Resistance Training Part 6

Last month, I explained how an understanding of Henneman’s size principle in association with research using the interpolated twitch technique (ITT) can collectively provide the answer to the age old question: How heavy a weight do I have to lift in the weight room? The size principle of motor unit recruitment indicates that the muscle fibers we want to activate during a resistance training session are those that reside at the upper end of a recruitment hierarchy. Consequently, during a resistance training set, it is important to force a drive through a significant portion of this spectrum. The results of ITT research provide important clues as to what is required to bring about this degree of activation. And this is often misunderstood, as explained by Dr. Ralph Carpinelli.

Many authors have suggested that it is necessary to lift heavy weights in order to progress through the recruitment hierarchy. In his article in the Journal of Exercise Science & Fitness, Carpinelli explains that this is simply not true. Instead, it is the intensity of the contractile activity during your set that determines how many motor units will be activated. In short, when it gets hard, more will be called into play, regardless of how heavy the actual weight is. Now, if a weight is relatively heavy (e.g., if it is an amount you can only lift for six reps), it will surely be hard when you reach the end of your set, assuming you continue your set long enough (e.g., until failure at rep six). But if you take a lighter weight (e.g., one you can lift 15 times), it will be equally as hard for you to complete repetition 15 as repetition six was with the heavier weight. Consequently, the drive for motor unit activation will be similar and a comparable training effect will be experienced.

The take-home message from Carpinelli’s review is that you don’t have to lift heavy, but you do have to lift hard. This means that continuing a set until your tank is pretty much empty is far more important than grabbing the heaviest weight you can find. There is, however, a limit. For example, if you picked up the lightest dumbbell in the gym and curled it continuously for 20 minutes, it might be pretty difficult at the end, but that wouldn’t represent the same stimulus. Simply stated, fiber activation is much different during long-duration fatiguing activities, so the time you can continue for is also an important thing to consider and in this regard, it’s probably safe to say that the weight must be sufficiently heavy such that you cannot move it continuously for more than two minutes. But as long as this criteria is met, there will be no advantage to using a heavier weight (for example, one you can only lift continuously for 60 seconds instead of 120) so long as you continue the set long enough for the reps to become difficult.

A technique that allows you to lift hard without lifting heavy is pre-exhaust supersets. These involve preceding a compound (multi joint) movement like the bench press with an isolation (single joint) movement that works the same major muscle group (in the case of the bench press, this would be the chest). For example, if you can bench press 100 pounds for six repetitions when you perform the lift first in your workout, you might only be able to complete six reps with 80 pounds when you do the movement directly after pre-exhausting your chest with flyes (moving your upper arm against resistance the same as you would during a bench press, but without associated movement at the elbow). But this weight reduction matters not because it will be equally as hard when you reach the sixth rep and, therefore, it will be equally as effective. And similarly, if you decided, instead, to bench 80 pounds without performing flyes first, you could also achieve the same degree of effort if you continued the set until you could no longer complete another repetition. The only difference is that this would occur at a higher repetition (number 10, for example) under these circumstances.

Understanding that intensity is the key aspect of contractile activity that drives motor unit recruitment is also important when training special populations. For example, a typical guideline is that patients with cardiovascular disease should use lighter weights to avoid potential adverse effects of resistance training (for example, a dangerously high blood pressure response). However, if a patient performs 20 repetitions with a weight they can lift 20 times (i.e., what would typically be considered a light weight), the degree of cardiovascular stress at rep 20 will be similar to what they would experience if they lifted a heavy weight (e.g., something they could complete six reps with) until the same end point (i.e., failure; e.g., in this case, rep six). In other words, the stress associated with failure is the stress associated with failure, no matter what rep it occurs at. Consequently, assuming no orthopedic concerns, the patient need not necessarily lift light weight, but must restrict repetitions with whatever weight they choose such that the excessive degree of effort associated with very difficult repetitions is avoided.

This article was originally published in New Living Magazine, which can be accessed on-line at






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