Experimental setup and protocol.

<p>Subjects stood on a balance board with a narrow beam of support and performed forward leans to move a cursor via their center of pressure to a target presented on a LCD screen. To add uncertainty, targets may jump forwards, backwards, or remain at the original target position. A top down view of the balance board and feet illustrates that stability margins represent the difference between the width of the beam and the range of medio-lateral center of pressure (CoP) excursion. The experimental protocol consisted of 200 null trials, 300 target-jump trials, and 100 post trials. During the target-jump block, each of the three possible target-jump distances were presented 100 times in a randomized order. Key time points for comparison were Late Null (diagonal hatch), Early Uncertainty (gray fill), and Late Uncertainty (black fill).</p

Group averaged muscle activity linear envelopes for late null and late jump-0.

<p>Linear envelopes were similar between late null (thin line) and late jump-0 (thick line). The dashed vertical line indicates when the cursor moved out of the home circle.</p

Center of pressure (CoP) measures (mean±s.d.) for all phases of the experiment.

<p>CoP excursion ranges in the anterior-posterior (AP) and medio-lateral (ML) directions for the early and late phases during the null, jump-0, jump-b, jump-f, and post blocks. Early consisted of the first 15 successful trials and late were the last 15 successful trials.</p><p>□Subjects had different target distances.</p><p>na = not applicable. Response times only calculated when subjects were forced to make a maneuver during the backwards target-jump.</p><p>Bold text highlights planned comparisons.</p><p>*Significantly different, p<0.05.</p

Maneuverability comes at the expense of stability.

<p>Group averaged (n  = 11) medio-lateral (ML) center of pressure time series profiles for late null (light gray) and late target-jump (dark gray). These conditions compared trials to the same target distance. Solid lines are group means and shaded areas are±s.d. The stability metric was the width of the stability margins which were larger for late null (diagonal hatch) compared to late target-jump (dark gray). Bars are the group means±s.e.m. of the stability margin for each time point.</p

Group averaged RMS EMG amplitudes for all 16 lower limb muscles.

<p>Thick lines are the narrow beam data while dotted lines are the wide beam data. Left muscles are black and right muscles are gray. Single asterisks indicate a significant increase in left muscle activity compared to the right muscle on the narrow board. Black double asterisks indicate a significant increase in muscle activity in the left muscle on the narrow beam compared to the wide beam board, while gray double asterisks indicate a significant increase in muscle activity in the right muscle. Error bars are standard error of the mean.</p

The percent of success and stability.

<p>Percent of successful trials in 0.1 cm bins of stability margin across the width of the beam of support. Thick line is the mean and the shaded area is±s.d. Larger stability margins corresponded with higher probabilities of success and implied increased stability. Smaller stability margins corresponded with lower probabilities of success and implied decreased stability. These data support the definition of stability as the probability of keeping the board level and of stability margin as a metric of stability.</p

Experimental set up and display.

<p>Subjects were seated in front of a screen, holding a handle mounted to a force transducer. By pushing/pulling on the handle subjects accelerated/decelerated the vertical motion of a cursor that moved horizontally across the screen. On the right edge of the screen was a color-coded target that depicted rewards decreasing quadractically from the central target.</p

Example subjects and model data in the <i>CLIFF</i> block (velocity profiles not shown).

<p>A), D) Across-subject/model average trajectories for position and force, respectively. B), E) The subject and model with the best fit. C), F) The subject and model with the poorest fit. In all panels solid and dashed lines indicate subject and model mean trajectories, respectively. Shaded regions represent either +/− SEM (panels A and D) or +/− standard deviation across trajectories (remaining panels).</p

Example subjects and model data in the <i>CLIFF+NOISE LARGE</i> block.

<p>A), D) Across-subject/model average trajectories for position and force, respectively. B), E) The subject and model with the best fit. C), F) The subject and model with the poorest fit. In all panels solid and dashed lines indicate subject and model mean trajectories, respectively. Shaded regions represent either +/− SEM (panels A and D) or +/− standard deviation across trajectories (remaining panels).</p

Example subjects and model data in the <i>NULL</i> block (velocity profiles not shown).

<p>A), D) Across-subject/model average trajectories for position and force, respectively. B), E) The subject and model with the best fit. C), F) The subject and model with the poorest fit. In all panels solid and dashed lines indicate subject and model mean trajectories, respectively. Shaded regions represent either +/− SEM (panels A and D) or +/− standard deviation across trajectories (remaining panels).</p