In vivo operating lengths of the gastrocnemius muscle during gait in children who idiopathically toe-walk.

Carla Harkness‐Armstrong,Constantinos Maganaris,Alfie Bass,Roger Walton,Vasilios Baltzopoulos,David M Wright,Thomas D O'Brien

doi:10.1113/ep089658

Abstract

What is the central question of this study? What are the in vivo operating lengths of the gastrocnemius muscle in children who idiopathically toe-walk? What is the main finding and its importance? Children who idiopathically toe-walk operate at more plantarflexed positions but at longer fascicle lengths than typically developing children during gait. However, these ranges utilised during gait correspond to where children who idiopathically toe-walk are optimally strong. This should be considered when prescribing clinical treatments to restore typical gait. Children who idiopathically toe-walk (ITW) habitually operate at greater plantarflexion angles than typically developing (TD) children, which might result in shorter, sub-optimal gastrocnemius fascicle lengths. However, currently no experimental evidence exists to substantiate this notion. Five children who ITW and 14 TD children completed a gait analysis, whilst gastrocnemius fascicle behaviour was simultaneously quantified using ultrasound. The moment-angle (hip, knee and ankle) and moment-length (gastrocnemius) relationships were determined from isometric maximum voluntary contractions (MVC) on an isokinetic dynamometer combined with ultrasound. During gait, children who ITW operated at more plantarflexed angles (Δ=20°; P=0.013) and longer muscle fascicle lengths (Δ=12mm; P=0.008) than TD children. During MVC, no differences in the peak moment of any joint were found. However, peak plantarflexor moment occurred at significantly more plantarflexed angles (-16 vs. 1°; P=0.010) and at longer muscle fascicle lengths (44 vs. 37mm; P=0.001) in children who ITW than TD children. Observed alterations in the moment-angle and moment-length relationships of children who ITW coincided with the ranges used during gait. Therefore, the gastrocnemius muscle in children who ITW operates close to the peak of the force-length relationship, similarly to TD children. Thus, this study indicates that idiopathic toe-walking is truly an ankle joint pathology, and children who ITW present with substantial alterations in the gastrocnemius muscle functional properties, which appear well adapted to the characteristic demands of equinus gait. These findings should be considered when prescribing clinical treatments to restore typical gait.

Highlights

Locomotion relies on joint moments to propel the body forwards, and so adequate contractile muscle force must be produced
Children who idiopathically toe-walk (ITW) operated at more plantarflexed angles, but at longer fascicle lengths than typically developing (TD) children
This coincided with observed differences in the peak of the moment–angle and moment–length relationships, measured from maximum voluntary contractions (MVC), which showed that children who ITW had the greatest strength at more plantarflexed angles and an Equinus gait has previously been linked with ankle, knee and hip extensor weakness in other clinical populations (Hampton et al, 2003; Kennedy et al, 2020; Morozova et al, 2017; Wiley & Damiano, 1998)

Summary

Introduction

Locomotion relies on joint moments to propel the body forwards, and so adequate contractile muscle force must be produced. The plantarflexor muscles, a vital source of the required mechanical power for gait (Winter, 1983), have been shown to operate quasi-isometrically and close to the region of optimal sarcomere length during gait (Fukunaga et al, 2001), thereby maximising the potential muscle force and promoting the economical production of this force. This relies on gait kinematics that are matched to the underlying muscle–tendon architecture and functional properties. To improve our understanding of how the altered gait kinematics may affect the ability of children who ITW to produce force economically, it is important to establish whether muscle strength, determined by the in vivo force–length and moment–angle relationships, may be contributing to the toe-walking gait characteristic in these children

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