Kinematics and kinetics of unanticipated misstep conditions: Femoral fracture implications in the elderly Mehmet Uygur, James G. Richards, Slobodan Jaric, Paulo B. de Freitas, David A. Barlow à Human Performance Lab, Department of Health, Nutrition, and Exercise Sciences, University of Delaware,11 Carpenter Sports Building, Newark, DE 19716, USA article info Article history: Accepted 11 March 2009 Keywords: Gait Stepping Hip Impact Inverse dynamics Stair descent abstract Most hip fractures are thought to occur after falling during everyday activities. We speculated that hip fractures might also occur because of excessive loading of the hip joint during an unexpected misstep consequently leading to a fall. The aims of this study were to explore the kinematics and kinetics of the lower extremity joints during missteps as compared with regular stepping, as well as to compare the magnitude of forces acting upon the hip joint with the threshold forces expected to fracture the hip. Fourteen healthy adults performed two forward steps on a 17.8cm high platform under the following four conditions: forward with and without vision, as well as a misstep (the box for the final step unexpectedly removed without participant awareness), and regular stepping down with eyes open. The results revealed no differences between stepping forward with and without vision. When compared with both stepping forward and regular stepping down, the misstep revealed altered joint positions accompanied by increased forces and moments acting upon the hip joint. For example, the peak vertical proximal thigh segment force was 3.0570.55 BW vs. 1.2370.14 BW and 0.9170.09 BW (po.001; misstep vs. regular stepping down and stepping forward, respectively), while the proximal thigh segment moment in frontal plane was 1.3970.70 Nm/kg vs. 0.1870.32Nm/kg of adduction and 0.1670.19 Nm/kg of abduction (po.001). When compared with the literature data, the forces during misstep were within the range of those forces that could result in hip fractures in the elderly. Therefore, it may be possible for the elderly to experience hip/proximal femur fractures during missteps prior to falling. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Hip fractures are one of the most devastating health problems for the elderly because of the associated mortality, immobility, and financial consequences. In the United States, around 340,000 people experience hip fractures annually (Kozak et al., 2002) and approximately 20% of them die within a year after their injury (Leibson et al., 2002). Due to the fact that the estimated elderly population in year 2040 will be three times higher than it was in 2000 (Wan et al., 2005), the accumulation of hip fracture injuries is expected to increase. Therefore, understanding the mechanisms involved in hip fractures may lead to the adoption of strategies that can reduce their prevalence in the elderly. It is a common belief that most hip fractures occur as a consequence of trauma (e.g. compression forces along the axis of the femoral neck) and more than 90% are fall related (Nyberg et al., 1996). In addition, some studies have reported spontaneous (without trauma) hip fractures occurring during daily ambulation (Michelson et al., 1995). In one survey, 24% of hip fracture patients expressed that a leg gave way prior to falling (Sloan and Holloway, 1981). Moreover, the occurrence of spontaneous hip fractures could be underestimated since the elderly are often incapable of explicitly report whether they broke their hip before or after falling (Sloan and Holloway, 1981; Youm et al., 1999). Therefore, we consider that hip fractures might not occur as a consequence of a fall, but due to the excessive loads applied in the hip joint during a misstep, which can happen in situations such as miscounting the number of steps during stair descending. During level walking or in expected stepping down, a high portion of the shock of impact is gradually absorbed in the lower extremity by pre-programmed muscle activity and positioning of lower extremity joints (McFadyen and Winter, 1988). However, during missteps, this anticipatory strategy may not function adequately leading to unusual loads applied to the skeletal and joint systems. For example, over extended leg joints can lead to excessive joint forces and toques originated from the ground reaction force (van Dieen et al., 2007). Due to the detrimental effects of aging on the musculoskeletal system (Frost, 1997), the possible excessive loading of the lower extremity joints during misstep caused by altered joint angles may increase the risk of skeletal fractures, principally in the hips (Keyak et al., 2001). ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics 0021-9290/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2009.03.022 à Corresponding author. E-mail address: Barlow@udel.edu (D.A. Barlow). Journal of Biomechanics 42 (2009) 1241–1245