The purpose of the study is to measure the functional effects of recombinant GH in skeletal muscle, in addition to growth promotion, in short prepubertal boys with either growth hormone deficiency or idiopathic short stature. Patients will be similarly short. The investigators will also compare these values in the short stature cohort to those obtained in testing performed in normally growing age-matched healthy control boys not on GH. The group on GH will be studied before and after 6 and 12 months of GH treatment.

Prepubertal boys with significant short stature (height SDS ≤-2.0) diagnosed with either GH deficiency or idiopathic short stature (ISS) who are identified as candidates for GH treatment will recruited. Subjects will have a battery of studies to assess skeletal muscle strength, agility, power and endurance, as well as assessment of body composition and energy expenditure before and after GH administration. GH (Somatropin) treatment at standard doses will be given as daily injections at bedtime. Subjects will be followed at three-month intervals per the clinical routine, when anthropometric measurements will be obtained. Baseline studies will be repeated at 6 and 12 months from initiation of treatment, each patient will serve as his own control pre and post GH.

A group of normally statured healthy boys will have the same testing as the study patients but without GH treatment to assess the impact of natural growth on the muscle measures above.

Inclusion Criteria:

Short Stature Group

1. 30 boys between 6-11 years of age
2. Prepubertal
3. Short stature (height ≤ -2SDS) due to either GH deficiency or idiopathic
4. Stable treatment of other pituitary hormone deficiencies
5. Naïve to GH therapy, or GH discontinued at least 6 months prior to study

Normal Stature Group

1. 15 boys between 6-11 years of age
2. Prepubertal
3. Normal height (10th to 90th%)

Exclusion Criteria:

1. Actively growing brain tumors
2. Chronic medical conditions that could affect study outcomes
3. Long-term steroid use
4. Intense regular physical training programs or organized team sports

Prepubertal boys with short stature defined as a height ≤-2 SDS with either GH deficiency (i.e., peak GH responses to pharmacologic stimuli <10ng/ml) or idiopathic short stature (i.e., normal GH, no identifiable pathology) will be studied.

Normally Statured Boys: A group of 15 healthy, normally statured (between 10th- 90th %), age-matched boys not on somatropin, preferably siblings (although not exclusively) will be studied similarly.