We have two main types of muscle fibers Type I (slow twitch) fibers, and Type II (fast twitch fibers). Type I fibers are used for repetitive, long endurance activity. They don't fatigue very easily and make the body seem leaner. Type II fibers are used for short bursts of energy. They are extremely powerful and also fatigue very quickly. They make the body look much thicker and muscular than Type I fibers.
Type I Fiber
Type II Fiber
The cyclists in the Tour de France have to engage both types of fibers during their competition. Their hours and hours of competing every day requires constant Type I fiber use, yet they must also sprint! This requires Type II fiber use. This style of sport is extremely intense and requires many years of training be done well.
Since we now know what our muscle fibers are specifically used for, let's use this information for our workouts. How do we train? Power lifters engage in short bursts of energy. They primarily engage Type II fibers. Marathon runners require repetitive motion for long periods of time. They are going to mostly use Type I fibers. Cyclists require both! What do you want to focus on? If your goal is overall fitness... Do everything. You can lift weights one day, run longer distances that next day and then take it from there. This style of Cross-training is also the best way to strengthen our hearts and muscles.
In case you are wondering... Each of us is born with a specific genetic ratio of Type I:Type II fibers. Some of us are natural sprinters (more Type II fibers) and conversely, others are natural long distance runners (more Type I fibers). Even though we can never change our fiber type ratio, that doesn't mean we cant train the way we love. Through consistent exercise, all of us can still strengthen the specific fibers we choose to train.