20-min hyperoxic (PO₂ of ~142 kPa) dive in warm water (31–32°C) with mild exercise can significantly increase oxidative stress (24). Further, single and repeated (9–12 dives) hyperoxic (PO₂ of ~120 kPa) dives can elevate inflammatory markers and may also influence physical performance (25). Therefore, prolonged exposure to ROS from hyperoxia may precondition skeletal muscle to a state of fatigue thereby reducing performance. Nonetheless, the present study expands on our previous findings by incorporating exercise during diving, along with comparing the effects of 100% oxygen to air. Collectively, these data suggest that repeated hyperoxic resting or exercising water immersions decrease muscular endurance.

Similar to the handgrip endurance findings, the particular breathing gas had a significant effect on aerobic run time to exhaustion. The OXY group exhibited performance decrements after both SD and DW, whereas the AIR group displayed overall decreases after diving. These findings suggest that although diving has a negative effect on time to exhaustion, breathing 100% O₂ may potentiate these effects. Notably, both tests of endurance performance rely on aerobic metabolism for optimal performance because of the time course of the test (>3 min) (26). On the other hand, tests of maximal strength, like the maximal handgrip test, likely rely on the phosphocreatine system, as each contraction lasts approximately 3 s (27). It is possible that repeated diving and hyperoxic exposure differentially affect these pathways, causing endurance decrements but not strength decrements. Although previous data on hyperoxia have found the inhibition of Kreb cycle enzymes and decreases in mitochondrial complex activity and respiration in animal and isolated cells (28,29), our previous work did not find changes in muscle oxidative capacity, as measured via near-infrared spectroscopy, during a DW with 100% O₂ (2). Therefore, alterations to mitochondrial function may not be a significant contributor to performance changes. As previously mentioned, alterations to calcium kinetics as a result of increased muscular ROS during hyperoxic environments may be a source of fatigue; however, this requires further investigation. Consequently, decreased aerobic performance could reduce a diver’s ability to carry required loads like equipment or assisting in carrying an injured individual.

Another potential factor that may have contributed to endurance performance decreases is fatigue due to the combination of long-duration diving and intermittent exercise. However, follow-up testing was completed 18 h after diving, which should have at least partially attenuated the potential effects of fatigue on endurance performance. Although fatigue from the study procedures likely influenced endurance performance decrement, the data strongly suggest that breathing 100% oxygen during the dives amplified the effect. The AIR group experienced a 2% increase and 12% decrease in handgrip endurance in the SD and the DW, along with a 13% and 19% reduction in aerobic endurance in the SD and the DW, respectively. Meanwhile, the OXY group experienced larger decreases in performance, especially during repeated diving, and displayed 3% and 25% decreases in handgrip endurance in the SD and DW, in addition to 20% and 48% reductions in aerobic endurance in the SD and DW, respectively.

Respiratory parameters

There were no changes to V_T, Rf, or V̇_E; although there was an interaction for V_T, follow-up pairwise comparisons found no differences. Moreover, it is unlikely that the observed changes in V_T have physiological relevance, as mean values fell within well-established exercise ranges for both trained athletes and untrained subjects (30). Indeed, previous data have demonstrated alterations in V_T and breathing frequency because of immersion-related fluid shifts (5–7). Additionally, hyperoxia during acute (between 5 and 10 min) exercise results in decreased respiratory rate and ventilation volume (13–15). Previous data on hyperoxic diving have demonstrated that shorter duration (20–147 min) dives, even on consecutive days, generally do not alter spirometry measured pulmonary function (24,31) but may result in extravascular water accumulation after repeated diving, which returns to baseline within 24 h (31). However, there is a well-established dose-dependent relationship with breathing hyperoxic gas and pulmonary complications, as our laboratory has shown that 6-h dives with 100% oxygen on five consecutive days increased pulmonary oxygen toxicity symptoms (e.g., inspiratory burning, coughing, chest tightens, dyspnea) and reduced lung function (10). It is possible that the 18-h window after diving before completing postdive testing in both post-SD and post-DW was ample time for the acute immersion or hyperoxic-influenced changes of the respiratory system to return to baseline. Another possibility is that pulmonary oxygen toxicity symptoms and spirometry measures are not related to aerobic exercise performance as measured in this study. Taken together, the data suggest that the mechanism for the attenuated performance after diving with O₂ is not based on appreciable respiratory system-driven changes.

Metabolic parameters

Interestingly, V̇O₂ decreased after the DW during the aerobic performance test in the AIR group, with no changes in the OXY group nor any other measured metabolic parameter. Although there are limited data on hyperoxic diving, reduced oxygen consumption after diving may be due to alterations in the autonomic nervous system and, consequently, vascular function. Previous work from our laboratory demonstrated that a 6-h water immersion while breathing air increased sympathetic nervous system activity, specifically leading to peripheral vascular resistance and a decrease in peripheral blood flow (i.e., forearm and calf muscles) (9). Interestingly, breathing 100% O₂ during the same conditions attenuated the decrease in peripheral blood flow compared with breathing air (32). It is possible that oxygen consumption during exercise after repeated long-duration diving with air was decreased as a result of decreased peripheral blood flow to the working muscles and, more generally, autonomic alterations after water immersions breathing air. Despite decreased oxygen consumption, the AIR group performed similarly in this test of aerobic performance. Therefore, the observed decreases in oxygen consumption may be inconsequential for performance in the aerobic test used in this study.

Additionally, there were no alterations in F_ECO₂ or RER and only slight changes in HR during the endurance run to exhaustion. Although diving can result in hypercapnia because of decreased V̇_E, increased physiological dead space, and the Haldane effect (33), we did not observe any changes during aerobic exercise. There were also no significant changes to F_ECO₂ between any of the dive scenarios. F_ECO₂ values were within the normal range of 3%–5%, which was indicative of normal respiratory drive. Moreover, there were no changes in exercise RER, and mean values are in accordance with many previously published studies (34,35), suggesting that there were not shifts in whole substrate for metabolism. There were modest changes in HR; however, the mean values throughout all dive conditions were within 5 bpm, and heart rate values were all within normal exercise limits.

Study design limitations

The results of this study should be interpreted with its limitations in mind. First, performance measurements were only taken after an SD and after five 6-h dives on consecutive days, and therefore, the exact point where performance decrements occurred could not be determined. Additionally, the time course for recovery of performance changes could not be obtained because performance measurements were only completed at 18 h after diving. Moreover, this study included only healthy young men. Although military personnel are primarily males, future research should focus on sex-related differences to ensure the applicability to a broader population, especially as the number of female military divers increases. Another consideration is that these dives were conducted in a controlled environment within an enclosed pool to a single depth, which does not represent open water conditions. However, this design allowed us to better isolate the effects of water immersion and breathing gas by controlling for environmental variables. Finally, food intake after completing the 6-h dives was not controlled; however, the standardized breakfast and lunch on dive days likely reduced much of the variation that might be attributed to food consumption.

CONCLUSIONS

The ability to perform aerobic endurance running is reduced after long-duration repeated diving, and breathing 100% O₂ seems to exacerbate the effects and diminish muscular endurance. These findings have important implications for divers, especially military and commercial diving populations who may be required to perform strenuous physical tasks, such as carrying equipment or injured personnel, after long-duration hyperoxic dives. Further research is warranted for mechanistic explanations for these decrements, as the data do not suggest respiratory or whole body metabolic parameters are causal. Additionally, there are very few data on the effects of long-duration diving on physical performance, and therefore, there may be age-, sex-, or training-related differences that are not yet recognized. Further, future research should consider the influence of water temperature or other environmental factors on both physical and cognitive performance.