Aged humans often exhibit an impaired defense of core temperature during cold stress. However, research documenting this response has typically used small subject samples and strong cold stimuli. The purpose of this study was to determine the responses of young and older subjects, matched for anthropometric characteristics, during mild cold stress. Thirty-six young (YS; 23 +/- 1 years, range 18-30) and 46 older (OS; 71 +/- 1 years, range 65-89) subjects underwent a slow transient cold air exposure from a thermoneutral baseline, during which esophageal (T(es)) and mean skin temperatures (T(sk)), O(2) consumption, and skin blood flow (SkBF; laser-Doppler flowmetry) were measured. Cold exposure was terminated at the onset of visible sustained shivering. Net metabolic heat production (M(net)), heat debt, predicted change in midregion temperature (DeltaT(mid)), and tissue insulation (I(t)) were calculated. Cutaneous vascular conductance (CVC) was calculated as laser-Doppler flux/mean arterial pressure and expressed as percent change from baseline (DeltaCVC(%base)). There were no baseline group differences for T(es), but OS M(net) was lower (OS: 38.0 +/- 1.1; YS: 41.9 +/- 1.1 W . m(-2), P < 0.05). T(es) was well maintained in YS but fell progressively in OS (P < 0.01 for all timepoints after 35 min). The skin vasoconstrictor response to mild cold stress was attenuated in OS (42 +/- 3 vs. 53 +/- 4 DeltaCVC(%base), P < 0.01). There were no group differences for T(sk) or I(t), while M(net) remained lower in OS (P < 0.05). The DeltaT(mid) did not account for the drop in T(es) in OS. Healthy aged humans failed to maintain T(es); however, the mechanisms underlying this response are not clear.

Evidence suggests that core temperatures of approximately 40 degrees C can induce fatigue, although this may be confounded by coincident elevations in skin temperatures and maximal cardiovascular strain. In an observational field study to examine core temperature threshold for fatigue, we investigated whether running performance is impaired when rectal temperature (T(re)) is >40 degrees C and skin temperature remains modest. Seventeen competitive runners (7/10 women/men: 8 km best 1,759 +/- 78/1,531 +/- 60 s) completed 8-km track time trials in cool (WBGT approximately 13 degrees C; n = 6), warm (WBGT approximately 27 degrees C; n = 4), or both (n = 7) conditions. T(re), chest skin temperature, and heart rate were logged continuously; elapsed time was recorded every 200 m. Running velocity for T(re) >40 degrees C was compared with that for T(re) <40 degrees C for each runner. Changes in running velocity over the last 600 m were compared between runners with T(re) >40 degrees C and <40 degrees C. Twelve runners achieved T(re) >40.0 degrees C with >or=600 m remaining (range 600-3,400 m). Average running velocity for T(re) <40 degrees C (282 +/- 27 m/min) was not different from that for T(re) >40 degrees C (279 +/- 28 m/min; P = 0.82). There were no differences in running velocity during the final 600 m between runners with final T(re) >40 degrees C or <40 degrees C (P = 0.16). Chest skin temperature ranged from 30 to 34 degrees C, and heart rate was >95% of age-predicted maximum. Our observation that runners were able to sustain running velocity despite T(re) >40 degrees C is evidence against 40 degrees C representing a "critical" core temperature limit to performance.

Sweating threshold temperature and sweating sensitivity responses are measured to evaluate thermoregulatory control. However, analytic approaches vary, and no standardized methodology has been validated. This study validated a simple and standardized method, segmented linear regression (SReg), for determination of sweating threshold temperature and sensitivity. Archived data were extracted for analysis from studies in which local arm sweat rate (m(sw); ventilated dew-point temperature sensor) and esophageal temperature (T(es)) were measured under a variety of conditions. The relationship m(sw)/T(es) from 16 experiments was analyzed by seven experienced raters (Rater), using a variety of empirical methods, and compared against SReg for the determination of sweating threshold temperature and sweating sensitivity values. Individual interrater differences (n = 324 comparisons) and differences between Rater and SReg (n = 110 comparisons) were evaluated within the context of biologically important limits of magnitude (LOM) via a modified Bland-Altman approach. The average Rater and SReg outputs for threshold temperature and sensitivity were compared (n = 16) using inferential statistics. Rater employed a very diverse set of criteria to determine the sweating threshold temperature and sweating sensitivity for the 16 data sets, but interrater differences were within the LOM for 95% (threshold) and 73% (sensitivity) of observations, respectively. Differences between mean Rater and SReg were within the LOM 90% (threshold) and 83% (sensitivity) of the time, respectively. Rater and SReg were not different by conventional t-test (P > 0.05). SReg provides a simple, valid, and standardized way to determine sweating threshold temperature and sweating sensitivity values for thermoregulatory studies.

: Physically demanding occupations (ie, military, firefighter, law enforcement) often use fitness tests for job selection or retention. Despite numerous individual studies, the relationship of these tests to job performance is not always clear. : This review examined the relationship by aggregating previously reported correlations between different fitness tests and common occupational tasks. : Search criteria were applied to PUBMED, EBSCO, EMBASE and military sources; scoring yielded 27 original studies providing 533 Pearson correlation coefficients (r) between fitness tests and 12 common physical job task categories. Fitness tests were grouped into predominant health-related fitness components and body regions: cardiorespiratory endurance (CRe); upper body, lower body and trunk muscular strength and muscular endurance (UBs, LBs, TRs, UBe, LBe, TRe) and flexibility (FLX). Meta-analyses provided pooled r's between each fitness component and task category. : The CRe tests had the strongest pooled correlations with most tasks (eight pooled r values 0.80-0.52). Next were LBs (six pooled r values >0.50) and UBe (four pooled r values >0.50). UBs and LBe correlated strongly to three tasks. TRs, TRe and FLX did not strongly correlate to tasks. : Employers can maximise the relevancy of assessing workforce health by using fitness tests with strong correlations between fitness components and job performance, especially those that are also indicators for injury risk. Potentially useful field-expedient tests include timed-runs (CRe), jump tests (LBs) and push-ups (UBe). Impacts of gender and physiological characteristics (eg, lean body mass) should be considered in future study and when implementing tests.

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