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Mechanical overload and skeletal muscle fiber hyperplasia: a meta-analysis

GEORGE KELLEY

Department of Physical Education, Northern

Illinois University, DeKalb, Illinois 60115-2854

Kelley, George. Mechanical overload and skeletal muscle fiber hyperplasia: a meta-analysis. J. Appl. Physiol. 81(4): 1584–1588, 1996.—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 mechani- cal overload (stretch, exercise, or compensatory hypertrophy) used to induce changes in muscle fiber number, and 5) sufficient data to accurately calculate percent changes in muscle fiber number. Across all designs and categories, statistically significant increases were found for muscle fiber number [15.00 6 19.60% (SD), 95% confidence interval 5 8.65–21.53], muscle fiber area (31.60 6 44.30%, 95% confi- dence interval 5 16.83–46.37), and muscle mass (90.50 6 86.50%, 95% confidence interval 5 61.59–119.34). When partitioned according to the fiber-counting technique, larger increases in muscle fiber number were found by using the histological vs. nitric acid digestion method (histological 5 20.70%, nitric acid digestion 5 11.10%; P 5 0.14). Increases in fiber number partitioned according to species were greatest among those groups that used an avian vs. mammalian model (avian 5 20.95%, mammalian 5 7.97%; P 5 0.07). Stretch overload yielded larger increases in muscle fiber number than did exercise and compensatory hypertrophy (stretch5 20.95%, exercise 5 11.59%, compensatory hypertrophy 5 5.44%; P 5 0.06). No significant differences between changes in fiber number were found when data were partitioned according to type of control (intra-animal 5 15.20%, between animal 5

    • 13.90

      %; P 5 0.82) or fiber arrangement of muscle (parallel 5

    • 15.80

      %, pennate 5 11.60%; P 5 0.61). The results of this

study suggest that in several animal species certain forms of mechanical overload increase muscle fiber number.

muscle mass; enlargement; hypertrophy

RECENTLY, A NARRATIVE REVIEW has suggested that in- creases in muscle fiber number (hyperplasia) in ani- mals occur as a result of stretch overload, whereas compensatory hypertrophy (ablation, tenotomy) does not generally change fiber number (8). In addition, it was also reported that exercise models in animals have led to mixed results with regard to increases in muscle fiber number (8). Although the above-mentioned review provided valuable information, it relied on the tradi- tional narrative approach, that is, chronologically ar- ranging and then describing studies. A need exists for the quantification of the magnitude and direction of changes in skeletal muscle fiber number as a result of different types of mechanical overload in animals. Thus the purpose of this study was to use the meta-analytic approach (12, 14, 20, 26) to examine the effect of different types of mechanical overload (stretch, exer-

cise, and compensatory hypertrophy) on skeletal muscle fiber number in animals.

METHODS

Literature search. The search for literature was limited to studies published in journals between January 1966 and December 1994. Studies in English-language journals were obtained from computer searches (Medline) as well as hand searches and cross-referencing. The search for studies in foreign- language journals was limited to computer searches (Medline) only. Specific inclusion criteria were 1) ‘‘basic’’ research studies published in journals, 2) animals (no humans) as subjects, 3) control group (intra- or between animal) included, 4) some type of mechanical overload employed (stretch, exercise, compensatory hypertrophy), and (5) sufficient data to calcu- late percent changes in muscle fiber number. Human studies were not included in this analysis for two reasons: 1) only one study providing quantitative data on humans is known to exist and 2) the methods used to examine muscle fiber number in humans are not as accurate as in animals (29).

Recording and classifying variables. All studies that met the criteria for inclusion were recorded on a recording sheet (available on request) that could hold up to 81 pieces of information. The major categories of information recorded included 1) study characteristics (year, journal, length of study, number of groups, number of subjects, type of study,

  • i.

    e., intra-animal or between animal, and muscle examined),

    • 2)

      physical characteristics of subjects (type of animal, age,

weight, and diet), 3) mechanical overload characteristics (length, frequency, intensity, duration, and mode), and 4) skeletal muscle changes (muscle mass, muscle fiber area, and muscle fiber number). To avoid bias in selecting and rejecting studies, the decision to include a paper was made by examin- ing the methods and results sections separately under coded conditions. A control group was defined as that group that did not receive any type of mechanical overload during the study. Two primary types of information were desired from the studies: outcomes and major variables that could affect outcomes. For this study, the major outcome was changes in skeletal muscle fiber number. In addition, changes in muscle mass and fiber area were also examined. Major variables that could potentially affect changes in fiber number included 1) fiber-counting technique used (histological analysis vs. nitric acid digestion), 2) type of mechanical overload employed (stretch, exercise, or compen- satory hypertrophy), 3) species used (avian vs. mammalian), 4) type of control (intra- vs. between animal), and 5) fiber arrangement of muscle (pennate vs. parallel).

Statistical analysis. In a meta-analysis, the mean results for each group from each study are recorded irrespective of whether or not the results from each study are statistically significant. For this study, descriptive statistics (percentages) were used to report changes in muscle fiber number as well as changes in muscle fiber area and mass. Percentages were calculated by dividing the treatment minus control group difference by the control group value. Ninety-five percent confidence intervals were then established for each of the three major outcome variables, i.e., fiber number, fiber area, and muscle mass. Because there was no relationship between number of subjects and changes in skeletal muscle, no

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0161-7567/96 $5.00 Copyright

1996 the American Physiological Society

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