Children who idiopathically toe-walk have greater plantarflexor effective mechanical advantage compared to typically developing children

David M. Wright,Alfie Bass,Vasilios Baltzoloulos,Roger Walton,Carla Harkness-Armstrong,Constantinos Maganaris,Thomas D. O’Brien

doi:10.1007/s00421-022-04913-7

Abstract

PurposeThe effective mechanical advantage (EMA) of the plantarflexor muscles is important for gait function and is likely different from typical in equinus gait. However, this has never been quantified for children who idiopathically toe-walk (ITW), despite being routinely altered through clinical intervention.MethodsThis study quantified the Achilles tendon and ground reaction force (GRF) moment arms, and the plantarflexor EMA of 5 children who ITW and 14 typically developing (TD) children, whilst walking on an instrumented treadmill.ResultsThere was no difference in the Achilles tendon moment arm length throughout stance between groups (p > 0.05). Children who ITW had a significantly greater GRF moment arm length in early stance (20–24% p = 0.001), but a significantly shorter GRF moment arm length during propulsion (68–74% of stance; p = 0.013) than TD children. Therefore, children who ITW had a greater plantarflexor EMA than TD children when active plantarflexion moments were being generated (60–70% of stance; p = 0.007). Consequently, it was estimated that children who ITW required 30% less plantarflexor muscle force for propulsion.ConclusionClinical decision making should fully consider that interventions which aim to restore a typical heel-toe gait pattern risk compromising this advantageous leverage and thus, may increase the strength requirements for gait.

Highlights

Self-selected walking speed did not differ between groups (0.76 ± 0.15 vs 0.86 ± 0.15 m·s−1; p = 0.186). Normalisedheight Achilles tendon moment arm length remained relatively constant throughout stance in both groups (Fig. 2a) and did not significantly differ between children who idiopathically toe-walk (ITW) and typically developing (TD) children
Children who ITW had, on average, a greater plantarflexor effective mechanical advantage (EMA) than TD children in the period of stance where meaningful plantarflexion moments were generated in both groups
Clinical decision making should fully consider that interventions which restore a typical heel-to-toe gait pattern risk compromising this advantageous leverage and may increase the plantarflexor muscle force requirements for gait

Summary

Introduction

Efficient locomotion requires the generation of adequate contractile muscle force to overcome the effects of external forces acting on the body. The required magnitude of contractile force is partly determined by the leverage about which the muscle and ground reaction forces (GRF) act on the skeleton For the plantarflexors, this leverage, known as effective mechanical advantage (EMA), is quantified as the ratio between moment arms of the internal Achilles tendon force and external GRF (Biewener et al 2004). Moment arms are defined as the shortest perpendicular distance between the line of action of force and the axis of rotation Both moment arms can be altered by anatomical variations and/or kinematic changes, which in turn greatly influence the muscle force required to generate adequate joint moments (Lee and Piazza 2009). Children who idiopathically toe-walk (ITW) walk in equinus despite no diagnosed orthopaedic or neurological disorder (Sala et al 1999)

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