Back Lift versus Leg Lift: A Dynamic Lifting Strategy Index

This paper presents an index that quantitatively characterizes different dynamic postural strategies employed during sagittal plane lifting. The dynamic lifting strategy is analyzed in the velocity domain as a strategy of partitioning postural changes between the torso and leg segments. The index consists of two parameters, assigned to two leg segments, which quantify their contributions relative to the torso. Parameter values, ranging from 0.1 to 10, are estimated through a simple enumeration search. The use of this index is illustrated by applying it to 24 lifting movements performed by six subjects using either a back-lift or a leg-lift strategy. Results suggest that a lifting strategy, in terms of whether the leg or the back is the prime mover, can be well characterized by this simple two-parameter index. It not only differentiates the two distinct general postural strategies, but also discerns non-distinct intermediate ones by quantifying the involvement of each segment in a lifting movement.