Control of Tibial Advancement by the Plantar Flexors during the Stance Phase of Gait Depends on Knee Flexion with Respect to the Ground Reaction Force

Abstract

During the stance phase of a normal gait, the triceps surae muscle controls the advancement of the tibia, which contributes to knee extension. Plantar flexor weakness results in excessive dorsiflexion, and consequently, the knee loses this contribution. However, increasing knee flexion is also seen in patients with cerebral palsy who do not have plantar flexor weakness. We aimed to understand this mechanism through the use of a musculoskeletal dynamic model. The model consists of solid segments connected with rotatory joints and springs to represent individual muscles. It was positioned at different degrees of ankle plantarflexion, knee flexion, and hip flexion. The soleus muscle was activated concentrically to produce plantarflexion and push the foot against the ground. The resulting knee extension was analyzed. The principal determinant of knee flexion or extension associated with ankle plantarflexion was the position of the knee joint center. When this was anterior to the line of action of the ground reaction force (GRF), the soleus contraction resulted in increased knee flexion. The knee extension was obtained when the knee was flexed less than approximately 25°. The relation between joint angles, anthropometric parameters, and the position of the GRF was expressed in a mathematical formulation. The clinical relevance of this model is that it explains the failure of plantar flexor control on knee extension in patients with cerebral palsy, when increased knee flexion can occur even if there is a normal or plantarflexed foot position.

Keywords: crouch gait; dynamic simulation; knee flexion in gait; plantar flexion-knee extension couple; plantar flexor control on tibial advancement.

Figure 1

Failure of AFO treatment. Knee kinematic graphs from an instrumented gait analysis of a patient with bilateral cerebral palsy mainly affecting the legs. Although a knee extension was possible up to 25 degrees of flexion (knee flexion contracture), his dynamic extension during gait was limited to 70 degrees of flexion bilaterally. The bilateral treatment with stiff AFOs to gain better knee extension had no effect. Blue = right, red = left, grey = healthy, continuous = barefoot, dotted = stiff AFO.

Figure 2

The multisegmental dynamic model.

Figure 3

Plantarflexion extends the knee (plantar flexion-knee extension couple). In each panel, the initial position of the model is depicted on the left, and the position achieved after ankle plantarflexion is reported on the right. The values of the initial hip and knee joint angles are reported as H and K, respectively. A hypothetical ground reaction force has been drawn as applied at the tip of the foot to show its relationship with the knee joint.

Figure 4

The GRF and the effect of the plantar flexors on knee extension. The legend is the same as in Figure 2. Here, the initial positions include increased knee flexion. It appears that, when the GRF lies posterior to the knee joint, increased knee flexion is produced instead of knee extension. In this case, the plantar flexors lose the competence to control tibial advancement.

Figure 5

A simplified diagram of the lower limb and the GRF (left). The external knee moment is extensor in this case and would be flexor if the GRF was posterior to the knee (PFKEC: plantar flexor-knee extension couple).

Figure 6

Examples of conditions enabling the plantar flexors to control tibial advancement (the GRF in front of the knee joint center) or preventing it (the GRF behind the knee joint center). Upper row: different hip joint angles, knee joint flexed at 30°; lower row: different knee joint angles, hip flexed at 20°. In these examples, the foot longitudinal axis is always oriented 30° towards the floor (foot–floor angle).

Figure 7

Fields of angle values that define the competence of the plantar flexors to control tibial advancement (functional or non-functional). They are separated by a line that depends on the ankle plantarflexion (see explanation in the text).