Abstract
Aim: To study the causes of locomotor dysfunction, estimate muscle forces, or understand the influence of altered sarcomere and muscle properties and behaviours on whole body function, it is necessary to examine the leverage with which contractile forces operate. At the ankle joint, current methods to quantify this leverage for the plantarflexors do not account for curvature of the Achilles tendon, and so may not be appropriate when studying equinus gait. Thus, novel methodologies need to be developed and implemented to quantify the Achilles tendon moment arm length during locomotion.Methods: Plantarflexor internal moment arm length and effective mechanical advantage of 11 typically developed young adults were calculated throughout stance, while heel-toe walking and voluntarily toe-walking on an instrumented treadmill. Achilles tendon moment arm was defined in two-ways: (1) assuming a straight tendon, defined between the gastrocnemius medialis myotendinous junction and Achilles tendon insertion point, and (2) accounting for tendon curvature, by tracking the initial path of the Achilles tendon from the calcaneal insertion.Results: When accounting for tendon curvature, Achilles tendon moment arm length and plantarflexor effective mechanical advantage did not differ between walking conditions (p > 0.05). In contrast, when assuming a straight tendon, Achilles tendon moment arm length (p = 0.043) and plantarflexor effective mechanical advantage (p = 0.007) were significantly greater when voluntary toe-walking than heel-toe walking in late stance.Discussion: Assuming a straight Achilles tendon led to a greater Achilles tendon moment arm length and plantarflexor effective mechanical advantage during late stance, compared to accounting for tendon curvature. Consequently, plantarflexor muscle force would appear smaller when assuming a straight tendon. This could lead to erroneous interpretations of muscular function and fascicle force-length-velocity behaviour in vivo, and potentially inappropriate and ineffective clinical interventions for equinus gait.
Highlights
In human locomotion, propelling the body forward necessitates the development of joint moments which exceed those caused by the ground reaction force (GRF)
External moment arm length in the voluntary toe-walking condition remained relatively constant throughout stance, whereas for the heel-toe walking condition, external moment arm length increased throughout stance (Figure 4)
We have developed a reliable method to account for Achilles tendon curvature in the assessment of internal moment arm length and effective mechanical advantage of the plantarflexors during locomotion
Summary
In human locomotion, propelling the body forward necessitates the development of joint moments which exceed those caused by the ground reaction force (GRF). The gastrocnemius medialis muscle of typically developed adults has been shown to operate quasi-isometrically and close to the region where optimal sarcomere overlap would be expected (Fukunaga et al, 2001) This behaviour favours the economical production of high contractile forces. Differences in sarcomere length and function (Ponten et al, 2007; Lieber et al, 2017), muscle stiffness (Barber et al, 2011), and lengthening characteristics (Kalkman et al, 2018) have been documented in those with CP, and associated differences in fascicle behaviour during gait observed (Barber et al, 2017) These may explain why these children find it difficult to generate the required forces for adequate moment development about the ankle joint
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