* Corresponding author. Tel.: #617-573-2731; fax: #617-573-2769. E-mail address: priley@partners.org (P.O. Riley). Journal of Biomechanics 34 (2001) 197}202 Propulsive adaptation to changing gait speed Patrick O. Riley*, Ugo Della Croce, D. Casey Kerrigan Spaulding CRS Rehabilitation Engineering Laboratory and Hospital, and Department of PM&R Harvard Medical School, 125, Nashua Street, Boston, MA 02114, USA Dipartimento Scienze Biomediche, Universita+ di Sassari, Italy Accepted 20 August 2000 Abstract Understanding propulsion and adaptation to speed requirements is important in determining appropriate therapies for gait disorders. We hypothesize that adaptations for changing speed requirements occur primarily at the hip. The slow, normal and fast gait of 24 healthy young subjects was analyzed. The linear power was analyzed at the hip joint. The anterior}posterior and vertical induced accelerations of the hip were also determined. Linear power and anterior}posterior-induced acceleration (IA) analyses of the hip reveal that the lower limb joint's moments contribute to body forward propulsion primarily during late swing and early stance. Propulsive adaptations to speed changes occur primarily at the hip and secondarily at the ankle. These analyses show that hip muscles, particularly the hip extensors, are critical to propulsion. They also show that ankle function is primarily for support, but is important to propulsion, especially at slow speeds.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Gait analysis; Biomechanics; Power; Induced accelerations; Speed 1. Introduction Clinicians seeking to rehabilitate patients with gait disorders must take into account a range of factors a!ect- ing patient safety, comfort, strength reserve and endur- ance. It is essential to provide the patient with adequate support and propulsion using the strength and control available to the patient. Joint powers, measured by gait analysis, are used to guide this aspect of rehabilitation (Winter, 1991). As a result, there has been a strong em- phasis on developing ankle plantar#exor `push-o!a power (Winter, 1983; Olney et al., 1990, 1991; Ounpuu et al., 1996). Winter (1983) reported that, in healthy adults, ankle joint power decreased with decreasing speed. How- ever, Chen et al. (1997) reported that, in healthy children, ankle joint power was nearly constant and contributed relatively less to propulsion as speed increased. More- over, joint powers indicate the net power developed by muscles acting about a joint; they do not provide in- formation as to the function of that power (Meinders et al., 1998). Several model-based techniques are available to eluci- date function, to determine the contribution of power developed at one joint to the kinematics and kinetics of all parts of the body. Buczek et al. (1994) suggested the usefulness of linear power analysis and Meinders et al. (1998) skillfully used a combination of joint power, linear power and segmental energy #ow analyses to examine the function of ankle `push-o!a power. Induced acceler- ation analysis has been used to study posture control (Zajac and Gordon, 1989) and gait (Meglan, 1991; Kepple et al., 1997; Riley and Kerrigan, 1999). A combination of linear power and induced acceleration analyses measured subtle di!erences in propulsion between healthy elders and young control subjects (Riley et al., in review). The latter study led us to conclude that linear power and induced acceleration analyses provided a profound insight into the biomechanics of propulsion. However, this work also brought out the necessity of determining how propulsive mechanisms vary with speed, which is the purpose of the present work. Our hypothesis is that adaptation to changing speed requirements occurs prim- arily by modulating the action of the hip #exors and extensors. 2. Methods Healthy young volunteers (16 female/8 male) between 18 and 40 years of age (23.9$4.4 years, mean$ 0021-9290/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 1 - 9 2 9 0 ( 0 0 ) 0 0 1 7 4 - 3