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The growing interest in accessing steep terrain is pushing the expansion of the operating range of ground-based machines through winch-assist technology. However, cable tensile forces remain a major concern in the advancement of this technology. In this study, cable tensile forces in a winch-assist harvesting operation based on an anchor machine configuration were investigated. A harvesting machine tethered on ground slopes of up to 103% (45.9°) was observed over four days. Cable tensile forces were recorded at both the harvesting and anchor machines, at a frequency of 100 and 10 Hz, respectively. Cameras and GNSS devices enabled the execution of a time and motion study and the recording of machine positions. At the harvesting machine, peak tensile forces of up to 400 kN were recorded. Downhill movements of the harvesting machine were the cause of the highest peaks which, however, never exceeded the endurance limit of the cable. Numerous local variations in cable tensile forces exceeded 50 kN for just a few hundredths of a second, with a maximum variation of 300 kN in less than a second. Movements of the harvesting machine were the main cause of the major local variations. Depending on work element and harvesting machine movements, cable tensile forces were higher at either the harvesting or anchor machines; however, the highest peak tensile forces per work element and the related working loads were always recorded at the harvesting machine. Some implementation steps for improving the safety of winch-assist harvesting operations using anchor machines are also discussed.