Impact of ankle foot orthosis stiffness on Achilles tendon and gastrocnemius function during unimpaired gait

Abstract

Ankle foot orthoses (AFOs) are designed to improve gait for individuals with neuromuscular conditions and have also been used to reduce energy costs of walking for unimpaired individuals. AFOs influence joint motion and metabolic cost, but how they impact muscle function remains unclear. This study investigated the impact of different stiffness AFOs on medial gastrocnemius muscle (MG) and Achilles tendon (AT) function during two walking speeds. We performed gait analyses for eight unimpaired individuals. Each individual walked at slow and very slow speeds with a 3D printed AFO with no resistance (free hinge condition) and four levels of ankle dorsiflexion stiffness: 0.25 Nm/°, 1 Nm/°, 2 Nm/°, and 3.7 Nm/°. Motion capture, ultrasound, and musculoskeletal modeling were used to quantify MG and AT lengths with each AFO condition. Increasing AFO stiffness increased peak AFO dorsiflexion moment with decreased peak knee extension and peak ankle dorsiflexion angles. Overall musculotendon length and peak AT length decreased, while peak MG length increased with increasing AFO stiffness. Peak MG activity, length, and velocity significantly decreased with slower walking speed. This study provides experimental evidence of the impact of AFO stiffness and walking speed on joint kinematics and musculotendon function. These methods can provide insight to improve AFO designs and optimize musculotendon function for rehabilitation, performance, or other goals.

Introduction

Ankle foot orthoses (AFOs) can be used to improve walking function (Figueiredo et al., 2008). Most AFOs are passive, exerting a torque about the ankle based upon the stiffness of the AFO’s structure. In unimpaired individuals, passive AFOs have been shown to reduce metabolic costs of walking by up to 7% (Collins et al., 2015). However, this reduction is less than that predicted by various models and researchers have hypothesized this discrepancy is due to complex musculotendon dynamics during gait (Collins et al., 2015, Sawicki and Khan, 2016). Since the amount of force a muscle can produce is dependent on muscle length and velocity (Hill, 1953, Hill, 1938), if an AFO changes a muscle’s length in unanticipated ways, AFOs may have counter-intuitive impacts on muscle force and metabolic costs. However, the impact of AFOs on the length of key muscles, such as the gastrocnemius, has not been experimentally evaluated.

The ankle plantarflexors play a critical role supporting and propelling the body during gait (Neptune et al., 2008, Steele et al., 2010). Previous studies have shown the contributions of the ankle plantarflexors to motion can be significantly altered when wearing AFOs (Collins et al., 2015, Delafontaine et al., 2017). For example, a case series of children with cerebral palsy evaluated the impact of AFOs on muscle contributions to mass center accelerations and found that AFOs significantly reduced gastrocnemius contributions to support and propulsion during gait (Ries, 2017). The gastrocnemius is a bi-articular muscle that generates large forces during activities of daily living (Finni et al., 1998, Sawicki and Ferris, 2009). It inserts into the Achilles tendon (AT), the thickest and strongest tendon in the human body (Muffulli, 1999, Giddings, 2000), and stores and releases mechanical energy during gait (Cavagna et al., 1977, Farris and Sawicki, 2012). Given the importance of the gastrocnemius and AT to efficient gait, it is important to understand how AFOs will impact their function.

Quantifying muscle and tendon length during dynamic tasks like walking is challenging. The overall musculotendon unit (MTU) lengths can be estimated with musculoskeletal modeling using information about an individual’s joint kinematics, musculoskeletal geometry, and musculotendon moment arms (Delp et al., 1990). Prior studies have evaluated the impact of various types of AFOs on MTU lengths in individuals with neurologic injuries, demonstrating AFOs can impact MTU stretching and shortening during gait, depending on AFO stiffness and other properties (Choi et al., 2017, Choi et al., 2015, Thompson et al., 2002). However, these methods are limited to estimating changes in overall MTU lengths, and do not differentiate between the relative muscle and tendon lengths.

Ultrasound can be used to experimentally monitor the relative length of the gastrocnemius and AT (Lichtwark and Wilson, 2006, Kalsi et al., 2016, Ishikawa et al., 2005). These methods have demonstrated that changes in AT length help reduce changes in gastrocnemius length, allowing it to operate near isometric conditions during gait (Fukunaga et al., 2001). If AFOs decrease ankle plantarflexor activity, they may alter the efficiency and function of the gastrocnemius and AT during gait (Figueiredo et al., 2008, Lichtwark and Wilson, 2006, Cronin et al., 2010, Kalsi et al., 2016). Using a mathematical model, one prior study suggested decreased gastrocnemius activity with an AFO may alter changes in muscle and tendon length, compromising muscle force production and metabolic costs during gait (Sawicki and Khan, 2016); however, changes in gastrocnemius function with AFOs or other assistive devices has not been experimentally investigated.

In this study, we integrated ultrasound and musculoskeletal modeling to evaluate changes in AT and gastrocnemius length during gait with AFOs. Based upon prior research, we hypothesized that (1) as AFO stiffness increases, gastrocnemius muscle activity decreases leading to greater stretching of the gastrocnemius and a decrease in peak AT length during gait, and (2) as walking speed decreases, the gastrocnemius lengthens at a slower rate during stance, decreasing the sensitivity of musculotendon dynamics to different AFO conditions. By examining the impact of both AFO stiffness and walking speed on AT and gastrocnemius function, this research may inform future AFO design and prescription for both unimpaired individuals and individuals with neurologic injuries.