The authors examined how the stability of the current total isometric force (F T) produced by four fingers is influenced by previous and expected voluntary changes in F T. The authors employed the synergy index obtained from the across-trial uncontrolled manifold analysis to quantify the stability of F T. The authors compared two tasks with similar histories of F T changes; one in which participants expected changes in F T in the future, and one in which they expected no changes in F T. The stability of F T was lower in the former task, indicating the existence of a novel type of anticipatory synergy adjustment. Disparate histories of F T changes yield inconsistent changes in stability, driven by individual differences in the covariation in the finger forces that leave F T invariant. Future research should focus on exploring these individual differences to better understand how previous and expected behavior changes influence the stability of the current motor behavior.
Mitchell Tillman and Satyajit Ambike
Satyajit Ambike, Daniela Mattos, Vladimir Zatsiorsky, and Mark Latash
Cyclic isometric finger-force patterns established using visual feedback show systematic drifts when the feedback is removed. Force changes at multiple time scales and in opposite directions have been reported. For further characterization of these drifts, healthy subjects produced isometric, cyclic finger force with and without visual feedback at various initial amplitudes and frequencies. We hypothesized that on feedback removal, the amplitude will be attracted toward a preferred value that is frequency dependent. We found that the amplitude always increased after feedback removal. The magnitude of the amplitude increase changed with initial frequency, but it was invariant over the explored range of initial amplitudes. Thus, the existence of a preferred amplitude of force oscillations was not supported. We interpret these results within the referent configuration and the referent configuration back-coupling hypotheses. These data will inform a mathematical model of finger-force drifts. However, currently, they raise more questions than they answer, and a coherent account of finger-force drifts remains a challenge.
Chuyi Cui, Brittney Muir, Shirley Rietdyk, Jeffrey Haddad, Richard van Emmerik, and Satyajit Ambike
Tripping while walking is a main contributor to falls across the adult lifespan. Trip risk is proportional to variability in toe clearance. To determine the sources of this variability, the authors computed for 10 young adults the sensitivity of toe clearance to 10 bilateral lower limb joint angles during unobstructed and obstructed walking when the lead and the trail limb crossed the obstacle. The authors computed a novel measure—singular value of the appropriate Jacobian—as the combined toe clearance sensitivity to 4 groups of angles: all sagittal and all frontal plane angles and all swing and all stance limb angles. Toe clearance was most sensitive to the stance hip ab/adduction for unobstructed gait. For obstructed gait, sensitivity to other joints increased and matched the sensitivity to stance hip ab/adduction. Combined sensitivities revealed critical information that was not evident in the sensitivities to individual angles. The combined sensitivity to stance limb angles was 84% higher than swing limb angles. The combined sensitivity to the sagittal plane angles was lower than the sensitivity to the frontal plane angles during unobstructed gait, and this relation was reversed during obstacle crossing. The results highlight the importance of the stance limb joints and indicate that frontal plane angles should not be ignored.