The relation between balancing performance and reaction time is investigated for human subjects balancing on rolling balance board of adjustable physical parameters: adjustable rolling radius
R
and adjustable board elevation
h
. A well-defined measure of balancing performance is whether a subject can or cannot balance on balance board with a given geometry (
R
,
h
). The balancing ability is linked to the stabilizability of the underlying two-degree-of-freedom mechanical model subject to a delayed proportional–derivative feedback control. Although different sensory perceptions involve different reaction times at different hierarchical feedback loops, their effect is modelled as a single lumped reaction time delay. Stabilizability is investigated in terms of the time delay in the mechanical model: if the delay is larger than a critical value (critical delay), then no stabilizing feedback control exists. Series of balancing trials by 15 human subjects show that it is more difficult to balance on balance board configuration associated with smaller critical delay, than on balance boards associated with larger critical delay. Experiments verify the feature of the mechanical model that a change in the rolling radius
R
results in larger change in the difficulty of the task than the same change in the board elevation
h
does. The rolling balance board characterized by the two well-defined parameters
R
and
h
can therefore be a useful device to assess human balancing skill and to estimate the corresponding lumped reaction time delay.