Force-grips: Completion of force-grip on effort trials.

<p>Force traces from trials in which a 12s grip was exerted in Experiment 2 (A,C,E,G) and Experiment 1 (B,D,F,H). <b>(A,B)</b> Trial-by-trial (top) and trial-average (bottom) percentage of participants that had reached the required effort level (color bar: 0 = no participant, 1 = all participants). <b>(C,D)</b> Group average 12s force trace for six different effort levels, expressed as percentage of participants’ maximum voluntary contraction (MVC; Experiment 1: equally sized bins between [0,1]; Experiment 2: [0.15 0.25 0.4 0.55 0.75 1];). There was an offset at t = 0 in Experiment 1 because the 12s-effort directly followed the response which was indicated by briefly squeezing the gripper of the corresponding hand at a level of 0.35 (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1004116#sec013" target="_blank">Materials and Methods</a>). <b>(E,F)</b> Within-trace average variability (STD = standard deviation) for the same six effort levels increases with increasing difficulty (group average ± SEM). <b>(G,H)</b> Required force levels are plotted against mean exerted force levels for every effort trial, showing a strong correlation in both experiments. The produced force consistently exceeded the required force level by a small amount because participants were instructed to maintain their force just above the target bar and because they kept a ‘safety margin’ to account for slight variations in force.