Force Control Characteristics for Generation and Relaxation in the Lower Limb

Abstract

We investigated the characteristics for force generation and relaxation using graded isometric contractions of the knee extensors. Participants performed the following tasks as quickly and accurately as possible. For the force generation task, force was increased from 0% to 20%, 40% and 60% of the maximal voluntary force (MVF). For the force relaxation task, force was decreased from 60% to 40%, 20% and 0%. The following parameters of the recorded force were calculated: error, time, and rate of force development. The error was consistently greater for force relaxation than generation. Reaction and adjustment times were independent of the tasks. The control strategy was markedly different for force relaxation and generation, this tendency was particularly evident for the lower limb compared to the upper limb.

Effect of switching force control direction on consecutive compared to discrete adjustment

“…The results of the 0% task showed the onset time of relaxation from the starting level of 20% or 60% MVF to be simply 0%. This result is consistent with that of a previous study according to the basic characteristics of force generation and relaxation 7,8) , which showed that the onset time of relaxation is shorter with a large magnitude of relaxation compared to a small magnitude. The results of the 60% task indicated that it has the same characteristics as the 0% task; the onset time is shorter with a large magnitude of relaxation rather than a small magnitude in the case of consecutive movement, previous studies have focused on sequence complexity and estimated it based on the following two points: 1) overall sequence of chunks and 2) internal structure of each chunk [18][19][20][21][22][23]26) .…”

“…The result of the 0% task showed the relaxation time of the adjustment from a starting level of 20% or 60% MVF to be simply 0%. This result is consistent with that of a previous study according to the basic characteristics of force generation and relaxation 7,8) ; relaxation time is longer when the magnitude of relaxation increases. Although the results of the relaxation time in the 20%, 40%, and 60% tasks were the same as those in the 0% task, the rate of force development was significantly different.…”

“…The average rate of force development (N/s) was then calculated over a 10 ms period following the 'go' signal. Subsequently, the force onset was defined as the first time point within this first 10 ms period during which the average rate of force development exceeded 50% of baseline force 8) , and the force offset was defined as the first time point within this first 10 ms period during which the average rate of force development decreased below 200 N/s 13) . The onset time was defined as the time interval between the 'go' signal and the force onset.…”

“…Regarding the quickness and accuracy of performance, studies have reported the characteristics of force control by utilizing isometric motor tasks or joint movement tasks and recording electromyographically (EMG). Almost all motor tasks in these previous studies were discrete adjustment tasks [4][5][6][7][8] or periodic adjustment tasks 3,[9][10][11] .…”

“…In this study, we defined discrete adjustment as just relaxing completely; and consecutive adjustment is defined as matching a target level after relaxing completely. Based on the previous studies of force control 7,8,13) and movement sequences [18][19][20][21][22][23] , we hypothesized the following.…”