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Muscle Fiber Recruitment and Gains

In the world of weight training, scientific principles usually take a back seat to real world experience, but one principle worth paying attention to is that of muscle fiber recruitment order.  Applying this principle correctly in the gym can make a huge difference in your muscle and strength gains.  In this post I’ll go over what you need to know without getting overly technical.

The first thing to know is that all muscle groups contain a mixture of slow-twitch and fast-twitch muscle fibers.  The slow-twitch fibers are small and weak but have good endurance.  Think long distance running.  The fast-twitch fibers are large and powerful but have poor endurance.  Think sprinting or powerlifting.  To gain muscular size and/or strength, you’ll want to primarily target the fast-twitch fibers because they have by far the most potential to grow.

The other thing to know is the order in which your muscle fibers are recruited.  When you lift a weight for a series of repetitions, your body starts by using the small slow-twitch fibers, and as you tire and the set gets more difficult, it will start using more fast-twitch fibers.  If you push the set all the way to failure, all the available fibers will be recruited.

Given the facts above, there are only three ways to stimulate your fast-twitch muscle fibers in the gym.  You’ll want to make sure you do at least one of them in your workouts:

  1. Lift a heavy weight.  If you lift a weight that’s over 80% of your 1 rep max, your body has no choice but to recruit the stronger fast-twitch fibers.
  2. Lift a light weight explosively.  If you push or pull a light weight with a lot of force, for example on a power clean, box jump, or any other explosive lift, your fast-twitch fibers will be recruited.
  3. Lift a light to medium weight to failure (or close to it).  Your body will start with slow-twitch fibers and then recruit an increasing number of fast-twitch fibers as the set gets more difficult.

You may notice that all three approaches require significant effort by the lifter.  I see a lot of people in the gym who take it easy and do something like 3 sets of 10 reps using a weight they could have lifted for 3 sets of 20 reps.  They’re largely wasting their time because the fast-twitch fibers are never recruited. 

All three approaches that I listed work for building strength and muscle, but you should consider the pros and cons of each when designing your workout program:

Lifting heavy is the most direct way to stimulate the fast-twitch fibers, and it’s a fairly easy way to train because the reps are low.  But lifting heavy is hard on the connective tissues and leads to burn out when done too often.  It can take experience to learn when to back off and train lighter for recovery.

Lifting a light weight explosively is a great way to hit the fast-twitch fibers without experiencing the overtraining and burn out that occurs when lifting heavy all the time.  However, it can be hard to gauge progress because we can’t easily measure bar speed.  It’s also less effective at building muscle than the other two approaches because time under tension, a requirement for stimulating muscular growth, is low.

Lifting a light to medium weight (e.g. 50-70% of 1 rep max) for multiple sets of as many reps as you can is a great way to stimulate both slow and fast twitch fibers.  It’s also easy on the joints.  For those reasons, it’s the preferred approach of most bodybuilders most of the time.  However, it’s is a hard way to train, and not everyone has the mental fortitude to do multiple sets of 10-20 reps with a challenging weight.  If you decide to use this approach, be sure to keep your rest periods short.  The easy reps of a set are not productive, so sitting around resting for several minutes to prepare for the next light set is a waste of time.  Rest only long enough for your breathing and heart rate to come back down – one to two minutes on most exercises – and go again.

If you’re an intermediate or advanced lifter, I’d highly recommend incorporating more than one of the above approaches in your workout routine.  For example, start your workout with a heavy or explosive lift and then finish off with lighter exercises taken to near failure.  Or you may train a body part heavy one day and lighter the next time you train it.  There are many ways to successfully organize your workouts that are beyond the scope of this post, but the main point to remember is to recruit your fast-twitch fibers through hard training and avoiding lifting heavy all the time.

Happy training and good luck!

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