Length-tension relationships in the calf muscles in children with cerebral palsy.

Investigators: Mrs Tandy Hastings-Ison, Prof Mary Galea, Prof David Morgan

Fifteen children with normal development and 15 ambulant children with cerebral palsy (spastic diplegia, spastic hemiplegia) aged between 3 - 9 years, were tested on an ankle ergometer (Fig. 1) to determine the length-tension relationship of their gastrocnemius muscle.

Figure 1 . Child positioned in the ankle ergometer

The children were required to sit on the machine in a partial weightbearing position, with knees extended and ankles strapped onto a moveable footplate.   Isometric plantarflexion contractions were performed at different points in range.   Visual biofeedback was given via a series of lights that lit up with increasing plantarflexion contraction.   Of the children with normal development, better length-tension curves (consistent measurements) were achieved in older children aged between 7 - 9 years.   The children with cerebral palsy had difficulty performing consistent maximal voluntary isometric contractions required for the test.   This was thought to be due to reduced selective motor control, weakness and the possible use of ankle-foot orthotics, which restrict active muscle contraction in the plantarflexion range.

The testing procedure on the machine was modified to allow more points in range to be tested.   An additional test was developed involving the use of an ankle-foot orthosis with a torque measuring device attached (Fig. 2).   In this test the children are able to perform a plantarflexion contraction through range against a constant compliance, eliminating the need for maximum voluntary contractions.   In this way, an indirect length-tension curve can be obtained.

Data collection is continuing on developing a reference sample for both devices as well as testing the abilities of children with cerebral palsy in this area.

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