Comparison of force and timing control in periodic isometric leg extension

Background: In daily life and sports activities, the regulation of muscle force and timing is often not controlled independently, rather the appropriate amount of force is controlled simultaneously with appropriate timing. However, which of the two variables, force or timing, is more difficult to control remains unclear. Objective: We aimed to investigate the difficulty in the accurate control of force and timing, simultaneously. By setting target levels for force and timing interval, with both target lines presented, none of them presented, or only one of the target lines for force or timing interval presented, we directly examined and identified which variable is more important. Methods: Participants were asked to produce periodic isometric knee extension force using their right leg. The following four tasks were established: 20 %MVF-1000 ms task (20 % maximum voluntary force [MVF] at 1000 ms intervals), 20 %MVF-2000 ms task (20 % MVF at 2000 ms intervals), 40 %MVF-1000 ms task (40 % MVF at 1000 ms intervals), and 40 %MVF-2000 ms task (40 % MVF at 2000 ms intervals). Moreover, the participants performed four tasks under the following four conditions based on target line presentation: Both variables condition (force and interval lines presented), force condition (only force line presented), interval condition (only interval line presented), and no variable condition (neither force nor interval lines presented). The recorded force data were analyzed. Results: Regarding the force factor, the error of the interval and no variable conditions was greater than that of the both variables and force conditions. As for the interval factor, the error was greater when the target interval line was not presented (force and no variable conditions) than when it was presented (both variables and interval conditions), and it exceeded the target interval in the 1000 ms task, whereas it was shorter than the target interval in the 2000 ms task. Except for the force condition, the force factor showed significantly greater absolute errors when the target level was set as 100 %, compared to the interval factor. Conclusions: The control of force was found to be more difficult than that of timing, based on aspects relating to accuracy and reproducibility.