Adaptation to unilateral change in lower limb mechanical properties during human walking.

To produce successful and safe walking movements, the locomotor control system must have a detailed awareness of the mechanical properties of the lower limbs. Flexibility of this control comes from an ability to identify and accommodate any changes in limb mechanics by updating its internal representation of the lower limb. To explore the ability of the locomotor control system to tune its representation of the lower limb, eight participants performed three 5 min trials (PRE, WEIGHT and POST) of treadmill walking. During the middle trial the participants wore a 2 kg mass around the leg segment of the left lower limb. Joint kinematics and kinetics were determined to assess changes in the walking movements. The modification of limb inertia by adding mass to the limbs (WEIGHT) required a substantive period of adaptation, which lasted between 45 and 50 strides, before individuals fully adjusted to their new lower limb mechanics to achieve steady-state joint kinematics. These movements were caused in part from an increase in hip flexor and knee extensor activity in early swing followed by an increase in hip extensors and knee flexor activity in late swing. Following the removal of the mass (POST), ankle, knee and hip flexion all increased above the levels that were observed in the PRE condition and returned the baseline levels within 20, 70 and 70 strides, respectively. The removal of the mass appeared to cause a greater disruption to walking than the addition of mass to the limb despite a quick return of the joint moments to the PRE condition. Both the changes following the addition of the mass and its subsequent removal may embody a recalibration of the internal limb representation. These changes were characterized by an integrated response consisting of primary recalibration to the modified mechanical parameters and secondary actions to main the integrity of locomotor objectives such as propulsion, balance, support and safe foot trajectories. These recalibration responses were similar to those demonstrated in upper limb movements in response to altered force environments. Understanding this recalibration process will have implications for the prevention of trips and falls as individuals encounter different movement environments or changes to mechanical properties of their limbs, especially for individuals with decreased proprioception or other neural challenges.