162 Mass and Mass Distribution of Below-Knee Prostheses: Effect oil Gait Efficacy and Self-Selected Walking Speed Justus F. Lehmann, MD, Robert Price, MSME, Ramona Okumura, CP, Kent Questad, PhD, Barbara J. de Lateur, MD, Alain N&retot, MA ABSTRACT. Lehmann JF, Price R, Okumura R, Questad K, de Lateur BJ, Negretot A. Mass and mass distribution of below-knee prostheses: effect on gait efficacy and self-selected walking speed. Arch Phys Med Rehabil 1998;79:162-8. Objective: To study mass and mass distribution effect on function of below-knee prostheses. Design: Design modifications were done to produce proximal center of mass location versus distal center of mass location variations, and prosthesis weight was modified from 42% to 70% of normal limb weight. Work across joints of affected and unaffected extremities was compared to assess the ability of the prosthesis to substitute for function loss. Setting: University biomechanics laboratory. Participants: Fifteen volunteers with below-knee amputa- tions, residual limb length greater than 8.3cm, but excluding Syme amputations. Interventions: Patients walked with all configurations at self- selected walking speeds and 120m/min. Main Outcome Measures: Self-selected walking speed and metabolic efficiency. Work across the joints of affected and unaffected sides was compared. Results: Proximal center of mass location produced a more efficient gait. Weight change from 42% to 70% of normal had no significant effect. Mechanical studies show that the prosthesis is a relatively poor substitute for the normal limb; most work is done by the nonamputated side. Particularly, the prosthesis failed to produce effective forward impulses on the body, re- sulting from push-off and deceleration of the swing leg. Conclusions: For a proximal center of mass, lightweight distal components (eg, feet) should be used; it is questionable whether further expenditure to develop ultralightweight prosthe- ses would be cost effective for level walking. 0 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabili- A LARGE POPULATION of younger, active, and older marginal ambulators with below-knee (BK) amputations exists. Nationally, approximately 164,000 persons have lower limb amputations’; 60,000 to 75,000 of these amputations occur below the knee.2.3 Walking speed and efficiency are important to both the active and the marginal ambulators. Approximately 16% of all persons with lower limb amputations fall within From the Department of Rehabilitation Medicine, University of Washington, Seattle. Submitted for publication February 6, 1997. Accepted in revised form May 20, 1997. Supported by the National Institute on Disability and Rehabilitation Research (NIDRR), US Department of Education (project no. Hl33G20138). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprint requests to Justus F. Lehmann, MD, Department of Rehabilitation Medicine, Box 356490, University of Washington, Seattle, WA 98195.6490. 0 1998 bv the American Conuess of Rehabilitation Medicine and the American Academy oi Physical Medicine-and Rehabilitation 0003-9993/98/7902-4382$3.00/O the 21- to 40-year age range where fast walking and running capabilities are desirable to pursue both vocational and recre- ational activities. Kald and associates4 found that of 100 persons with lower limb amputations, approximately 60% were active in sport~.~ These active persons reported that the inability to run and jump was the most significant physical limitation to their participation in sports activities. Fully 50% of the subjects believed better prostheses were needed to improve their ability to participate in recreational activities. According to Kald,4 ap- proximately 61% of persons with lower limb amputations are over 51 years of age; reducing metabolic requirements may be an important criterion for optimal gait in this age group, espe- cially for marginal ambulators, where the metabolic efficiency will tip the balance between effective use and nonuse of the limb. In a study of the gait patterns of 19 persons with unilateral below-knee amputations (BKAs), Robinson and colleagues5 found asymmetries of movement between the prosthetic and intact limbs. The duration of the stance phase on the intact side was greater than that of the prosthetic stance phase. The intact step length was shorter than the prosthetic step length, sug- gesting a lack of effective push-off on the prosthetic side.6 The shorter step length contributed to a slowing of walking speed; persons with BKAs had a mean self-selected walking speed (SSWS) of 64m/min compared to a mean of 9lm/min for a group of people without amputations studied by Murray and coworkers.’ The energy requirements of ambulation by persons with amputation were examined by Molen.’ Of the 57 persons with BKAs studied, 37 were able to achieve and maintain walk- ing speeds of 50 to 90m/min, but they consumed approximately 20% more oxygen than able-bodied persons ambulating at the same speeds. Waters and associates’ found that persons with walking disabilities walked at slower SSWSs. This apparently is done in order to maintain the rate of oxygen consumption as close to normal limits as possible,” but it leads inevitably to a lower efficiency. Waters’ found an increase in milliliters of oxygen per kilogram meters (mL O&g-m) of approximately 33% for persons with traumatic BKAs and 80% for these with vascular amputations over the values for normal control sub- jects. From this review, there are definite opportunities to reduce kinematic asymmetries, improve metabolic efficiency, and in- crease the speed of ambulating persons with BKAs by different prosthetic design. Very few studies of the effect of prosthesis weight and the location of its center of mass are available. Conflicting concepts exist regarding the influence of the mass properties of prostheses on gait and its efficiency. Czemiecki and coworkers” found that adding .68kg and 1.34kg to the shank of the above-knee prosthesis, without changing the center of mass of the shank, did not change the metabolic requirements significantly; nor did SSWS change. Foerster and colleagues’* found no correlation between mass added or mass location and metabolic energy requirements in persons with above-knee amputations. Changes in mass did not consistently produce changes in metabolic efficiency. Ralston and Lukin,” on the other hand, found in normal persons that weight added to the feet produced a markedly Arch Phys Med Rehabil Vol79, February 1998