Ten Years of Growth Hormone (GH) Replacement Normalizes Muscle Strength in GH-Deficient Adults

Abstract

GH replacement for 1-5 yr improves, but does not fully normalize, muscle strength. OBJECTIVE, DESIGN, AND PATIENTS: In this single-center, open-labeled, prospective study, the effects of 10 yr of GH replacement on muscle strength and neuromuscular function were followed in 109 consecutive adults (61 men; mean age 50.0 yr; range 22-74 yr) with adult-onset GH deficiency. The mean initial GH dose of 0.88 mg/d was gradually lowered to 0.47 mg/d. The mean IGF-I sd score increased from -1.54 at baseline to 1.12 at study end. GH replacement induced a sustained increase in lean mass and isometric knee flexor strength (60 degrees). In most other measures of upper leg and handgrip strength, there were transient increases during the first half of the study (0-5 yr), whereas during the second half (5-10 yr), the absolute values of muscle strength decreased and returned to or even below the baseline values. However, after correction for age and gender using observed/predicted value ratios, there were sustained and, until 7 yr, even progressive increases in the measures of muscle strength. At study end, knee flexor strength had increased to 104-110% of predicted, knee extensor strength to 93-108%, and handgrip strength to 88-93%. Measurements of neuromuscular function showed reduced voluntary motor unit activation after 10 yr. Ten years of GH replacement therapy increased muscle strength during the first half of the study and thereafter partly protected against the normal age-related decline in muscle strength and neuromuscular function, resulting in approximately normalized muscle strength after 10 yr.