RESEARCH ARTICLE Jeremy W. Noble Æ Stephen D. Prentice Adaptation to unilateral change in lower limb mechanical properties during human walking Received: 19 May 2005 / Accepted: 16 July 2005 / Published online: 17 November 2005 Ó Springer-Verlag 2005 Abstract 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 las- ted 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 re- moval of the mass appeared to cause a greater disrup- tion 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 re- calibration 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 envi- ronments or changes to mechanical properties of their limbs, especially for individuals with decreased propri- oception or other neural challenges. Keywords Adaptation Æ Gait Æ Kinematics Æ Load Æ Internal model Introduction In order for the central nervous system (CNS) to effec- tively control the movement of the lower limbs during locomotion it must have some implicit knowledge of the mechanical properties of these body segments. This knowledge is necessary to achieve the desired lower limb kinematics during locomotion by recruiting the appro- priate muscles at the appropriate magnitudes. This awareness has been referred to as an internal model which has been described as a system capable of pre- dicting the next state of the motor apparatus, based on the current state of the motor apparatus and the motor command issued (Scott 2004). Therefore an internal model enables the CNS to link the issued motor com- mands and the expected movements of the limb. A de- tailed internal representation of lower limb mechanics would allow for a more skillful and energetically efficient recruitment of muscles since the CNS could better pre- dict how the limbs will respond to the motor commands issued. Walking is a movement pattern that involves signifi- cant amounts of motion at all three major joints of the lower limb and requires the ability to keep the upper body balanced. In addition to maintaining balance of J. W. Noble Æ S. D. Prentice (&) Gait and Posture Laboratory, Department of Kinesiology, University of Waterloo, 200 University Ave. W., Waterloo, ON N2L 3G1, Canada E-mail: sprentic@healthy.uwaterloo.ca Tel.: +1-519-8884567 Fax: +1-519-7466776 Exp Brain Res (2006) 169: 482–495 DOI 10.1007/s00221-005-0162-3