2.3. Interventions

The 6-week biofeedback training program focused on improving loading and movement symmetry during bilateral squatting in biweekly intervention sessions on non-consecutive days (12 sessions). The biofeedback training program was designed to provide sensory (visual and tactile) feedback to the participant to heighten awareness of asymmetrical movement strategies (e.g. load shift, decreased movement symmetry) and neuromuscular control during a squat. The two exercises that were completed during the biofeedback training program were a visual feedback squat and a resisted squat (tactile feedback). Each of these tasks was completed 30 (3 sets of 10 repetitions) times per session. We provided a 20 s rest between trials, and a 10-min break between the visual and tactile feedback exercises to decrease the effects of fatigue. Prior to the biofeedback intervention session, each participant completed a 5-min warmup on a stationary bicycle.

Visual Feedback: The simplest way to provide biofeedback during a squat is through visual feedback of load. Under this approach, participants were asked to stand on force plates, which measure the ground reaction forces (load) beneath each foot. Shoulder width for each participant was measured as the distance between acromioclavicular joints. This distance was measured on the force plates and 2 pieces of tape were placed this distance apart (one on each force plate) and participants were asked to stand with one heel on each piece of tape. Stance width was recorded on the data collection sheet and then entered in the REDCap [24,25] database so that this distance could be used during each subsequent training and testing session and foot position could be measured and marked prior to participant arrival. Participants faced a projection screen that displayed 2 bar graphs of the vertical ground reaction force, depicting each foot's load. Participants were asked to stand with their feet shoulder width apart (one foot on each force plate) with their hands in front on them with the shoulder flexed to 90° for counterweight. The foot width was standardized to ensure that foot placement was consistent for both the squatting trials and during the biofeedback training. The participants were asked to squat until their thighs were parallel with the ground or until their heels begin to come off the ground, whichever occurred first. A stool was placed behind the participant (Fig. 2) and was set to the height where the participant's thighs were parallel to the ground if they sat down (stool was placed at the height of the popliteal fold), so that the participant would know the deepest position they have to achieve. If the participant was able to achieve a squat position where the thigh was parallel to the floor they were instructed to squat until they barely touch down on the bench and then slowly stand back up without transferring any weight to the bench (Fig. 2). Participants were asked to stand on the force plates and transfer weight between their feet and watch the visual biofeedback during bilateral squatting to see the change in load beneath each foot. After completing the practice bilateral squat, participants completed all subsequent squats with the goal of keeping the bars level on the graph or maintaining an LSI ≥90% (symmetric load on both feet) (Fig. 2). This process was completed a total of 30 times (3 sets of 10 repetitions each) during each of the training sessions with the same goal each time of maintaining the bars at an equal level. During all squatting tasks, participants were instructed to squat to the pace of a metronome that is set at 30 beats per minute and the participants were asked to complete one squat every two beats.

Participant completing the visual feedback training.

Tactile Feedback: The second set of exercises at each biofeedback session was a series of resisted squats. Participants were asked to squat while an external force was applied to the side of the knee (Fig. 3) requiring the participant to work against this resistance to maintain balance and complete the squat. The band was placed on the surgical limb of each patient and was pulled at approximately a 45-degree angle toward the contralateral side. Pulling the participant toward the non-operative limb (one that is typically displaying higher loads) required the participant to pull toward the surgical limb and maintain good frontal plane knee position by resisting frontal plane valgus. A handheld dynamometer (Rolyan Smart Handle, Performance Health, Warrenville, IL) was used to maintain a consistent load across trials and sessions. The clinician who completed the biofeedback session set this load based on their clinical judgement. This is a typical exercise utilized in the clinic to aid in equal weight bearing and active hip abduction. The squat position that was used during these exercises was the same as the squat position used in the visual biofeedback task and during the biomechanical testing. The participants were asked to stand with their feet at the same standard width as described in the visual feedback section and squat until they contacted the stool that was positioned behind them during the biofeedback session. Participants were asked to complete 30 tactile feedback squats (3 sets of 10 squats) during each of the biofeedback sessions.

Participant completing the tactile feedback intervention.

The 6-week attention control program focused on providing educational information to the participants related to the clinical and sports expectations as they are released to return to sport. These participants were asked to meet 6 times, once a week, during the 6-week intervention period. Three of these visits were completed in person and three were completed using an online educational module (6 sessions in total). The online sessions were completed in week 1, week 3, and week 5, while the in-person sessions were completed during week 2, week 4, and week 6. The content of these sessions focused on providing information on ACL reconstruction, athlete expectation as they return to sport, incidence, and risk factors for second ACL injuries, as well as some suggestions on the gradual progression back into sport. A short quiz was administered at the completion of each educational session to assess understanding of the content from each session.