Common modelling assumptions affect the joint moments measured during passive joint mobilizations

Area of Science:

• Biomechanics
• Rehabilitation Engineering
• Human Movement Science

Background:

• Joint resistance to passive mobilization is crucial for understanding joint mechanics.
• Current estimations often rely on simplified models, with unknown impacts on accuracy.
• Accurate assessment is vital for diagnosing and treating conditions affecting joint mobility.

Purpose of the Study:

• To develop and validate a reference 3D inverse dynamics protocol for estimating joint resistance.
• To compare the accuracy of this reference method against five common simplified approaches.
• To quantify the discrepancies introduced by various simplifications in joint resistance calculations.

Main Methods:

• A passive testing protocol was conducted on typically developing children and children with cerebral palsy.
• Applied forces and moments were measured using a 3D handheld dynamometer.
• Simultaneous 3D kinematics of the dynamometer and body segments were recorded.
• Joint resistance was calculated using a reference 3D inverse dynamics approach and five simplified methods.

Main Results:

• Simplified approaches resulted in non-negligible differences compared to the reference 3D inverse dynamics method.
• Discrepancies ranged from 3-32% for the ankle, 4-34% for the knee, and 1-58% for the hip.
• The extent of error varied significantly depending on the specific simplification applied.

Conclusions:

• A complete 3D kinematics and dynamics modeling approach is recommended for accurate estimation of joint resistance.
• Simplified modeling techniques can lead to substantial inaccuracies in joint resistance measurements.
• This study provides a validated reference method and highlights the importance of comprehensive modeling in biomechanical assessments.