Introduction and Aims: We investigated the hypothesis that the effects of muscle-tendon surgery could be controlled or ‘dosed’ by varying the location of intramuscular tenotomy (IMT) or fascial striping within the muscle-tendon unit (MTU). We performed a series of randomised trials in paired cadaver MTUs of tibialis posterior, semitendinosus, gracilis and semimembranosus.

Method: Following dissection of 10 paired cadaver MTUs of the above-mentioned muscles, we performed a series of randomised trials in which each pair of MTUs received a low or high IMT. ‘Low IMT’ was defined as an IMT performed two centimetres proximal to the distal musculotendinous junction. ‘High IMT’ was performed two centimetres distal to the start of the first tendinous fibres in the proximal muscle belly. The force-length characteristics were then determined by tensile load testing until failure on an Instron machine. The load and lengthening at failure for each pair of MTUs were compared by paired t test.

Results: As expected, there were significant differences in the load versus length curves for different muscles and for different simulated surgeries (IMT versus fascial striping). The mean load at failure was significantly lower for all low IMTs compared to high IMTs in all MTUs tested e.g. tibialis posterior: mean difference low versus high = 13N (95% CI 6.8, 19.2 p< 0.001). The lengthening at failure was also greater for low IMTs than for high IMTs. The difference reached statistical significance only in tibialis posterior.

Conclusions: The site of the intramuscular surgery or fascial striping has a direct bearing on the force versus lengthening curve. We hypothesise that the same principle applies during muscle tendon surgery in children with spastic contractures and that it may be possible to graduate surgical lengthening, according to the correction required.