# Functional data analysis of lower-limb joint kinematics during badminton lunges under fatigue

## TL;DR

This study used advanced data analysis to show how fatigue changes lower-limb joint movements during badminton lunges, potentially increasing injury risk.

## Contribution

The novel use of Functional Data Analysis and FPCA reveals specific fatigue-induced kinematic adaptations in badminton lunges.

## Key findings

- Fatigue reduced hip and knee flexion range of motion but increased ankle dorsiflexion.
- Hip and knee abduction-adduction decreased, while ankle inversion-eversion increased under fatigue.
- A compensatory 'proximal-stiffening, distal-mobility' pattern emerged to maintain performance.

## Abstract

This study applied Functional Data Analysis (FDA) to investigate the effects of fatigue on lower-limb joint kinematics during badminton lunges.

Seventeen elite male badminton players executed both forward and 45° sideways forehand lunges before and after a fatigue protocol. Three-dimensional kinematic data of the lower limbs were collected using a motion capture system synchronized with force plates. Functional principal component analysis (FPCA) was employed to reduce dimensionality and extract key features from the time-series curves of hip, knee, and ankle joint angles. Paired-samples t-tests were used to compare the principal component scores before and after the induction of fatigue.

Fatigue significantly altered lower-limb movement patterns. Sagittal-plane analyses revealed a decreased range of motion in hip and knee flexion, contrasted with an increased range of ankle dorsiflexion (p < 0.05). In the coronal plane, hip and knee abduction-adduction ranges decreased, while ankle inversion-eversion ranges increased (p < 0.05).

Fatigue triggered a “top-down” compensatory response: proximal joints stiffened to stabilize, while the ankle enhanced flexibility to compensate. This “proximal-stiffening, distal-mobility” pattern maintained performance but may have redistributed loads, potentially raising the risk of ACL and lateral ankle ligament injuries. These findings help guide fatigue-specific training and injury-prevention strategies.