Volunteers pedaled a stationary electromagnetically
braked ergometer (Excalibur Sport, Lode BV; Groningen,
the Netherlands) adapted with a custom-fabricated pedal
and crank system. Constant resistance was set at 15 Nm,
and volunteers controlled cadence at 90 rotations/min
based on a handlebar-mounted tachometer. The combination
of workload and cadence meant power output was
~150 W. This is a common load and cadence combination
in motor control literature using cycling [18]. The
baseline condition was with the crank arm lengths set to
172 mm (control condition), which is typical for road
cycling, while the experimental condition had the crank
arm length on the amputated side set at 162 mm
(CRANK condition). Shortening the crank on the amputated
side reduces the geometric asymmetries between
the two lower limbs of an individual with TTA. The ankle
in the contralateral limb actively plantarflexes at the bottom
of the pedal stroke and dorsiflexes at the top, giving
a total ROM of ~20 [25]. Using average human anthropometrics,
this means the shank and ankle (with the
cycling cleat at the base of the first metatarsal) are effectively
lengthening and shortening a total of 19.8 mm
[30]. Therefore, shortening the crank 10 mm on the
amputated side (half the linear ROM) brings the pedal
closer at the bottom and farther away at the top of the
stroke to decrease kinematic asymmetries and offers a
method to account for ankle joint movement of the contralateral
limb [26]. The linear ROM of the crank spindle
relative to the ankle joint marker was calculated on the
contralateral limb during cycling to verify whether shortening
the crank arm 10 mm was sufficient for these
subjects.