Thermoregulation and markers of muscle breakdown in malignant hyperthermia susceptible volunteers during an acute heat tolerance test.

OBJECTIVES: The study was undertaken to compare the thermal and biochemical responses to a heat tolerance test (HTT) of malignant hyperthermia (MH) susceptible individuals, volunteers who have suffered heat illness (HI) and control volunteers. METHODS: Three groups of male volunteers (n=6 in each group) were recruited to the study: MHS - civilian volunteers previously diagnosed as MH susceptible; EHI - military volunteers with a history of exertional HI; CON - military volunteers with no history of HI or MH. For the HTT, volunteers walked on a treadmill at 60% maximal oxygen uptake in a hot environment. Measurements were made of core and skin temperatures, heat flow, whole body sweat rate and serum lactate, creatine kinase and myoglobin concentrations. RESULTS: There were no differences in deep body temperature, oxygen uptake or serum lactate and creatine kinase concentrations between the three groups. One MHS volunteer and two EHI volunteers failed to achieve thermal balance with rectal temperature continuing to rise throughout the test and reaching 39.5°C, the rectal temperatures of the other volunteers plateaued at a mean (SD) of 38.7 (0.4)°C demonstrating thermal tolerance on this test. Serum myoglobin concentration and the increase in serum myoglobin was higher in MHS than EHI and CON Post HHT (P<0.05). CONCLUSION: MH susceptibility does not always predispose an individual to heat intolerance during an acute HTT, but does appear to increase muscle breakdown. The inclusion of serum myoglobin measurements to a HTT may help to distinguish patients that are potentially MHS, and who otherwise demonstrate thermal tolerance.

Rebreather Unit to Prolong Underwater Survival Time.

INTRODUCTION: This study investigated whether the timing of activation affects the utility of an emergency underwater rebreather unit (RBU) when submerged in cold water. METHOD: On two successive occasions, 16 male UK Royal Marines were submerged in stirred water at 12.2°C for up to 78 s. The subjects were lowered (taking 18 s) into the water in a seated position and were instructed to take a large breath in, activate the unit, breath-hold for as long as possible, exhale into the unit, and breathe normally to and from the unit for the remainder of submersion. On one occasion the subjects were instructed to activate the RBU when the water reached chest height (Condition-1) and, on the other, prior to the feet entering the water (Condition-2). Measurements were made of the duration of breath-hold, rebreathing and submersion, exhaled oxygen and carbon dioxide concentrations, skin temperature, and heart rate. RESULTS: In 16 of the 32 submersions, the breath-hold was released before the subject became fully submerged and in 8 submersions the subject requested early withdrawal from the water. Mean (SD) breath-hold duration was 14.0 (13.8) s and the duration of rebreathing was 45.9 (21.9) s. The duration of breath-hold once completely submerged was longer in Condition-1 (9.1 s) than Condition-2 (4.1 s). CONCLUSIONS: The study indicates the RBU should be activated just before the mouth becomes submerged rather than before entering the water, and that the RBU will prolong underwater stay time, thereby increasing survival prospects. House CM, Shaw AM, Roiz de Sa DG. Rebreather unit to prolong underwater stay time, thereby increasing survival prospects.

The impact of a phase-change cooling vest on heat strain and the effect of different cooling pack melting temperatures.

Cooling vests (CV) are often used to reduce heat strain. CVs have traditionally used ice as the coolant, although other phase-change materials (PCM) that melt at warmer temperatures have been used in an attempt to enhance cooling by avoiding vasoconstriction, which supposedly occurs when ice CVs are used. This study assessed the effectiveness of four CVs that melted at 0, 10, 20 and 30 °C (CV0, CV10, CV20, and CV30) when worn by 10 male volunteers exercising and then recovering in 40 °C air whilst wearing fire-fighting clothing. When compared with a non-cooling control condition (CON), only the CV0 and CV10 vests provided cooling during exercise (40 and 29 W, respectively), whereas all CVs provided cooling during resting recovery (CV0 69 W, CV10 66 W, CV20 55 W and CV30 29 W) (P < 0.05). In all conditions, skin blood flow increased when exercising and reduced during recovery, but was lower in the CV0 and CV10 conditions compared with control during exercise (observed power 0.709) (P < 0.05), but not during resting recovery (observed power only 0.55). The participants preferred the CV10 to the CV0, which caused temporary erythema to underlying skin, although this resolved overnight after each occurrence. Consequently, a cooling vest melting at 10 °C would seem to be the most appropriate choice for cooling during combined work and rest periods, although possibly an ice-vest (CV0) may also be appropriate if more insulation was worn between the cooling packs and the skin than used in this study.

User trial and insulation tests to determine whether shock-absorbing insoles are suitable for use by military recruits during training.

A user trial was undertaken to determine whether a shock-absorbing insole is suitable for military use. Two thicknesses of insole (3 mm and 6 mm) were studied and were issued to 38 Royal Marine recruits to wear in their military boots for weeks 12 to 30 of training. Biomechanical measurements showed that both thicknesses of insole significantly (p < 0.05) attenuated the peak pressures generated at heel strike and during forefoot loading when new (relative to a no-insole condition) and that this was well maintained after wear. This was supported by mechanical tests conducted on the insoles. It was concluded that the insoles are sufficiently durable for military use. The main user complaint was that water retention reduced the comfort of the insoles; however, insulation tests conducted with a foot manikin indicated that switching from the current-issue Saran insoles to the trial insoles would not increase the risk of recruits sustaining nonfreezing cold injuries to their feet.

Biomechanical analysis of running in military boots with new and degraded insoles.

PURPOSE: The purpose of the present study was to investigate the influence of degradation using repeated impacts on the ability of different shock-absorbing insoles to reduce peak impact loading during running in military boots. METHODS: Four insole types were degraded mechanically to simulate typical running loads that occur during approximately 100 km of running. The influence of insole mechanical degradation on stiffness and impact-absorbing ability was assessed using standard test procedures. The ability of new and degraded insole samples to reduce peak impact loading during running was assessed by monitoring peak impact force and rate of loading. In addition, the influence of insoles on sagittal plane kinematics was quantified by measurement of hip, knee, and ankle joint flexion. RESULTS: Insole mechanical degradation resulted in an increase in mechanical stiffness and a decrease in ability to reduce mechanical impacts for all test insoles. Measurements taken during running indicated that only one insole type reduced peak impact loading when new, as indicated by a significant (P< 0.05) reduction in peak rate of loading. The ability of this insole type to reduce peak rate of loading during running was maintained after mechanical degradation. This insole was also found to significantly (P< 0.05) reduce peak ankle dorsiflexion. CONCLUSION: The present study identifies an insole type that reduces peak rate of loading during running both when new and when mechanically degraded. It is suggested that this indicates an insole that could potentially reduce the frequency of overuse injuries. Based on these results, this insole is recommended for use in the investigation of the practical use of insoles by military recruits, particularly for study of the influence on injury occurrence.

The influence of simulated wear upon the ability of insoles to reduce peak pressures during running when wearing military boots.

Mechanical degradation of three types of shock absorbing insoles equivalent to 100-130 km of running did not reduce their ability to attenuate the peak pressures generated during running when wearing military boots. Pressure measurements at the heel and forefoot were recorded with pressure measuring insoles placed in the boots of nine subjects. Two of the three insoles tested reduced the peak pressures (P<0.05) generated at the heel and forefoot relative to the no-insole (control) condition. The most effective insole reduced the peak pressures at the heel by 37% and at the forefoot by 24%.