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The current analysis evaluates cumulative benefits after year 2 of a school-based resistance training intervention. Adolescent girls were enrolled and measured at the beginning of sixth grade (baseline [BL]) and again at first follow-up (Y1 end) and second follow-up (year 2 end). School gym classes met alternate school days. Site 1 had standard gym classes (CON). Site 2 gym classes included 8–12 min of resistance training (INT); INT girls were categorized based on observed participation effort and time (LO, HI). Measurements included the following: 1) height and weight, 2) questionnaires to assess extracurricular exercise and diet (calcium, vitamin D), and 3) dual-energy X-ray absorptiometry (Lunar Prodigy). Whole body less head scans yielded bone mineral content (BMC) and body composition. Lumbar spine (L1–L4) and femoral neck (FN) scans yielded BMC and areal bone mineral density (BMD); radius scans yielded ultradistal and 1/3 BMD. ANCOVA compared group means for percent gains from BL to second follow-up, accounting for biological maturity, BL height, height change, interscan interval, organized activity, calcium, and vitamin D. In 62 girls (21 CON, 41 INT), intention-to-treat analyses detected INT versus CON advantages for L1–L4 BMC and BMD (4.1%, 5.6%: P < 0.05). HI effort participants (n = 19) demonstrated advantages for BMC and BMD at L1–L4 and FN (5.7% to 8.2%, P < 0.01) versus CON. Over two school years, this resistance intervention yielded lumbar spine advantages; enthusiastic participation (HI) yielded lumbar spine and FN advantages. Further work is warranted to evaluate benefit persistence after intervention cessation.