JOINT ENERGY BALANCES: THE COMMITMENT TO THE SYNCHRONIZATION OF MEASURING SYSTEMS

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Our study quantifies the amount of error induced in the calculated energy balances of the joints if a trigger offset between the measurement of ground reaction force and of video data occurs. Joint energy balances constitute the basis for an adequate interpretation of muscular activity. An estimation of the amount of this error introduced by deficient synchronization has not been published so far but currently seems to be essential in the face of commercial providers offering complete solutions from data acquisition up to inverse dynamics analyses. As an example, we applied an inverse dynamics process to a data set of the contact phase of human running where the synchronization was disturbed artificially. We compared the amount of error for different methods of inverse dynamics. We found that a time offset of 5 ms results in almost 100% error (compared to zero offset) in the energy balance of each joint (up to 28 J in the hip). A kinematic event appearing later on the time scale than the respective kinetics shifts the calculated main source of energy production from the ankle to the hip, and vice versa if appearing precipitate. This 5 ms synchronization error is even higher than the methodical error introduced when synchronizing correctly but using the static torque equilibrium instead of complex inverse dynamics for the calculation of joint torques. We conclude that when buying professional analysis systems a strong urge to prove exact synchronization should be put on the provider.