The First Lactate Threshold Is a Limit for Heavy Occupational Work

Abstract

Long-term heavy physical work often leads to early retirement and disability pension due to chronic overload, with a need to define upper limits. The aim of this study was to evaluate the value of the first lactate threshold (LTP₁) as a physiological marker for heavy occupational work. A total of 188 male and 52 female workers performed an incremental cycle ergometer test to determine maximal exercise performance and the first and second lactate (LTP₁; LTP₂) and ventilatory thresholds (VT₁; VT₂). Heart rate (HR) recordings were obtained during one eight-hour shift (HR_8h) and oxygen uptake was measured during 20 minutes of a representative work phase. Energy expenditure (EE) was calculated from gas-exchange measures. Maximal power output (P_max), maximal oxygen consumption (VO_{2 max}) and power output at LTP₁ and LTP₂ were significantly different between male and female workers. HR_8h was not significantly different between male and female workers. A significant relationship was found between P_max and power output at LTP₁. HR_8h as a percentage of maximum HR significantly declined with increasing performance (P_max:r = -0.56; p < 0.01; P_LTP1:r = -0.49; p < 0.01). Despite different cardio-respiratory fitness-levels; 95.4% of all workers performed their usual work below LTP₁. It is therefore suggested that LTP₁ represents the upper limit for sustained heavy occupational work; which supports its use to determine work capability and assessing the limits of heavy occupational work.

Keywords: heavy work; individualization; intensity threshold; maximum oxygen uptake; work strain.

Figure 1

Time course of blood lactate concentration (La) during an incremental cycle ergometer test and the lactate shuttle mechanisms (modified from Tschakert and Hofmann [20]). M—muscle; S—system; P—lactate production; E—lactate elimination; EM—muscular elimination; PM—muscular production; ES—systemic elimination.

Figure 2

Relationship between power output at the first lactate threshold (P_LTP1) and maximum power output (P_max) for male (m) and female (f) workers.

Figure 3

Correlation between the maximum power output (P_max) related to the target standard P_max (P_max-target) and percentage of maximum heart rate (% HR_8h) during 8 h occupational work for male (m) and female (f) workers. Dotted lines represent age-predicted maximum target cycle ergometer power output (black) and heart rate at the first lactate turn point (LTP₁, red).

Figure 4

Eight-hour average heart rate (HR, shaded red) and 20-min representative oxygen uptake (VO₂, shaded green) during 8 h of occupational work related to HR and VO₂ from incremental cycle ergometer exercise. Phases of metabolism are divided by lactate turn points (LTP₁, LTP₂) according to Hofmann et al. [29]. Workloads below LTP₁ represent occupational work, intensities above LTP₁ represent sport activities (N = 240).

Figure 5

Published limits for heart rate (HR) as a percentage of the maximum HR (% HR_max, red) and oxygen uptake as a percentage of the maximum oxygen consumption (% VO_{2 max}, green) for heavy occupational work related to cycle ergometer exercise reference data. Lactate thresholds (LTP₁, LTP₂) were determined according to Hofmann et al. [29], from original data [5]. Workloads below LTP₁ represent occupational work, strain levels above LTP₁ are defined as sport activities.