Background and Objective:
All previous studies assessing the physical workload of skidder operators have merely reported a single overall average value, and little research has been conducted on the differences in physiological strain across the individual components of skidding operations. However, skidding with a rubber-tired skidder is a cyclic process consisting of six distinct and sequential work elements (travel empty, maneuvering, winching, travel loaded, unloading, and sorting/piling). Due to differences in movement patterns, vibration exposure, required concentration, and physical effort, each of these elements imposes varying cardiovascular strain on the operator. Given this research gap, the primary objective of the present study was to introduce and test a novel methodological approach based on the disaggregated analysis of skidding work elements to more accurately assess the physical workload of skidder operators and to identify critical workload peaks for targeted ergonomic interventions.
Materials and Methods:
This study was conducted in loblolly pine (Pinus taeda) plantation forests in western Guilan Province (Compartment 12, Pilambra Series, Rezvanshahr County), Iran. Tree felling was performed using the whole-tree method with chainsaws. One rubber-tired skidder (Timberjack 450C) was used for skidding operations. One male skidder operator (aged 52 years) voluntarily participated in this study. Heart rate was measured using a portable Polar H10 device, which continuously recorded heart rate at one-minute intervals over three non-consecutive days, each comprising 8 hours of net work. Resting heart rate was measured after 15 minutes of sitting in the skidder cab with the engine off. Maximum predicted heart rate was estimated based on the operator's age using the standard formula (220 − age). The percentage of heart rate reserve (HRR%) was used as a standard index to quantify physical workload. Skidder operator activities were recorded using the continuous time study method, and each complete skidding cycle was divided into six standard work elements. In total, data and time measurements were collected for 100 complete work cycles. For comparing the mean heart rate among the skidding components linear mixed-effects model and the Bonferroni test were used. Pearson's correlation coefficient was applied to examine the relationship between elapsed time from the start of work and heart rate changes.
Results:
A total of 100 complete skidding cycles were analyzed over three non-consecutive working days. The mean skidding distance was 195.32 ± 103.97 m, the mean load volume was 3.99 ± 0.79 m³ per cycle, and the mean net productivity was 18.41 ± 3.69 m³·h⁻¹. Results showed that the highest mean heart rate (97.73 bpm) was recorded during the travel loaded element, while the lowest (84.34 bpm) occurred during unloading. The overall mean heart rate across all elements was 93.12 bpm. linear mixed-effects model results indicated a statistically significant difference in heart rate among skidding work elements. Bonferroni's test revealed that the travel loaded element had the highest mean heart rate, whereas unloading had the lowest. The operator's physical workload was calculated as 32%, which falls into the "moderately heavy" category according to standard classification. Given the sample size limitation, these findings should be considered as an exploratory study, and their generalizability to the entire population of skidder operators requires further research with a larger sample size. Pearson correlation results showed that during travel empty, maneuvering, winching, and travel loaded, heart rate significantly increased over the workday. This increasing pattern, while consistent with the fatigue accumulation hypothesis, may also be influenced by other factors such as rising ambient temperature during the day, decreased body fluid levels, and potential variations in work intensity. In contrast, no significant correlation with time was observed for unloading and sorting/piling. This lack of significance cannot be attributed solely to the lighter nature of these elements; it may also be due to their short duration (which limits the opportunity for a linear increasing trend to emerge) or a change in activity type (the operator exiting the cab and performing light manual tasks). Further studies with simultaneous recording of additional physiological indicators are recommended to precisely identify the cause of this differing pattern.
General Conclusion:
This study revealed statistically significant differences in physical workload among different skidding work elements. Travel loaded was identified as the heaviest element, while unloading was the lightest. The overall mean physical workload (32%) fell into the moderately heavy category. The main novelty of this study, compared to previous research that only reported overall averages, lies in its detailed disaggregation and comparison of individual work elements. The findings demonstrate that reporting a single overall average conceals workload variations and peaks. Accordingly, the travel loaded element was identified as a critical point for ergonomic interventions. It is recommended that short breaks (5–10 minutes) be incorporated into the work schedule after every 2–3 travel-loaded cycles. Furthermore, the observed increase in heart rate over time suggests the need to revise the one-hour midday rest period and replace it with shorter, more frequent breaks.