Difference Between Intentional and Reactive Movement in Side-Steps: Patterns of Temporal Structure and Force Exertion

Abstract

Intentional and reactive movements are dissimilar in terms of execution time. Previous studies reported that reactive movements are faster than intentional movements ("Bohr's law" or "Gunslinger effect"), however, these studies focused only on hand-reaching tasks, such as pressing buttons. No studies assessed whole-body movements involving movement of the center of mass (CoM). This movement is characterized by many degrees of freedom because it involves many joints and requires more force than the hand-reaching movement. In this study, we determined the differences in the patterns of temporal structure and force exertion to elucidate the mechanism of "Bohr's law" in whole-body movement involving movement of the CoM. Ten participants performed a sidestepping task, which requires at least two steps: (1) an intentional movement, in which the movement started with the participants' own timing; and (2) a reactive movement, in which the movement started the moment a light-emitting diode bulb in front of the participants lit up. We collected data on the ground reaction forces and coordinates of 20 body points. The time of movement onset was calculated and defined based on the ground reaction force, which has the earliest onset compared with velocity and position. The execution time was significantly shorter in the reactive movement condition than in the intentional movement condition (772 vs. 715 ms, p = 2.9 × 10^-4). We confirmed that Bohr's law was applicable not only in hand-reaching tasks but also in whole-body movement. Moreover, we identified three phases, including the velocity reversal phenomenon associated with the produced mechanism of Bohr's law, and provided the temporal structure. The difference in the pattern of force exertion accompanying the two styles of motor planning with different accuracies was strongly associated with this motor characteristic. These findings may serve as important basic data to scientifically clarify the mechanism of complex physical tactics implemented in one-on-one dueling in various sports.

Keywords: externally triggered movement; internally initiated movement; kinetics; movement time; onset time; whole body.

FIGURE 1

Experimental setup. Participants prepare by standing with one foot each on the two force plates. Thereafter, they take steps laterally toward (“x” component) the right and reach the marked target line at a distance equal to their own body height. The light-emitting diode bulb placed in front lit up only under the reactive movement condition (RMC).

FIGURE 2

Typical example of sidestepping motion and characteristic phases (viewpoint: back). From the top, lateral ground reaction force (GRF; F_x), vertical GRF (F_z), lateral velocity, and lateral position. Phase (A) is the preparation phase, phase (B) is the takeoff of the leading foot, phase (C) involves reaching the peak force for the trailing foot, and phase (D) involves landing of the leading foot. These data represent one of the IMC results.

FIGURE 3

Movement time for each participant. Points connected by lines indicate the average from a single individual. The reaction is shorter than the intention in all participants, and there is a significant difference between two conditions (p = 2.9 × 10^–4). ***p < 0.001.

FIGURE 4

A typical example of lateral velocity (“x” component) in two conditions. The peak velocity (black dots) indicates the moments for the takeoff of both legs in the first step. The intentional movement (solid line) slowly reaches the peak velocity and a gentle curve is obtained, whereas the reactive movement (dashed line) is linear. The time difference (t₁–t₂) generated until the peak velocity is reached is largely related to the final MT difference.

FIGURE 5

Conceptual scheme of the temporal structure according to the three phases including the velocity reversal phenomenon. The shaded gray area shows the conclusive generated time difference in the MT (reactive advantage). Phase 1: the reactive movement leads the intentional movement until the peak velocity is reached, generating greater differences in the MT (reaction advantage phase). Phase 2: the intentional movement, which generated a greater peak velocity than reactive movement, reduced the difference in the moving distance from the reactive movement (intention advantage phase). Phase 3: the reactive movement holds the difference in the MT generated until the peak velocity is reached, leading to a difference in the final MT (still intention moving phase). The scale below shows the actual values, and the standardized value when the MT of IMC is 100 (all values are average).