Abstract
BackgroundThe relationship between normal and tangential force components (grip force – GF and load force – LF, respectively) acting on the digits-object interface during object manipulation reveals neural mechanisms involved in movement control. Here, we examined whether the feedback type provided to the participants during exertion of LF would influence GF-LF coordination and task performance.MethodsSixteen young (24.7 ±3.8 years-old) volunteers isometrically exerted continuously sinusoidal FZ (vertical component of LF) by pulling a fixed instrumented handle up and relaxing under two feedback conditions: targeting and tracking. In targeting condition, FZ exertion range was determined by horizontal lines representing the upper (10 N) and lower (1 N) targets, with frequency (0.77 or 1.53 Hz) dictated by a metronome. In tracking condition, a sinusoidal template set at similar frequencies and range was presented and should be superposed by the participants’ exerted FZ. Task performance was assessed by absolute errors at peaks (AEPeak) and valleys (AEValley) and GF-LF coordination by GF-LF ratios, maximum cross-correlation coefficients (rmax), and time lags.ResultsThe results revealed no effect of feedback and no feedback by frequency interaction on any variable. AEPeak and GF-LF ratio were higher and rmax lower at 1.53 Hz than at 0.77 Hz.ConclusionThese findings indicate that the type of feedback does not influence task performance and GF-LF coordination. Therefore, we recommend the use of tracking tasks when assessing GF-LF coordination during isometric LF exertion in externally fixed instrumented handles because they are easier to understand and provide additional indices (e.g., RMSE) of voluntary force control.
Highlights
The relationship between normal and tangential force components acting on the digits-object interface during object manipulation reveals neural mechanisms involved in movement control
If we find differences in task performance and, mainly, in grip force (GF)-load force (LF) coordination we would have to take into consideration this fact before selecting one type of feedback that interfere less with GF-LF coordination
The horizontal force components acting perpendicularly to the handle contact areas (i.e., FC recorded by the single-axis and FY recorded by the multi-axis force and torque (F/T) transducer) were used to calculate the grip force [GF = (FC + (||FC-FY||))/2], i.e., the average force exerted against the two sides of the handle [9,14]
Summary
The relationship between normal and tangential force components (grip force – GF and load force – LF, respectively) acting on the digits-object interface during object manipulation reveals neural mechanisms involved in movement control. The close relationship established between GF and LF during object manipulation is a striking evidence of the central nervous system’s (CNS) ability to predict the effects of individuals’ own actions [2,4,5,6] This coupling has been widely investigated during manipulation tasks that involve lifting a grasped object [1], moving a handheld object upward and downward discretely [7] or continuously [5,8], and isometrically applying sinusoidal LF profiles on an externally fixed object [4,6,9]. During tasks in which LF changes (e.g., lifting, transporting, and shaking a handheld object) this parallel change of GF and LF ensure an economical exertion of GF [1,4,5,7,9]
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