Whole-body angular momentum during stair walking using passive and powered lower-limb prostheses*

Abstract

Individuals with a unilateral transtibial amputation have a greater risk of falling compared to able-bodied individuals, and falling on stairs can lead to serious injuries. Individuals with transtibial amputations have lost ankle plantarflexor muscle function, which is critical for regulating whole-body angular momentum to maintain dynamic balance. Recently, powered prostheses have been designed to provide active ankle power generation with the goal of restoring biological ankle function. However, the effects of using a powered prosthesis on the regulation of whole-body angular momentum are unknown. The purpose of this study was to use angular momentum to evaluate dynamic balance in individuals with a transtibial amputation using powered and passive prostheses relative to able-bodied individuals during stair ascent and descent. Ground reaction forces, external moment arms, and joint powers were also investigated to interpret the angular momentum results. A key result was that individuals with an amputation had a larger range of sagittal-plane angular momentum during prosthetic limb stance compared to able-bodied individuals during stair ascent. There were no significant differences in the frontal, transverse, or sagittal-plane ranges of angular momentum or maximum magnitude of the angular momentum vector between the passive and powered prostheses during stair ascent or descent. These results indicate that individuals with an amputation have altered angular momentum trajectories during stair walking compared to able-bodied individuals, which may contribute to an increased fall risk. The results also suggest that a powered prosthesis provides no distinct advantage over a passive prosthesis in maintaining dynamic balance during stair walking.

Introduction

Ascending and descending stairs is frequently required during the completion of activities of daily living. Walking on stairs is biomechanically demanding and requires the generation of large joint moments relative to level walking (Andriacchi et al., 1980), which necessitates greater muscle output. The increased physical requirements may explain why populations with muscle weakness or impaired balance, such as the elderly, more frequently experience serious injury due to falling on stairs than able-bodied (AB) individuals (Startzell et al., 2000). Similarly, individuals with transtibial amputation (TTA) are characterized by reduced walking ability and altered muscle function compared to their able-bodied counterparts (Silverman and Neptune, 2012). Further, individuals with TTA have impaired balance (Jayakaran et al., 2012), have a high risk and fear of falling (Miller et al., 2001), and report a decreased ability to walk on stairs (De Laat et al., 2013). The increased biomechanical demand of stair walking combined with greater fall risk in individuals with TTA make walking on stairs a significant safety concern for this patient population. Understanding how balance is maintained during stair walking is critical to ultimately reducing the incidence of falls in individuals with TTA.

Whole-body angular momentum ($\overrightarrow{H}$) is tightly regulated by AB individuals during level walking (Herr and Popovic, 2008) to maintain dynamic balance. Regulation of $\overrightarrow{H}$ is achieved through the generation of an external moment about the body center-of-mass (COM), which is a function of the ground reaction forces (GRFs) applied to the feet and the external moment arm representing the distance of the center-of-pressure (COP) relative to the body COM (Fig. 1). Muscles are the primary contributors to the net external moment about the body COM (Neptune and McGowan, 2011) and to the net joint moments. Thus, analysis of the joint kinetics can provide insight into potential mechanisms used to regulate $\overrightarrow{H}$.

The ankle plantarflexor muscle group, which includes the soleus and gastrocnemius, is the only muscle group that contributes to the regulation of $\overrightarrow{H}$ throughout the gait cycle during level walking (Neptune and McGowan, 2011). The ankle muscles are also important for trip recovery (Pijnappels et al., 2005a, Pijnappels et al., 2005b), and fallers can be distinguished from non-fallers by decreased peak ankle plantarflexion moments (Simoneau and Krebs, 2000). The importance of the plantarflexors in the regulation of $\overrightarrow{H}$ and fall prevention may partially explain why TTA have a greater risk and fear of falling relative to AB.

The ankle plantarflexors are also critical in stair walking. During stair descent, the plantarflexors are predominantly active during touch-down at the beginning of the stance phase (Spanjaard et al., 2008). However, a conventional passive energy-storage-and-return prosthesis cannot provide active ankle plantarflexion and the associated energy absorption through controlled dorsiflexion that occurs during early stance in stair descent (Sinitski et al., 2012). During stair ascent, the inability of the passive prosthesis to generate plantarflexion power during late stance (i.e., as the trailing limb) is evident in the decreased ankle moment and work output, and the intact limb generates increased plantarflexion power (Sinitski et al., 2012).

Recently, powered prostheses with motorized ankle joints have been developed (Au et al., 2007). These devices aim to restore natural gait by performing positive net work at the ankle joint over the gait cycle and have shown promising results in reducing metabolic costs and increasing preferred walking velocity during level-ground walking (Herr and Grabowski, 2012). However, these prostheses are not explicitly designed for stair walking (Eilenberg et al., 2010), and individuals with TTA have similar kinematics and kinetics when walking with both powered and passive prostheses on stairs (Aldridge et al., 2012). In addition, the influence of powered prostheses relative to passive prostheses on maintaining dynamic balance is unknown.

Therefore, the purpose of this study was to investigate the influence of passive versus powered prostheses on maintaining dynamic balance, quantified by $\overrightarrow{H}$, during stair walking. GRFs, external moment arms, and joint powers were used to help interpret differences in the $\overrightarrow{H}$ trajectories. Level walking was also analyzed as a baseline for comparison with the stair walking conditions. We hypothesized that individuals with TTA using both passive and powered prostheses would have a decreased range and maximum vector magnitude of $\overrightarrow{H}$ relative to AB during stair descent. This result was expected due to the increased fear and risk of falling in participants with TTA, possibly leading to a more conservative strategy. Passive and powered prostheses were expected to have similar performance during stair descent as neither device provides the ankle power absorption that occurs during stair descent in AB (Sinitski et al., 2012). We also hypothesized that individuals with TTA using passive and powered prostheses would have an increased range and maximum vector magnitude of $\overrightarrow{H}$ relative to AB during stair ascent, similar to previous results on level ground (Silverman and Neptune, 2011). Due to the ability of the powered prosthesis to generate net positive power at the ankle joint and more effectively emulate a biological ankle, we expected that $\overrightarrow{H}$ would be similar between individuals with TTA using a powered prosthesis and AB during stair ascent. Quantifying differences in $\overrightarrow{H}$ between individuals with TTA using different prostheses relative to AB will provide insight into the influence of powered and passive prostheses on gait mechanics and strategies used to maintain dynamic balance during stair walking.