Biofeedback Training Targeting at Knee Proprioception

Fourteen healthy young adults between the ages of 18-35 years were recruited from the student body of the department, friends and families of the students, and by word of mouth. Participants reported to the research laboratory for baseline testing. A randomized crossover design was utilized for this study. Inclusion criteria required that participants had at least one prior injury or trauma to the knee sustained no less than one year prior to enrollment, with any formal physical therapy concluding no less than 6 months prior. Participants were also required to be in generally good health, able to understand English, and capable of following verbal instructions. Exclusion criteria included a current neurological pathology, pregnancy, or any other lower extremity injury or traumatic event within the past year besides the index knee injury.

Upon arriving at the research laboratory, the study procedures were explained to the participants in detail, and they were allocated time to ask questions and decide on participation. Once informed consent was obtained, initial testing proceeded. A unique identification number (e.g., Knee01) was assigned to each participant for data de-identification and tracking. Participants first completed the Tampa Scale of Kinesiophobia (TSK-17) to quantify movement-related fear or fear of re-injury. Anthropometric measurements, including height and weight, were collected using a stadiometer, followed by leg length and lower leg length measurements via a tape measure. The dominant limb was determined by asking the participant to kick a soccer ball, and the history of the injured leg was documented. Participants were instructed to wear the same pair of athletic footwear during all subsequent laboratory and training sessions.

To evaluate knee joint proprioception, joint position sense (JPS) was assessed using an isokinetic dynamometer and a hand-held manual goniometer, with the more involved limb tested first. While seated on the isokinetic dynamometer, the machine passively moved the participant's limb to a target flexion angle (20° or 50°) with eyes open. The participant was instructed to memorize the joint position in space before the machine returned the limb to the starting resting position. Participants were then asked to actively recreate the target angle with eyes closed and hold the position while the dynamometer recorded the final angle. This protocol was repeated for three trials at each target angle (20° and 50°) on both limbs. For the manual goniometry assessment, participants sat on a treatment table with the knee joint line and lateral malleolus marked for consistent alignment. One research assistant positioned the goniometer to the target angle (20° or 50°) while a second assistant passively moved the participant's leg to allow them to perceive the reference position. The leg was returned to the resting position, and the participant actively recreated the target angle with eyes closed. The research assistants recorded the angle using the goniometer across three trials per target angle for both limbs.

Following JPS testing, seven Inertial Measurement Unit (IMU) sensors were secured via straps to the surface of the body (both feet, shanks, thighs, and the sacrum) to capture lower-body kinematics. Smart Insoles were placed inside the participants' shoes to record ground reaction force data. Participants executed a series of functional tasks: a double-leg quiet stance for 30 seconds, a single-leg quiet stance for 1 minute per limb (involved side first), three jump-landing trials, and the Y-Balance Test (involved side first). Participants then completed a dynamic action-perception coupling task starting with the involved limb. Participants were instructed to tap the top of hurdles set at varying heights (6-inch, 12-inch, and 18-inch) positioned either directly in front of or behind them. They performed the tapping task twice per hurdle with eyes looking forward, and then repeated the task with eyes closed. Finally, participants stood in front of the 18-inch hurdle with eyes closed while a research assistant moved them one step sideways. Participants were required to accurately locate and tap the hurdle with eyes closed in the correct direction for 10 consecutive trials. The entire testing session lasted approximately 1.5 hours.

Following baseline testing, participants were randomly assigned to either: 1) an independent home-based training program (Control group), or 2) a laboratory-based biofeedback training program (Feedback group). Both programs were completed twice weekly for 3 weeks.

For the Feedback group, training sessions took place in the research laboratory with seven surface IMUs attached to track real-time knee kinematics. Visual and auditory thresholds were configured at specific target angles (30°, 60°, or 90° of knee flexion) during a standardized exercise circuit. The circuit consisted of a 30-second squat hold (3 trials), 5-meter lateral monster walks to the involved and uninvolved sides, 8 single-leg retro-squats per limb, and 10 double-leg squats. Participants were instructed to move close to the target threshold angle without exceeding it ; exceeding the threshold immediately triggered a visual red bar warning on the monitor and an auditory beep sound. A two-minute rest was provided between joint angle conditions, resulting in a total training session time of approximately 20 minutes.

The Control group performed the identical exercise circuit twice weekly for 3 weeks at home. Rather than numerical biofeedback thresholds, they were instructed to perform the exercises at qualitative "shallow, medium, or deep" knee flexion depths. Research personnel conducted weekly compliance checks using a tracking sheet to monitor home exercise adherence. Missed sessions for either group were permitted to be made up during a designated fourth week.

Post-training testing sessions, matching the baseline protocol exactly, were scheduled within one week of the final training session. Following a strict two-week washout period to eliminate carryover effects, participants crossed over to the alternate training group to repeat the baseline testing, 3-week training intervention, and final post-training assessments.