The largest torque generated from two maximum isometric contractions was used to establish the
child’s maximum voluntary torque (MVT) at the respective joints. Maximum torque was normalized by
body weight (kg) prior to comparison. The child then performed two steady-state isometric contractions at
20% of their MVT. This value was determined by pilot investigation which determined that children with
CP had difficulty in reliably sustaining a steady-state isometric contractions below 20% MVT. The target
and the torque exerted by the child was displayed as a bar graph on a large monitor that was positioned
~1 meter away from the subject at eye level. The child was given ample time to practice achieving the
target torque before the two actual trials were recorded. These two trials were then averaged together for
all data measures. The voltage output from the torque motor was read by custom LabVIEW (National
Instrument Inc., USA) software and sampled at 1 kHz by a 14-bit National Instruments analogue-to-digital
converter. The voltage output from the Biodex dynamometer was converted to Nm and displayed in real time as a bar graph in the custom software interface. The maximum on the vertical scale of the bar graph
was twice the target value (Kouzaki & Shinohara, 2010). Each steady-state contraction was performed for
30 s while the sEMG was simultaneously collected.
For the ankle plantarflexion task the children were seated with the knee extended and ankle at
90°. During the knee extension task the children were seated with the hip and knee at 90°. During the hip
extension task the children were supine with the hip at 90°. Between each 30 s contraction the children
had approximately 30 s of rest, and between each of the test conditions the children received about 1
minute of rest while the appropriate attachment for the next test condition was connected to the Biodex
dynamometer. Additionally, the order of the task conditions was randomized and the protocol was well
tolerated, with all subjects able to complete the tasks.
The coefficient of variation (CV = [standard deviation of torque/mean torque] x 100) was used to
assess the amount of variability present in the middle 15 s of the steady-state torque. A lower CV value
was an indication of greater motor control of the joint steady-state torque (Christou & Tracy, 2006).
Approximate entropy (ApEn) was also calculated to assess the regularity of the middle 15 s of the steadystate
torque. ApEn evaluates the likelihood that similar patterns in the time series will be present at a later
time period. The ApEn calculation results in a score ranging from 0 to 2, a value closer to zero indicated
that the time series had a more consistent pattern, while a value closer to 2 indicated a less consistent
pattern. ApEn was calculated using the following equation: