Modelling the triceps surae muscle-tendon complex for the estimation of length changes during walking.

The triceps surae muscle-tendon complex has been modelled by many authors seeking to estimate the change in muscle length that occurs in locomotion. The objective of the present study is to assess to what extent the commonly adopted assumptions of foot rigidity and pure sagittal motion are acceptable. A model of the triceps surae muscle-tendon complex was implemented by taking into account all possible movements between forefoot and rear foot. Length and velocity curves from a 3-dimensional gait analysis were obtained from six normal subjects. The angle between forefoot and rear foot proved to be changeable with stride (11.8 degrees +/- 4.7 SE). The effect on the length and velocity estimation was analysed by comparing the curves obtained by our model to those obtained by a model in which the foot is considered to be a rigid body. Significant differences were found for the soleus muscle length at late stance/early swing and late swing phases, and for the soleus muscle velocity at early stance phase. The length and velocity curves were also compared to curves calculated on a pure sagittal projection. No changes were observed, except for an offset of 1-3 mm caused by the general external rotation of the foot (which is also present in standing). The curves appeared superimposable when referred to the standing upright position. Care needs to be taken, however, when extending the above results to the clinical application, where foot deformity and deviation from a normal pattern of motion can occur.