Cardiac frequency in miners recorded during four to five work shifts

Abstract

Cardiac frequency (f _c) was recorded in 101 coal-face miners [mean age 32.7 (range 21–49) years, mean height 169.6 (range 150–185) cm, mean body mass 76.9 (range 54–106) kg, mean maximal cardiac frequency (f _cmax) 180.2 (range 154–197) beats min⁻¹, mean maximal oxygen uptake$(\dot VO_{2max} )$ 2.93 (range 1.9–4.0) l · min⁻¹] with a small-size, nonintrusivef _c counter, during the five (n = 76) or at least four (n = 25) work-shifts in a week. Thef _cmax and$\dot VO_{2max}$ were determined during a progressive test to exhaustion on a treadmill. Overall (four to five work-shifts) mean work-timef _c (f _c) was 97.7 (range 74.9–122.4) beats · min⁻¹, restingf _c (f _{c rest}) 59.1 (range 50–75) beats · min⁻¹, work-time increase inf _c (f _c —f _{c rest}) 38.5 (range 21.1–55.8) beats · min⁻¹ and percentage off _c reserve used (f _{c reserve} =f _{c max} —f _crest)32.0(range 18.5–50.9). Multiple regression analysis showed thatf _c andf _c —f _{c rest}, as dependent variables, correlated with predicted percentage of CO lung diffusion capacity, (D _LCO_SB%) (r = −0.334 andr = −0.273, respectively) but not with age,f _{c max},$\dot VO_{2max}$,$\dot VO_{2max}$ · kg⁻¹ body mass, effort test Δoxygen uptake/Δf _c or percentage of forced expiratory volume in 1 s as independent variables. The percentage off _{c reserve} used as dependent variable, correlated withD _LCO_SB (r = − 0.265) andf _{c max} (r = − 0.227) but not with any of the other variables listed. Individual differences in worktimef _c are thus large and virtually unpredictable. Other physiological variables not taken in account here (i.e. sense of effort, fatigue perception) as well as psychological ones (work satisfaction, motivation) may have played a role in those differences. At peaks of effort, some subjects reachedf _c values within the range off _{c max}.