1. The rate of paraspinal (multifidus) muscle protein synthesis was measured bilaterally at the top, apex and bottom of the thoracic curve in nine children with an idiopathic scoliosis, using the stable-isotope-labelled amino acid l-[1-13C]leucine.

2. No significant difference was observed in rates of muscle protein synthesis between the two sides of the spine, at the levels of the first vertebrae in neutral alignment at the top and bottom of the curve. However, in every patient, at the apex of the spinal curve, synthesis was higher on the convexity than on the concavity (0.077 ±0.04 %/h convex, 0.052 ±0.02 %/h concave, means ±sd, P < 0.01).

3. Muscle RNA activity (μg of protein synthesized h−1 μg−1 of RNA) was lower at the curve apices on the concave than the convex side (0.019 ± 0.09 μg h−1 μg−1 convex apex, 0.016 ±0.06 μg h−1 μg−1 concave apex, P < 0.05). Activities were similar on the two sides at the top and bottom of the curve.

4. Differences in muscle histology between the two sides were also observed only at the apex, with a lower type I fibre diameter (50.9 ±8.5 μm convex, 38.3 ±2.4 μm concave, P < 0.05) and a lesser proportion of type I fibres (63 ± 12% convex, 49 ±9% concave, P < 0.05) on the concavity.

5. The results are consistent with effects on muscle protein turnover secondary to an increased muscle contractile activity on the curve convexity and functional immobilization of the muscle on the curve concavity.

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