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Introduction
Movement abnormalities, which are symptoms of many neuromuscular diseases and conditions, can be caused by aberrations in one or several of the parameters governing muscle excitation, activation, and mechanics. Of the parameters governing muscle mechanics, the optimal fiber length of a muscle is the most important mechanical parameter in determining how much isometric force it can produce [1].
Contracture is a condition in which muscle fibers are permanently shortened to 45% or more of normal length [5]. Several populations, including post-stroke victims, patients with Muscular Dystrophy, and patients with Cerebral Palsy suffer from contracture [8][,2]. Contracture is often present alongside muscle weakness and abnormal muscle excitation.
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This study aims to perform a similar sensitivity analysis on human gait, wherein the parameter of interest is optimal fiber length. This study examines the effects of isolated contracture of the gastrocnemius (both medial and lateral) as well as contracture of the entire triceps surae (gastrocnemius and soleus), as these are two conditions frequently encountered in Cerebral Palsy patients, and two situations meriting different tendon transfer surgical strategies . [5].
Questions to answer:
- What is the effect of calf muscle shortening on tracking normal walking gait kinematics?
- What are the compensatory activation strategies of the remaining muscles?
- How much do total muscle forces and total muscle stresses increase with calf muscle shortening?
Methods
The The Gait2392 OpenSim full-body muscle model was used along with Chand John walking data [3]. This experimental data is provided through OpenSim resources as an example for use with the Gait2392 model. An Umberger metabolic probe was added to the model for later analysis. The study process described below is also outlined in Figure 1.
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Computed Muscle Control: Computed Muscle Control [6] was used to find the optimal muscle activations that would enable the model to reproduce the experimental kinematics. The gastrocnemius excitation controls were set to a maximum of 0.65 so that the simulation could circumvent limitations in the CMC Tool and more closely match measured EMG data during walking. Kinematics were filtered at 6 Hz.
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Notable results from the contracture studies, including passive forces, activations, reserve actuators, and metabolic costs are shown in Figures 2-52–5. All plots represent one gait cycle as defined by the time between consecutive right foot heel strike.
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As a first effort to compensate for excessive passive plantarflexion forces in the calf muscles, the tibialis anterior (dorsiflexor) muscle is increasingly activated (Figure 3). HoweverHowever, at a certain point, the tibialis anterior cannot sufficiently compensate, and the Computed Muscle Control tool must make use of the generalized ankle reserve forces to allow the diseased model to still execute the desired normal gait kinematics without large error (Figure 4). Considering that maximum required ankle moment for this simulation was about 40 Newton-meters, and that reserves greater than 5% of the total force needed are conventionally an indicator of failure to track, these results generally indicate that a patients with isolated gastrocnemius contracture beyond 20-30% and patients with combined gastrocnemius and soleus contracture beyond 10-20% are unlikely to track normal gait. However, the baseline case with no shortening utilized reserves slightly above this 5% criterion, so caution should be used in affixing “successful” or “unsuccessful” tracking to these different conditions.
What can be observed is the following. Heel strike/early stance phase (at the beginning and very end of both plots) are difficult for patients exhibiting both kinds of contracture. This effect reflects clinical observations of Cerebral Palsy patients [9]. However, tracking the foot to be flat on the ground rather than plantarflexed during the stance phase requires enormous reserves for only the combined contracture case. This implies that combined contracture is more likely than isolated contracture to lead to equinus.
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Figure 6: Optimal tendon transfer surgery as simulated in [5], applied to tracking normal walking gait. Differential Differential tendon lengthening dramatically reduces passive forces in the triceps surae and thus the use of ankle reserves.
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Conclusions
In conclusion, a sensitivity study was performed in order to better understand the robustness of human gait to shortened calf muscles. Combined Combined contracture seems to inhibit normal walking more than isolated contracture when compared across similar fiber shortening percentages. Based Based on the relative weakness observed throughout the gait cycle, the tip toe walk (equinus), which arises from a difficulty in dorsiflexion during the stance phase of gait, is more likely a result of combined contracture than of isolated gastrocnemius contracture. The The results of these simulations are available for further study on simtk.org. Further, a framework has been presented to include information on specific types and degrees of contracture into biomechanical modeling and simulation in order to enable the design of personalized tendon transfer surgery techniques for patients with movement disorders.
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References
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