We’ve all seen someone working out at a fast pace. I don’t mean a sprinter or a cyclist because those are fast paced sports, but perhaps you’ve seen someone at the gym doing bicep curls or leg presses at a breakneck speed and with heavy weights. Did they have bulky muscles or leaner ones?  Have you ever known someone who was working out to slim down only to hear them complain that they are getting bigger?  Maybe this is even happening to you. Rapid repetitions at a heavy weight or even at a lighter weight are relatively useless for leaning out as they actually promote bulkiness. If you are looking for slim muscle tone, then slow and controlled is the name of the game. 

There are two main muscle categories and they are described below:

Slow Twitch Muscle Fibers – Also known as type I muscle fibers, they are responsible for high endurance but low intensity work; i.e. long distance running. Slow Twitch muscles also receive more oxygen, which allow them to work longer before fatigue or failure. Slow Twitch muscle fibers also burn more fat than the other muscle fiber category.

Fast Twitch Muscle Fibers – These are broken down into two categories of type IIa and type IIb. These muscles are used in more powerful movements but because they carry less oxygen they fatigue much faster. These muscles are recruited when the body needs maximum effort in short bursts; i.e. sprinting. 

Fast Twitch muscles burn more energy but lead to gaining muscle mass. So those with the bulkier muscles, like body builders tend to recruit manly Fast Twitch. This might be why you won’t see a body builder running a marathon. Their bodies Slow Twitch muscle fibers burn out fast because they aren’t used enough thus they don’t have a tremendous amount of endurance.

Exercises like Pilates are designed to primarily recruit the Slow Twitch fibers and then work into the cross over area into Fast Twitch. This is some of the reasoning why lighter weight and slow deliberate pace is recommended for leaning out. Those that insist on using heavier weighted springs in Pilates and those who speed through the exercises often find that they bulk up. When they want to know why, my answer is always to slow down and take the weight down. If they insist, I might let them keep the weight higher, but make them do the exercises slower. 

So, while I have no judgments about whether or not you want bulky big muscles. If that’s what you are looking for, I say go for it.  But if you want to lean out then you have to slow it down.  — Jessica Kuiken

Jessica Kuiken is a Los Angeles based Pilates instructor.