Decreased capillarization and a shift to fast myosin heavy chain IIx in the biceps brachii muscle from young adults with spastic paresis.

Muscle spasticity and paresis are conditions that occur secondary to upper motor neuron lesions. The co-existence of decreased motor unit recruitment and intermittent over-activity generates confusion concerning the effect on muscle fiber characteristics. In order to increase the knowledge about the effect of upper motor lesion on capillarization and muscle fiber composition, the biceps brachii muscle from seven young adults with long duration of spastic paresis and seven age-matched controls were analyzed using morphological and enzyme- and immuno-histochemical techniques. The spastic muscles had a 38% lower capillary density (p=0.002), 30% fewer capillaries around each muscle fiber (p=0.02), and 16% fewer capillaries when related to the fiber size (p=0.04). The frequency of fibers expressing myosin heavy chain (MyHC) IIx increased (30% vs. 4%, p=0.006), while the percentage of fibers expressing MyHC I and MyHC IIa, respectively, decreased (22% vs. 46% and 7% vs. 29%, p<0.01). The high proportion of muscle fibers with low oxidative capacity and low capillary supply indicates that biceps brachii muscle from patients with upper motor lesions fatigue more easily than normal controls. We also observed a significantly higher variability in fiber size for fibers expressing MyHC I (p<0.04), and, in three of the subjects, a small amount of small fibers expressing developmental MyHCs was found. These results suggest that, although intermittent stretch reflex contractions might have an impact on the muscle characteristics in spastic paresis, the muscle phenotypic properties are more adapted to decreased voluntary motor unit recruitment.