Slow twitch muscles are the thinnest muscles fibres in the body and the weakest. They also work the longest. Think long marathon runners.

Medium twitch muscles are in the middle, they can usually work for around 20-30 minutes however they are not the strongest muscles you have. Think fit rock climbers.

Fast twitch muscles are the strongest and bulkiest fibres, they also react the fastest. However these muscles only work for around 30s before they give out and the medium ones kick in. Think sprinters, body builders, etc.

You build muscle mass by targeting the fast muscles primarily, while toning the medium ones. One of the more widely used method is to use the fast muscles (heavy weight lifting) to the point of exhaustion for a short period of time (this will depend on person, but basically you’d rather be dead than lift more heavy things cause it hurts so much. This usually happens after a couple of days of crazy workout for most people), then they lift light weights and wait for the muscle to repair (grow) while eating a lot of protein (the thing needed for muscle growth). As protein passes through your body easily and only a small percentage is used, many builders chose to drink protein drinks to give the body a higher concentration.

People who want to tone usually target medium twitch muscles while still doing some work on the fast. The reason for this being the fast ones are the most noticeable and provide pretty definitions. Short heavy weight lifting (sets of 10) between more manageable weight sets are recommended, as is Cardio (usually classes are around 20 minutes) and constant movement classes with quick action like kick boxing. A normal diet is fine for this, no special eating is required

I don’t have a cure for major weight loss (think 400 pounds to 200) however if you want to lose some diameter in a specific area this can be done in my experience. You’ll be targeting the slow muscle with some medium muscle work. The reason why you want to avoid the fast ones are that you want to lose the fat, and not replace it with muscle.
Your work out emphasis will be on semi easy repetitive motions for around an hour at a time with light weights. Fast walking or riding on machines is also recommended, the trick is to do it for longer than 30 minutes and keep at it to deplete and build your slow muscles. As dieting when exercising is not recommended, it is still advised to avoid fatty foods and sugars.

May be NSFW.
[youtube]DZFnCpNQ4jU[/youtube]