1237 JRRD JRRD Volume 45, Number 8, 2008 Pages 1237–1248 Journal of Rehabilitation Research & Development Regulation of angular impulse during fall recovery Witaya Mathiyakom, PT, PhD; 1–3* Jill L. McNitt-Gray, PhD 3–5 1 Department of Physical Therapy, California State University, Northridge, CA; 2 Physical Medicine and Rehabilitation Service, Department of Veterans Affairs Greater Los Angeles Healthcare System, Los Angeles, CA; Departments of 3 Kinesiology, 4 Biomedical Engineering, and 5 Biological Sciences, University of Southern California, Los Angeles, CA Abstract—Maintaining balance and postural stability while performing functional activities is critical to an individual’s independence and quality of life. When individuals are unable to maintain their total-body center of mass (COM) within the base of support, a loss of balance may result, leading to a fall. Effective interaction between the environment and the neuro- muscular and musculoskeletal systems allows an individual to generate the ground reaction forces relative to the COM neces- sary for maintaining and recovering balance during expected and unexpected situations. This article reviews the role of the swing and support legs in regulating angular impulse during fall recovery and contrasts the balance recovery strategies used by younger adults and older adult nonfallers and fallers. Multi- joint dynamics and neuromuscular control used during fall recovery are discussed at the total-body, joint, and muscle lev- els. Understanding the fall recovery mechanisms successfully used by younger and older adults will allow us to begin to identify effective intervention strategies that target specific populations. Key words: aging, angular impulse, balance control, center of mass, fall recovery, ground reaction force, linear impulse, lower-limb kinetics, multijoint control, muscle activation pattern. INTRODUCTION Maintaining balance and postural stability while per- forming functional activities is critical to an individual’s independence and quality of life. Effective interaction between the environment and the neuromuscular and musculoskeletal systems allows an individual to generate the ground reaction forces (GRFs) necessary for main- taining and recovering balance during expected and unexpected situations. As one ages, alterations in the ner- vous and/or musculoskeletal system capabilities intro- duce age-related deficits that may influence how an individual plans and executes activities of daily living. Postural control during bipedal gait imposes a signifi- cant challenge to the nervous and musculoskeletal sys- tems. In general, normal bipedal human gait consists of a series of single- and double-leg stance phases. The dura- tion of the single-leg stance is approximately 40 percent of a gait cycle [1]. During the single-leg stance, the total- body center of mass (COM) is vertically positioned at approximately one-half of the body’s standing height above the foot and moves along a horizontal trajectory of approximately 0.5 to 0.6 m within a step cycle. During single-leg stance, the support leg simultaneously supports the weight of the upper body (head, arms, and trunk) [1], helps control upper-body trajectory in both anteroposterior and mediolateral directions [2–3], and generates and redi- rects the GRF to propel the COM in the desired direction Abbreviations: COM = center of mass, GRF = ground reac- tion force, NJM = net joint moment. * Address all correspondence to Witaya Mathiyakom, PT, PhD; Department of Physical Therapy, California State University, Northridge, 18111 Nordhoff St, Northridge, CA 91330; 818-677-3108; fax: 818-677-7411. Email: witaya.mathiyakom@csun.edu DOI: 10.1682/JRRD.2008.02.0033