Exercise capacity is dependent on both muscle strength and cardiovascular performance. Several studies have addressed exercise performance in GH-deficient adults using cycle ergometry (42,43). In these studies, values for maximum oxygen uptake were significantly reduced, being on average 72-82% of those predicted for age, sex, and height (42). Maximum oxygen uptake increased significantly (42) following six months GH treatment reaching predicted values (42). Evidence suggests that the increased performance is largely attributable to increased muscle mass (43) although observed increases in cardiac output, extracellular fluid volume, and red cell mass following GH replacement may all contribute. Thus adults with GH-deficiency have a reduced exercise performance capacity, which can be improved and probably normalized with six months physiological GH replacement.
The decreased LBM of GH-deficiency results in a mild to moderate reduction in muscle strength. Isometric quadriceps force has been shown to be reduced in GH-defi-cient adults compared with matched normal controls (43). The effects of GH-replace-ment on muscle strength has been investigated in several studies (43-45). These have demonstrated, an increase in limb girdle force after six months GH treatment, but neither isometric quadriceps force nor quadriceps torque increased significantly in any of the studies. This was despite marked increases in thigh muscle cross-sectional area. Only after more prolonged GH treatment (at least 12 mo) has a significant increase in quadriceps force been demonstrated with a further increase and normalization seen after three years (23). It is likely that the difficulties inherent in measuring muscle strength contribute to the difficulty in demonstrating statistically significant effects when small numbers of patients are studied.
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