Visual Feedback

The hand task was to track a waveform steadily and accurately with the finger movement. Subjects' right arm and hand were fixed on a Versa-Form pillow (Tumble Form, Bolingbrook, IL) for support. A small light emitting diode (LED) tracer was attached to the tip of the right index finger to track the movement of the finger in 3-D space using an Optotrak motion capture system (Northern Digital Inc., Ontario, Canada). Sensor outputs from the finger movements were displayed on the GUI in real time to provide visual feedback. Voluntary maximum extension of the index finger was determined by having subjects extend their index finger to maximum extension and recording that value relative to their neutral (level) position. For the step waveform, the amplitude of the waveform was set to 50% of voluntary maximum extension. For the sinusoidal wave, the peak amplitudes of the target sine wave were set to 50% of the voluntary maximum extension. Subjects started with their index finger level and extended and flexed their finger to track the assigned waveform as steadily and accurately as possible.

The experimental setup consisted of a vertically positioned custom-made ankle ergometer (OT Bioelettronica, Turin, Italy). Participants were seated on a chair with the dominant leg held in the ankle ergometer with straps at the foot, ankle, and knee. The hip and the knee were flexed at ∼90° and the ankle was placed at ∼100° of plantar flexion. The foot and the ankle were maintained with straps on an adjustable footplate connected in series with a calibrated load cell (CCt transducer s. a.s. Turin, Italy). The signal recorded with the force transducer was amplified (x200) and sampled at 2048 Hz with an external analog-to-digital (A/D) converter (OT-Bioelettronica, Turin, Italy). The force signal, recorded with OTbiolab software (OT-Bioelettronica), was synchronized with the electromyogram. Visual feedback of both the guided pattern and the performed force signal was displayed on a monitor positioned 1 m from the participants’ eyes.

The skin was exfoliated and cleaned with 70% isopropyl alcohol and Ag–AgCl cloth electrodes (Kendall^™ H59P, Covidien, Mansfield, MA, United States) were placed on the skin surface of the MG, LG, SOL, and tibialis anterior (TA) according to surface electromyography non-invasive assessment of muscles (SENIAM) guidelines (Hermens et al., 2000 (link)), with an interelectrode distance of 2 cm. The EMG signals were amplified (100 ×) and band-pass filtered (8–150 Hz) using a Coulbourn instrument unit (Allentown, Pennsylvania, PA, United States), sampled at 1,000 Hz, and converted from analog to digital using the Power 1401 (CED, Cambridge, England). Ground electrodes were placed on bony prominences of the femur and tibia.
To obtain EMG measures of maximal muscle activation for the plantar flexors (MG, LG, and SOL) and dorsiflexors (TA), isometric maximal voluntary contractions (MVC) were performed with participants seated in a commercially available dynamometer (System 4 PRO^™, Biodex Medical Systems, Shirley, NY, United States) with the ankle of the more affected leg (PD) or dominant leg (older adults without PD) secured to the dynamometer footplate with inelastic straps while the other leg rested on a foot rest. Participants sat with hips at 95° and the knee of the tested leg extended to ~ 160° (180° being terminal knee extension). The foot was secured to the dynamometer with the lateral malleolus aligned with the axis of rotation of the dynamometer and the ankle positioned at the participant’s standing ankle angle (~ 96° plantar flexion), measured at the medial malleolus as the angle between the tibia and first metatarsal, with 90° being a neutral ankle angle. The lateral malleolus of the tested leg was aligned with the axis of rotation of the dynamometer. Torque (Nm) was sampled from the Biodex at 1,000 Hz using a 16-bit Power 1401 (Cambridge Electronic Design (CED), Cambridge, England), stored for offline analysis using Spike 2 v7.12 (CED), and subsequently converted offline to Newtons (N) of force using the lever arm length of the footplate. Visual feedback of the torque signal was provided in real-time using a 52 cm monitor positioned 1 m in front of participants at eye level.
Standing balance tasks were performed on the rigid force plate (Length: 46.4 cm, Width: 50.8 cm, Height: 10.2 cm; Advanced Medical Technology, Inc., Watertown, Model OR6–5, Newton, MA, United States) under two conditions (firm and compliant). The firm surface consisted of the force plate alone, and the compliant surface consisted of a foam pad (10.2 cm thick; density, 0.016 g/cm³) placed on the force plate, with dimensions equivalent to the area of the force plate. A full-body safety harness was worn for all trials in the event of a loss of balance. Signals from the force plate were converted from analog to digital using a 16-bit Power 1401 (CED, Cambridge, England) at a sampling frequency of 1,000 Hz.