Day-to-day reliability of basal heart rate and short-term and ultra short-term heart rate variability assessment by the Equivital eq02+ LifeMonitor in US Army soldiers.

INTRODUCTION: The present study determined the (1) day-to-day reliability of basal heart rate (HR) and HR variability (HRV) measured by the Equivital eq02+ LifeMonitor and (2) agreement of ultra short-term HRV compared with short-term HRV. METHODS: Twenty-three active-duty US Army Soldiers (5 females, 18 males) completed two experimental visits separated by >48 hours with restrictions consistent with basal monitoring (eg, exercise, dietary), with measurements after supine rest at minutes 20-21 (ultra short-term) and minutes 20-25 (short-term). HRV was assessed as the SD of R-R intervals (SDNN) and the square root of the mean squared differences between consecutive R-R intervals (RMSSD). RESULTS: The day-to-day reliability (intraclass correlation coefficient (ICC)) using linear-mixed model approach was good for HR (0.849, 95% CI: 0.689 to 0.933) and RMSSD (ICC: 0.823, 95% CI: 0.623 to 0.920). SDNN had moderate day-to-day reliability with greater variation (ICC: 0.689, 95% CI: 0.428 to 0.858). The reliability of RMSSD was slightly improved when considering the effect of respiration (ICC: 0.821, 95% CI: 0.672 to 0.944). There was no bias for HR measured for 1 min versus 5 min (p=0.511). For 1 min measurements versus 5 min, there was a very modest mean bias of -4 ms for SDNN and -1 ms for RMSSD (p≤0.023). CONCLUSION: When preceded by a 20 min stabilisation period using restrictions consistent with basal monitoring and measuring respiration, military personnel can rely on the eq02+ for basal HR and RMSSD monitoring but should be more cautious using SDNN. These data also support using ultra short-term measurements when following these procedures.

'Super boots' for soldiers: theoretical ergogenic and thermoprotective benefits of energetically optimised military combat boots.

Soldiers typically perform physically demanding tasks while wearing military uniforms and tactical footwear. New research has revealed a substantial increase of ~10% in energetic cost of walking when wearing modern combat boots versus running shoes. One approach to mitigating these costs is to follow in the footsteps of recent innovations in athletic footwear that led to the development of 'super shoes', that is, running shoes designed to lower the energetic cost of locomotion and maximise performance. We modelled the theoretical effects of optimised combat boot construction on physical performance and heat strain with the intent of spurring similarly innovative research and development of 'super boots' for soldiers. We first assessed the theoretical benefits of super boots on 2-mile run performance in a typical US Army soldier using the model developed by Kipp and colleagues. We then used the Heat Strain Decision Aid thermoregulatory model to determine the metabolic savings required for a physiologically meaningful decrease in heat strain in various scenarios. Combat boots that impart a 10% improvement in running economy would result in 7.9%-15.1% improvement in 2-mile run time, for faster to slower runners, respectively. Our thermal modelling revealed that a 10% metabolic savings would more than suffice for a 0.25°C reduction in heat strain for the vast majority of work intensities and durations in both hot-dry and hot-humid environments. These findings highlight the impact that innovative military super boots would have on physical performance and heat strain in soldiers, which could potentially maximise the likelihood of mission success in real-world scenarios.