MECHANICAL OVERLOAD AND SKELETAL-MUSCLE FIBER HYPERPLASIA - A METAANALYSIS

With use of the meta-analytic approach, the purpose of this study was to examine the effects of mechanical overload on skeletal muscle fiber number in animals. A total of 17 studies yielding 37 data points and 360 subjects met the initial inclusion criteria: 1) ''basic'' research studies published in journals, 2) animals (no humans) as subjects, 3) control group included, 4) some type of mechanical overload (stretch, exercise, or compensatory hypertrophy) used to induce changes in musc le fiber number, and 5) sufficient data to accurately calculate percen t changes in muscle fiber number. Across all designs and categories, s tatistically significant increases were found for muscle fiber number [15.00 +/- 19.60% (SD), 95% confidence interval = 8.65-21.53], muscle fiber area (31.60 +/- 44.30%, 95% confidence interval = 16.83-46.37), and muscle mass (90.50 +/- 86.50%, 95% confidence interval = 61.59-119 .34). When partitioned according to the fiber-counting technique, larg er increases in muscle fiber number were found by using the histologic al vs. nitric acid digestion method (histological = 20.70%, nitric aci d digestion = 11.10%; P = 0.14). Increases in fiber number partitioned according to species were greatest among those groups that used an av ian vs. mammalian model (avian = 20.95%, mammalian = 7.97%; P = 0.07). Stretch overload yielded larger increases in muscle fiber number than did exercise and compensatory hypertrophy(stretch = 20.95%, exercise = 11.59%, compensatory hypertrophy = 5.44%; P = 0.06). No significant differences between changes in fiber number were found when data were partitioned according to type of control (intra-animal = 15.20%, betwe en animal = 13.90%; P = 0.82) or fiber arrangement of muscle (parallel = 15.80%, pennate = 11.60%; P = 0.61). The results of this study sugg est that in several animal species certain forms of mechanical overloa d increase muscle fiber number.