Implicit and explicit flexibility are inversely related during perturbed human walking in healthy young adult males

Effective locomotion depends on the ability to adapt to changing environmental constraints. Flexibility, defined as the ability of the locomotor system to adopt alternative movement solutions to achieve the same task outcome, is widely considered a fundamental property of adaptable movement. Despite its theoretical importance, flexibility in locomotion remains rarely characterized. This study investigated the relationship between implicit flexibility and explicit flexibility during perturbed walking. Healthy young males walked on a treadmill while unexpected unilateral perturbations were introduced via sudden belt accelerations or decelerations. Implicit flexibility was quantified using baseline Continuous Relative Phase variability (bCRPV). Explicit flexibility was quantified using joint angle relaxation time following perturbation. Spearman's rank correlations were used to examine the relationship between implicit flexibility, represented by bCRPV, and explicit flexibility, represented by joint angle relaxation time. Moderate positive correlations were observed between bCRPV and relaxation time across three of the four segment couplings. Individuals exhibiting greater bCRPV required more strides to return to regular walking patterns following sudden perturbations. Contrary to the hypothesis, greater bCRPV was associated with longer joint angle relaxation times. Under the present operational definitions, greater implicit flexibility was associated with lower explicit flexibility during perturbed walking. These findings indicate that higher bCRPV does not necessarily correspond to faster locomotor reorganization following perturbation. The present study provides an empirical investigation of the relationship between implicit and explicit flexibility during human walking and demonstrates the value of characterizing flexibility using complementary measures derived from different levels of movement organization.