The use of intra-subject variability as a means of identifying performance enhancement interventions

Background: The most common method used to identify performance determining biomechanical factors has been to compare differences between individuals. This group analysis approach assumes that the movement strategy for all individuals is the same. However, not every athlete has the same neuromuscular capacity, anthropometries and muscle morphology. It may be more appropriate to make inferences about an individual’s movement strategy by treating each individual as their own experiment group, examining differences between repetitions of an individual’s own performance, referred to as individual analysis. The aim of the study is to identify the performance determining biomechanical factors of countermovement vertical jumping, at both a group level and individual level and to highlight the commonality and differences between the two approaches. The study also aims to determine whether drop jumps (DJs) overload the performance driving kinetic factors of the countermovement jump (CMJ), thereby assessing the appropriateness of DJs as a training method. Experiment: Eighteen male university students performed 15 countermovement jumps (CMJ), 15 drop jumps from 0 30m (DJ30) and 15 from 0 50m (DJ50). From ground reaction force and motion data, kinematic, kinetic and coordination parameters were calculated for the whole body, hip, knee and ankle Correlation analysis was used to identify the biomechanical factors that may explain differences in jump height achieved, both between individuals (inter-subject) and within repetitions of an individual (intra-subject). Differences in kinetic factors between the CMJ, the DJ30 and the DJ50 at a group level was assessed using a two-way repeated measures ANOVA, and at an individual level using a single subject repeated measures ANOVA. Results: A number of biomechanical factors were found to be significantly correlated with CMJ height at a group level. These however, were not always correlated at the individual level, and visa versa Opposing relationships were evident at the individual level, both between individuals and compared to the group analysis Knee kinetic parameters were significantly greater in the DJ than the CMJ at a group level and for the majority of subjects at an individual level. In contrast, both ankle and hip kinetics were not overloaded in the DJ at the group level, although an overload of ankle kinetic parameters was achieved by a number of individuals. Conclusion: Results from a group and individual analysis are not always comparable. A considerable amount of important information may be lost regarding individual performance strategies when only a group analysis is employed. However, the use of solely an individual analysis based approach would not reveal any performance factors relating to differences between subjects, and a case for a group based analysis to be used to supplement an individual analysis therefore exist. Knee kinetic factors were overloaded in the DJ compared to the CMJ, while hip kinetics factors were not and the overload of ankle kinetic factors was found to be dependent on the DJ technique employed. Key words vertical jump performance, countermovement jump, drop jump, mtrasubject variability, individualised technique, rate of force development.