Relationship of HR to VO2 to determine the validity of measuring methods in the field
Heart rate increases linearly as a function of workload intensity and is closely related to oxygen uptake (Arts and Kuipers
1994; Gastinger et al.
2010). Nevertheless, the value of the HR/VO
2 relationship can vary between individuals due to metabolic stress or physical training level and therefore should be ascertained individually (Skinner et al.
2003). Similarly, interindividual differences are observed when CPX and field measurements during work are compared. To determine the physiological workload of physically demanding work, we investigated the relation between HR and oxygen uptake under field conditions. We could demonstrate that the HR/VO
2 relationship was linear not just during the incremental cycle exercise test (CPX) but also in their usual working environment with climatic and other factors. Nevertheless, the range of the correlation coefficients shows that HR is more strongly correlated to VO
2 during CPX (
R 0.89,
p < 0.001) than during field measurement (
R 0.78,
p < 0.001) (Fig.
3a, b). Yet, there is a significant correlation between HR and VO
2 in the field measurement, and furthermore, a congruent increasing and decreasing profile could be demonstrated (Fig.
5).
Due to different proportions between HR and VO
2, a method based on heart rate reserve (%HR
R) and VO
2 reserve (%VO
2,R) is widely used for the comparison of relative values. Swain and Leutholtz (
1997) and recent studies by Lounana et al. (
2007) have shown that %HR
R data at group level are consistent with %VO
2,R. We aimed to find out whether this correlation can also be validated for our group in CPX and especially in spiroergometric field measurement under working conditions. For the incremental exercise testing, we can confirm a substantial correlation of %HR
R and %VO
2,R with
R 0.96 (
p < 0.01) (Fig.
4a). For spiroergometric field measurement, we found a lower correlation of
R 0.78 (
p < 0.001). Both regressions show that %HR
R does not overestimate %VO
2,R as their intercepts are close to 0 (Fig.
4b). Possible reasons for lower field correlations between HR and VO
2 could be malfunctioning in gathering the individual values, different load shapes, and the varyingly high intensity of physical strain of muscle groups with differing efficiency. During work as a refuse collector, especially arm work is performed, while the incremental cycle exercise consists mostly of legwork. A better equivalence between %HR
R and %VO
2,R for legwork than for arm work has been described by (Rotstein and Meckel
2000). Additionally, HR can be impaired by further factors, such as temperature, emotion, and physical fitness status (Achten and Jeukendrup
2003). We nevertheless could demonstrate an equivalence between absolute values of HR and VO
2, and equally in relative calculations to HR
R and VO
2,R in dynamic work, even if it was measured in the field.
Fitness and workload capacity evaluated by various thresholds and aspects
Because the VT reflects the workload threshold beyond which endurance exercise will not lead to anaerobic metabolism, it can therefore be regarded as the upper limit of intensity during the endurance performance (Binder et al.
2008). The present study showed a high endurance performance for the entire sample during 1 h of work and also during the whole work shift, depending on the HR measurement with a mean of 86.7 % HR
VT and 79.4 % HR
VT, respectively. The percentage of VO
2 during 1 h of work in percentage of VO
2,VT was likewise, but lower, with a mean of 60 % VO
2,VT. In Fig.
5, which shows a representative measurement from the field tests, the subject’s HR well exceeded most of the time the individual HR
VT. Similarly, VO
2,VT was exceeded several times. For individual values relative to the VT (%HR
VT, %VO
2,VT), our data show that %HR
VT may overestimate the real workload; %VO
2,VT seems to be more realistic (see Table
5). Furthermore, VT not only differs between individuals but also varies depending on the state of training and the type of exercise protocol (Faude et al.
2009). Therefore, the question arises whether %HR
VT is comparable to %VO
2,VT. We would recommend to determine VT and likewise the HR
VT and VO
2,VT, by CPX in the laboratory. This will enable an accurate estimate of %VO
2,VT during the field measurement.
In our sample, the CPX results are close to the individual predicted and age-dependent values (Table
2). Kroidl et al. (
2014) have described the requirements for high, normal, and pathological endurance performance, based on values at VT > 80 %, around 60 %, and <40 % of maximal values, respectively. In comparison, our subjects also reached performance levels in the upper range of normal endurance (Table
2). In the present study, workers show normal ranges of individual fitness. Long work periods with a high level of physical activity did not lead to an increase in maximal oxygen uptake, and only slightly better endurance performance was observed in them. This seems to be compatible with results from previous studies which also investigated workers with heavy workload (Ilmarinen et al.
1991; Søgaard et al.
1996).
It is commonly suggested that 33–40 % of the individual’s VO
2,max should be the capable workload for 8 h of physical work (Åstrand et al.
2003; Ilmarinen
1992). But %VO
2,max depends on the type of exercise performed. According to Kemper et al. (
1990), the acceptable limit for refuse collecting work in particular, which mainly consists of arm work combined with legwork, should be at 30 % VO
2,max for an 8-h shift. To describe the exercise intensity in our sample, we took HR
sh at a given %VO
2,max. This method is according to Skinner et al. (
2003); they have demonstrated that once VO
2,max and the relationship among HR and VO
2 are known, the corresponding HR is a good estimate for relative workload. Taking the mean HR values of the 41 subjects in our study who had undergone HR
sh measurement, there was a slight exceedance (mean HR 100.2 b/min) of the standards of calculated mean HR value at 30 % VO
2,max (96.6 b/min); ns). Here, 66 % of the individuals had mean HR
sh values above 30 % VO
2max. Frings-Dresen and Kemper 1995, under laboratory conditions, showed that 33–59 % of the subjects, depending on the waste collector activity (bags, different container volumes), exceeded the 30 % of VO2max.
Comparing these results with the oxygen uptake of the 13 individuals from the 1-h VO2 measurement, the means even exceeded the reference of 30 % VO2,max significantly (mean VO2,1h 1103 ml/min vs. calculated VO2 at 30 % VO2,max of 737.3 ml/min, p < 0.05). All subjects achieved a mean VO2, which was above the reference limit of 30 % VO2,max, with a total range of 35–69 % VO2,max. These results are consistent with the relation between HR1h and HRsh as the 1-h values were slightly but significantly higher than HRsh. Nevertheless, in both specifications (HR and VO2), very high values have been found, which reflects the high continuous work load of refuse collectors.
In general, exercises that are performed with a HR
R > 30 % for an 8-h shift are assumed to be at high cardiovascular load (Ilmarinen et al.
1991; Shimaoka et al.
1998). With long-term HR measurement for a work shift of 6.7 h, 39 % of residual waste collectors, 33 % of organic waste collectors, and 39 % of the street cleaners had %HR
R,sh values that were higher than 30 % HR
R. These findings are consistent with Kuijer et al. (
1999), who found 36.4 %HR
R for refuse collectors and 22.6 %HR
R for street sweepers. Therefore, we can conclude that refuse collectors and street cleaners have high endurance performance and high cardiovascular load during work.
Åstrand et al. (
2003) specified easy, moderate, and heavy work during an 8-h work shift on the basis of oxygen consumption at <600, 600–1000, and >1000 ml/min VO
2, respectively, and required a maximum VO
2 for work at 40 %VO
2,max at <1500, <1500–2500, and >2500 ml/min, respectively. When compared to Åstrand’s requirements of workload, the refuse collectors in our study had a mean VO
2,1h of 1103 ml/min during work corresponding to 46 % VO
2,max (Table
4) and a mean VO
2,max of 2623 ml/min during CPX corresponding to oxygen uptake under heavy physical work. This confirms Åstrand’s findings; the workload of refuse collectors can be classified in the upper field of heavy work. Whether the relatively high physical endurance is a health risk for the refuse collectors remains open. In our initial cross-sectional study, we found no evidence to this.
Comparison with other occupations
Compared to jobs which are commonly referred to as physically heavy, the relative workload found in this study was rather high. The means for HR
sh and %HR
max (Table
3) during one work shift are consistent with Wultsch et al. (
2012) findings for workers from waste processing (activities were not differentiated). They found mean HR
sh 100 b/min for male and 120 for female, 59 and 65 % HR
max, respectively. Compared to the other investigated professions (workers in metal industry, slaughterhouse work, or healthcare business) referred in this study (Wultsch et al.
2012), our findings on the physical demand of refuse collectors were higher. Compared to a study with housekeepers which also used a portable spiroergometric system for field measurements (MJ Fröhlich, personal communication), we found similar values at HR
1h and VO
2,1h to those they determined with 112 b/min and 1.06 l/min, respectively. However, compared to portable spiroergometric measurements with lumberjacks (Hagen et al.
1993)—their job is considered to be the hardest form of physical work (with 49 % VO
2,max for the younger, 53 % VO
2,max for the older, and a HR
sh of 138 and 126 b/min, respectively)—our measurement results were rather low.
Other studies with refuse collectors have also reported similar HR values to those found in our study. Kemper et al. (
1990) have found a mean HR
sh of 99.5 b/min in Dutch refuse collectors during one work shift, and—compared to the threshold value of 30 % VO
2,max calculated over HR
sh—30 % of their participants had exceeded that limit. Furthermore, they also established a linear relationship between HR and VO
2 during work, but they did not describe this correlation further. In a recent study with Brazilian refuse collectors, Anjos et al. (
2007) outlined a mean HR for the total working time at 97.6 b/min, 53.4 % HR
max, and 32.8 % HR
R; nevertheless, their results were partially lower than those found in the present study. In addition, they identified HR values during the actual working time which can be compared with our values for 1 h of continuous work. A recent Japanese study by Tsujimura et al. (
2012) found mean HR values for garbage collectors of 97.5 b/min, which were similar to the Brazilians but lower than our findings. These studies of refuse collectors, however, determined the workload only by the HR without VO
2 field measurements.