Quantification of lifting capacity has become a major component in clinical and occupational evaluations of people with or at risk for low back pain. The ideal test of maximum acceptable lifting would include an index of subject effort. Heart rate response appears to have significant limitations as an indicator of effort. In this study a device that records vertical lifting force measured peak force/weight ratios (PF/W) and differences (PF-W) for individual lifts. Seventeen men without low back pain lifted the device with progressively heavier loads. The PF/W, PF-W, and heart rate responses were compared at half-maximum and maximum effort levels. The PF/Ws were higher, while the PF-Ws and heart rate responses were lower during half-maximum efforts. As an indicator of subject effort, the relationship between peak force and weight may play a key role in isoinertial lifting evaluations.