Multi-Finger Interaction and Synergies in Finger Flexion and Extension Force Production

Abstract

The aim of this study was to discover finger interaction indices during single-finger ramp tasks and multi-finger coordination during a steady state force production in two directions, flexion, and extension. Furthermore, the indices of anticipatory adjustment of elemental variables (i.e., finger forces) prior to a quick pulse force production were quantified. It is currently unknown whether the organization and anticipatory modulation of stability properties are affected by force directions and strengths of in multi-finger actions. We expected to observe a smaller finger independency and larger indices of multi-finger coordination during extension than during flexion due to both neural and peripheral differences between the finger flexion and extension actions. We also examined the indices of the anticipatory adjustment between different force direction conditions. The anticipatory adjustment could be a neural process, which may be affected by the properties of the muscles and by the direction of the motions. The maximal voluntary contraction (MVC) force was larger for flexion than for extension, which confirmed the fact that the strength of finger flexor muscles (e.g., flexor digitorum profundus) was larger than that of finger extensor (e.g., extensor digitorum). The analysis within the uncontrolled manifold (UCM) hypothesis was used to quantify the motor synergy of elemental variables by decomposing two sources of variances across repetitive trials, which identifies the variances in the uncontrolled manifold (V_UCM) and that are orthogonal to the UCM (V_ORT). The presence of motor synergy and its strength were quantified by the relative amount of V_UCM and V_ORT. The strength of motor synergies at the steady state was larger in the extension condition, which suggests that the stability property (i.e., multi-finger synergies) may be a direction specific quantity. However, the results for the existence of anticipatory adjustment; however, no difference between the directional conditions suggests that feed-forward synergy adjustment (changes in the stability property) may be at least independent of the magnitude of the task-specific apparent performance variables and its direction (e.g., flexion and extension forces).

Keywords: anticipatory synergy adjustment; finger extension; finger flexion; multi-finger synergy; uncontrolled manifold hypothesis.

Figure 1

Top-down view of the experimental setup (A). The subject's wrist was held stationary with Velcro straps. A wooden cylinder supported the palm, and the force sensors were attached to a frame. The feedback screen displayed the real-time finger forces and showed the templates during single-finger ramp tasks (B) and quick pulse force production tasks (C).

Figure 2

The sample data of total force (gray line) and variance of the total force (z-transformed ΔV, black line) during quick pulse force production tasks. Flexion forces are presented as negative, and extension forces are presented as positive. t_ASA, t₀, t_ch, and t_peak stand for the time of anticipatory synergy adjustment (ASA), the time of initiation of total force(F_TOT) change, the time of the direction of force changed, and of the peak pulse force, respectively. ΔV_SS represents average ΔV at a steady state. ΔΔV_t0 and ΔΔV_peak stand for the change in the synergy index between steady state and t₀ and between the steady state and negative peak of ΔV after t₀, respectively.

Figure 3

The index of enslaving (EN) of the index (I), middle (M), ring (R), and little (L) fingers during flexion (filled bars) and extension conditions (open bars). Average values are presented with standard error (SE) bars. EN_I, EN_M < EN_L < EN_R during finger flexion (p < 0.05) and EN_I < EN_M, EN_R, EN_L during extension (p < 0.05).

Figure 4

Average (AvgT_peak) (A) and standard deviation (SdT_peak) (B) of the time to reach peak pulse force across repetitive trials when the direction of steady state force was flexion (filled bars) and extension (open bars). Two capital letters above the bars represent the experimental conditions. The first letter represents the force direction at the steady state, and the second letter stands for the direction of quick pulse force. “F” and “E” stand for flexion and extension, respectively (e.g., FF represents “flexion” to “flexion”). Averaged across subjects data are shown with standard error bars. The asterisks (^*) show statistically significant differences between conditions (p < 0.05).

Figure 5

The total force (F_TOT, thin gray lines) and z-transformed synergy index (ΔV_Z) during FF (gray dotted line, flexion to flexion), FE (black-dotted line, flexion to extension), EF (black solid line, extension to flexion), and EE (gray solid line, extension to extension) in mode space (A) and force space (B). Averages across subjects are presented for ΔV_Z. The times of ASA initiation (t_ASA) are shown with the arrows.

Figure 6

The variances in the UCM (V_UCM) (A) and orthogonal space (V_ORT) (B) when the direction of steady state force was flexion (filled bars) and extension (open bars). Two capital letters above the bars represent the experimental conditions. The first letter represents the force direction at the steady state, and the second letter stands for the direction of quick pulse force. “F” and “E” stand for flexion and extension, respectively (e.g., FF represents “flexion” to “flexion”). Averages across subjects data are shown with standard error (SE) bars. The asterisks (^*) show statistically significant differences between conditions (p < 0.05).

Figure 7

The difference in the synergy index between steady state and negative peak (ΔΔV_peak) for four conditions of FF (flexion to flexion), FE (flexion to extension), EF (extension to flexion), and EE (extension to extension) in mode space (A) and force space (B). Averaged data across subjects with standard error (SE) bars are presented. The asterisks (^*) show statistically significant differences between conditions (p < 0.05).