The effects of practice under conditions of contextual interference on the interjoint kinematics of lifting

Purpose. The practice of exercises and functional tasks in a physical therapy session, typically involves the use of blocked practice schedules (low contextual interference, LCI), in which each variation of a task is practiced separately. Research has shown that the use of a blocked practice schedule results in superior acquisition performance, conversely, random practice (high contextual interference, HCI) results in enhanced retention and transfer of a motor skill (Magill and Hall, 1990). Lifting utilizing a squat technique is commonly practiced in orthopedic rehabilitation. The purpose of this study was to examine the effect of practice organization (HCI vs. LCI) on the starting posture and the relative timing between the hip and knee when lifting loads of increasing weights. Methods. Ten subjects, without history of low back pain and no experience with industrial lifting, were randomly assigned to either blocked or random practice groups. Reflective markers were attached to the lateral mallelous, lateral femoral condyle, greater trochanter, and inferior to the acromion process to define segment and joint angles of the knee and hip. Three dimensional motion analysis was performed while the subjects lifted a crate containing loads 15% to 75% of each subject's maximum dynamic lifting capacity (MDLC). Acquisition practice totaled 36 lifts with 12 trials at each load of 30%, 45% and 60% MDLC presented in either blocked or random order. Two retention tests (post 15 minutes and post 7 days acquisition) of 3 trials at each practiced load were performed. Two transfer tests (15% and 75% MDLC) were given following the second retention test. Dependent variables for the hip and knee included: starting posture; angular excursion; midpoint difference; index of coordination and lift duration. Index of coordination was derived from the correlation between the hip and knee risetime difference (the difference in time required for the hip and the knee to competed 12%–88% of their total excursion) and midpoint difference (the difference in the time at which the hip and the knee complete 50% of their excursion). Results . Repeated measures analysis of variance found no significant difference between groups for the dependent variables. However, significance was found for the main effect of load during retention for lift duration (=.001), and knee excursion (p = .035), and during transfer for lift duration (p = .001), knee excursion (p = .040), hip excursion (p = .007), midpoint difference (p = .033) and index of coordination (p = .010). Conclusion/Implications . Following this single practice session, performance in lifting technique was consistent between practice groups. Interestingly, at higher loads, subjects in both groups modified their lifting strategy to synchronize movement between the hip and knee. Further research is needed to assess the effects of multiple practice sessions under varied practice conditions in motor skill acquisition of lifting.