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When conducting an experiment, extraneous variables are factors other than the independent variable that might affect the dependent variable. For instance, in a study about the effect of creatine on muscle development, variables like age, weight, lean body mass, and initial lifting capacity could interfere with the results.

To mitigate their impact, researchers try to make these extraneous variables constant across the study's participants. The strategy, as applied in the creatine experiment, is to select men of approximately the same age, weight, and fitness level. By doing so, researchers aim to isolate the effect of creatine from those potential confounders, ensuring that differences in muscle fiber development are attributed more directly to the supplement and not to varying personal characteristics.

All experiments should take such factors into account to enhance the validity of their findings. Failing to control for extraneous variables can lead to erroneous conclusions and reduced reliability of the experiment's outcomes.

Within an experimental design, a control group serves as a benchmark to measure how the other tested subjects do against a standard condition. The control group in the creatine study was the set of men who received a fake preparation instead of creatine.

The role of the control group is crucial because it shows what would happen without the experimental intervention. In this case, any changes in muscle development in the control group can be attributed to the regular workout regime or other factors not related to creatine consumption. The comparison between the creatine group and the control group thereby helps researchers to determine the actual effect of the supplement.

Having a control group rules out other variables that might be causing the observed effect, adding credibility to the argument that the independent variable (in this case, creatine) is responsible for the observed changes in the dependent variable (muscle fiber development).

A double-blind experiment is a method to remove both experimenter and subject bias from the results. Neither the participants nor the researchers know who has received the treatment or the placebo. This method was not used in the creatine study, and as a result, the researchers' expectations might inadvertently influence their measurements or interpretations of the data.

Implementing a double-blind experiment means that the outcomes are more likely to be attributed purely to the treatment's effects, without the influence of psychological factors. For instance, if the trainers administering the workouts knew who was receiving creatine, they might unconsciously encourage those individuals more, thus skewing results. Similarly, if the participants knew, they might push themselves harder (or not), which would affect the gains measured.

In conclusion, double-blinding enhances the credibility of experimental results by minimizing biases, leading to stronger, more reliable scientific conclusions. Integrating this method into the described creatine study would indeed provide a firmer ground to attribute changes in muscle fibers specifically to the supplement's effect.