Development of a Biomechanical Evaluation Protocol for Return to Sport in Athletes Following ACL Reconstruction Using Motion Capture and Force Platforms

Abstract

Introduction

Anterior cruciate ligament reconstruction is commonly followed by a long rehabilitation process before athletes are medically cleared to return to sport. However, even after medical clearance, residual biomechanical differences may still be present, including inter-limb asymmetries, altered movement patterns, and differences in force application. These alterations are not always detected through traditional clinical assessments. For this reason, biomechanical analysis using motion capture systems and force platforms can provide objective and quantitative information to complement return-to-sport evaluation after ACL reconstruction.

Objective

The objective was to develop a biomechanical evaluation protocol for return-to-sport assessment in athletes following ACL reconstruction. The protocol was based on kinematic and kinetic variables obtained from motion capture and force platform data.

Methodology

This study was designed as a retrospective, descriptive, and exploratory pilot study. For this we recruited 4 subjects that had ACL surgery and had concluded the rehabilitation and strength training protocol stablished by the Sports Medicine medical team at the Instituto Nacional de Rehabiltación. Data acquisition was performed using a Vicon motion capture system with 16 cameras synchronized with three AMTI force platforms. The Plug-in Gait biomechanical model was used to obtain the kinematic variables.

Five functional return-to-sport tasks were analyzed: countermovement jump, drop jump, deceleration, lateral movements, and cutting. These tasks were selected because they involve jumping, landing, braking, propulsion, lateral stability, and change-of-direction mechanics. Data processing involved marker correction and interpolation in Vicon Nexus, tasks segmentation, normalization and data analysis was performed in Visual3D. The extracted kinematic variables included maximum knee angle and ankle, hip, and knee range of motion. The kinetic and force-related variables included first impact, peak force, take-off force, and force vector behavior. Force-related variables were normalized by body weight to allow comparison between participants. Limb symmetry index percentage was calculated to compare the injured and healthy limbs.

Results

The results showed that asymmetry was both task-dependent and variable-dependent. In the kinematic analysis, the countermovement jump presented relevant asymmetry values in maximum knee angle (28.54%), ankle range of motion (14.93%), and knee range of motion (21.23%). Cutting showed the highest hip range of motion asymmetry (16.20%). In the kinetic analysis, take-off force showed the greatest inter-limb difference, especially during the countermovement jump, reaching 62.44%. In contrast, peak force asymmetry remained in a range between 1.90% and 5.37% for all tasks. While first impact asymmetry had similar values for most of the tasks (range: 0.41% to 6.02%), with exception of the lateral movement with an asymmetry of 11.04%.

Conclusions

This pilot study demonstrates that athletes who have received medical clearance following ACL reconstruction may still present biomechanical asymmetries above the clinically relevant 10% threshold in both kinematic and kinetic variables. These residual asymmetries, particularly the large takeoff force differences observed during the CMJ, show the additional objective layer that a biomechanical assessment could provide. Addressing these asymmetries through targeted rehabilitation may contribute to reducing re-injury risk. Due to the pilot nature and small sample size, future work should validate this protocol.