Intra-operatively measured spastic semimembranosus forces of children with cerebral palsy

The knee kept forcibly in a flexed position is typical in cerebral palsy. Using a benchmark, we investigate intra-operatively if peak spastic hamstring force is measured in flexed knee positions. This tests the assumed shift of optimal length due to adaptation of spastic muscle and a decreasing force trend towards extension. Previously we measured spastic gracilis (GRA) and semitendinosus (ST) forces. Presently, we studied spastic semimembranosus (SM) and tested the following hypotheses: spastic SM forces are (1) high in flexed and (2) low in extended positions. We compared the data to those of GRA and ST to test (3) if percentages of peak force produced in flexed positions are different. During muscle lengthening surgery of 8 CP patients (9 years, 4 months; GMFCS levels = II-IV; limbs tested = 13) isometric SM forces were measured from flexion (120°) to full extension (0°). Spastic SM forces were low in flexed knee positions (only 4.2% (3.4%) and 10.7% (9.7%) of peak force at KA = 120° and KA = 90° respectively, indicating less force production compared to the GRA or ST) and high in extended knee positions (even 100% of peak force at KA = 0°). This indicates an absence of strong evidence for a shift of optimal muscle length of SM towards flexion.