Spinal Loads During Activities of Daily Living: Influences of Unilateral Lower-Limb Amputation

The prevalence of low back pain (LBP) among persons with unilateral lower limb amputation (ULLA) has become a major health concern, as there are nearly twice as many self-reported incidences of LBP among this population than among the general population.1-4 Moreover, studies have shown LBP to be the most important factor that adversely affects the physical independence and quality of life in persons with ULLA.3, 5 Given the projected two-fold increase in the number of persons with amputation by 20506, it is important to investigate the underlying mechanism(s) responsible for the elevated prevalence of LBP in this cohort.7, 8 As such, this project will contribute to quantitative-based strategies for mitigating the increased risk for LBP, thereby helping to promote and improve long-term functional outcomes and quality of life for individuals with ULLA. Persons with ULLA demonstrate increased and asymmetric trunk movements during locomotion as compared to able-bodied individuals. These differences in trunk motion have been suggested to result from a motor strategy that uses trunk weight/inertia to assist with forward progression and/or stabilization of the body.9 The manner in which muscles respond to a given task demand internally and the way the task is performed externally are two major factors that impose mechanical loading on joints within the musculoskeletal system. Therefore, it is desirable to recover the physical function of persons with ULLA such that the imposed mechanical load from internal and external sources on the joint is minimized. However, our preliminary analyses showed that the larger and asymmetric trunk motion of persons with ULLA during gait corresponded with a muscle recruitment pattern in the lower back that resulted in substantial increases (up to ~65%) in their spinal loads relative to controls. Given the important role of spinal loading in the development of LBP, it is critical to account for the impact of the adopted neuromuscular patterns on spinal loading during physical rehabilitation of persons with ULLA. Such considerations, however, are typically overlooked in rehabilitation processes, in part due to the absence of appropriate quantitative measures. In recent years, the Biomechanics Laboratory at Walter Reed National Military Medical Center (WRNMMC) has had unprecedented access to a large population of service members with ULLA. Using standardized data collection protocols, the Biomechanics Lab, has built a large database of accurate and reliable data from which we can launch targeted retrospective data analyses. Given availability of this large database (see letters) along with our innovative finite element (FE) model of the lower back, we can quantitatively determine the impact of altered trunk movements and neuromuscular responses on spinal loads in persons with ULLA during several common daily activities. Thus in this project, we will conduct secondary analyses of kinematic data collected at WRNMMC for individuals with and without ULLA (ntotal=142) during at least one of the following tasks: 1) walking at self-selected and controlled speeds, 2) sitting-to-standing and standing-to-sitting, and 3) stairs ascending and descending. We hypothesize that the trunk movement strategies adopted by individuals with ULLA to cope with physical demands of daily activities are associated with a complex pattern of internal muscle responses that will result in larger loads on the spine compared with able-bodied individuals. Specific Aim 1: Determine differences in trunk muscle responses between persons with and without ULLA. The required forces in 56 muscles of the lower back for completing the above mentioned activities will be estimated for individuals with (n=110) and without (n=32) ULLA using our FE framework. Our working hypothesis is that, compared to able-bodied individuals, persons with ULLA adopt a different muscle recruitment pattern, primarily due to co-activation of several antagonistic muscles, in response to stability and equilibrium requirements of the spine. Specific Aim 2: Determine differences in spinal loads between persons with and without ULLA. The resultant spinal loads (i.e., compression as well as anterior-posterior and medio-lateral shear forces) at all levels of lumbar spine will be calculated using the estimated muscle responses (from Aim 1). Our working hypothesis is that individuals with ULLA experience substantially larger spinal loads compared to controls when performing the above activities. We predict this is due to differences in muscle recruitment patterns, as well as differences in trunk motion strategies of these two groups under various daily activities. This research on task-specific alterations in the neuromuscular control of trunk motion following amputation, and their contribution to spinal load, can help guide the design of rehabilitation programs aimed to minimize risk for the onset and/or recurrence of LBP. The immediate outcomes from this pilot project will be to provide a platform for the design of future projects wherein the impact of specific interventions (e.g., trunk neuromuscular training, passive tissue stretching, and balance training) for controlling trunk motion and spinal load will be investigated. While focusing on individuals with ULLA, longer-term impacts of such research will have implications for understanding LBP among other at-risk populations with lower limb neuro/musculoskeletal impairments (e.g., anterior cruciate ligament injuries and knee osteoarthritis).