Inertial-Based 3D Knee Joint Angular Kinematics Estimation: Effect of Number of Repetitions During Functional...

Area of Science:

• Biomechanics
• Human Movement Analysis
• Wearable Sensor Technology

Background:

• Inertial Measurement Units (IMUs) offer a cost-effective and compact solution for joint kinematics.
• Functional calibration using repeated movements is standard for defining anatomical reference frames with IMUs.
• The impact of reduced calibration repetitions on kinematic accuracy remains under-investigated.

Purpose of the Study:

• To develop and evaluate an anatomical calibration approach using fewer movement repetitions for IMU-based joint kinematics.
• To determine the minimum number of repetitions required for accurate functional calibration.
• To assess the trade-off between calibration duration and kinematic estimation accuracy.

Main Methods:

• Anatomical reference frames were defined using progressively shorter calibration trials (1, 3, and 10 repetitions).
• Three-dimensional knee joint kinematics were estimated using IMUs based on these calibration methods.
• Kinematic data were compared against a gold-standard optoelectronic system, with errors quantified using RMSE and MAPE.

Main Results:

• Ten repetitions yielded the best accuracy (RMSE: 2.1° FE, 3.8° AA, 4.6° IE).
• Results with three repetitions were not statistically different from ten repetitions for flexion/extension (FE).
• Significant differences were observed for internal/external (IE) rotation between three and ten repetitions.

Conclusions:

• Reduced functional calibration time for IMU-based kinematics is feasible without substantial loss of accuracy for certain movements.
• Fewer repetitions improve patient comfort and minimize fatigue during calibration.
• Clinically relevant joint kinematics can be reliably obtained with optimized IMU calibration protocols.