Muscle architecture and passive lengthening properties of the gastrocnemius medialis and Achilles tendon in children who idiopathically toe‐walk

Abstract

Children who idiopathically toe‐walk (ITW) habitually operate at greater plantarflexion angles and thus, at shorter muscle‐tendon unit (MTU) lengths than typically developing (TD) children. Therefore, it is often assumed that habitual use of the gastrocnemius muscle in this way will cause remodelling of the muscle‐tendon architecture compared to TD children. However, the gastrocnemius muscle architecture of children who ITW has never been measured. It is essential that we gain a better understanding of these muscle‐tendon properties, to ensure that appropriate clinical interventions can be provided for these children. Five children who ITW (age 8 ± 2 years) and 14 TD children (age 10 ± 2 years) participated in this study. Ultrasound was combined with isokinetic dynamometry and surface electromyography, to measure muscle architecture at common positions and passive lengthening properties of the gastrocnemius muscle and tendon across full range of motion. Regardless of which common condition groups were compared under, both the absolute and normalised to MTU muscle belly and fascicle lengths were always longer, and the Achilles tendon length was always shorter in children who ITW than TD children (p < 0.05; large effect sizes). The passive lengthening properties of the muscle and tendon were not different between groups (p > 0.05); however, passive joint stiffness was greater in children who ITW at maximum dorsiflexion (p = 0.001) and at a joint moment common to all participants (p = 0.029). Consequently, the findings of this pilot study indicate a remodelling of the relative MTU that does not support the concept that children who ITW commonly experience muscle shortening. Therefore, greater consideration of the muscle and tendon properties are required when prescribing clinical interventions that aim to lengthen the MTU, and treatments may be better targeted at the Achilles tendon in children who ITW.

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

“…Despite being more plantarflexed, the moment-fascicle length relationship shifted significantly towards longer fascicle lengths in children who ITW (Figure 2b). This is consistent with recent findings showing that the passive gastrocnemius fascicle lengths were longer in children who ITW than TD children (Harkness-Armstrong et al, 2021), and corresponded with the ranges used during gait. Consequently, children who ITW and TD children both operated at similar joint angles and fascicle lengths relative to their respective measured optimum.…”

“…Therefore, we should not assume that the muscle functional properties are similarly affected in children who ITW. Indeed recently, we showed that children who ITW had longer resting absolute and normalised gastrocnemius medialis muscle belly and fascicle lengths than typically developing (TD) children (Harkness-Armstrong et al, 2021). This is the exact opposite effect to that found in children with CP.…”

Gut Microbiome Dysbiosis as a Potential Risk Factor for Idiopathic Toe-Walking in Children: A Review

“…Thus, additional research is also required on the involvement of the intestinal microbiota of children with IWT in the development of Achilles tendon stiffness. The development of Achilles tendon stiffness and shortening in several children with ITW, together with habitual use of the gastrocnemius muscle and changes in the acting load, can lead to remodeling of the muscle-tendon architecture and the formation of equinus contracture [172][173][174].…”

Comments on Harkness‐Armstrong et al. (2021) ‘In vivo operating lengths of the gastrocnemius muscle during gait in children who idiopathically toe‐walk’

“…For the contracture model, the optimal fibre lengths and tendon slack length were obtained by minimising the root mean square error between the gastrocnemius and soleus passive moment-angle curve of the model and the 20 • shifted curve Musculotendon parameters were initially scaled such that the original force-length curves were maintained (Modenese et al, 2016). Next, for ITWs the generic passive ankle moment-angle curves for the gastrocnemii and soleus were shifted by 20 • into plantarflexion (Figure 1), because the maximum passive dorsiflexion range in ITWs was ∼20 • smaller than for TD children (Harkness-Armstrong et al, 2021b). When replicating the moment-length curves from Harkness-Armstrong et al (2021a; Figure 2a) using simulations with maximum activations at all joint angles, the moment-length curves did not reach a descending limb and stayed below optimal fibre length (Figure 2b).…”