Intensity or Effort: Which is it?

There is an old adage that says “heavy weights and low reps build strength; light weight and high reps build endurance”.  This belief has been a foundational tenet of strength training for at least 50 years and is evident in every modern resistance training program.  The practical application of the adage in strength and bodybuilding programs is encompassed in the following beliefs:  To build maximum strength you need to use a weight that allows a maximum of 3-5 reps to be performed.  If you are bodybuilding then reps of 8-12 produce the greatest increase in muscle hypertrophy (muscle size).  Doing more than about 15 reps is “endurance” training and won’t appreciably increase muscle strength or size.

The idea that heavy weights & low reps (i.e. 3-5 reps) is a requirement for maximal strength is completely entrenched in strength training lore.  The vast majority of strength training experts recommend 3-5 reps as necessary in order to fully activate and train the fast twitch fibers and to optimally improve strength.

Recently some scientists have questioned the validity of the belief that heavy weights and low reps (i.e. 3-5 reps) are required in order to build maximum strength (Carpinelli 2008; Jungblut 2009, Burd et al 2010).  In particular Carpinelli and Jungblut challenge the idea that heavy weight and low reps are necessary for increasing strength; they propose that “effort” is the key strength building ingredient, not the amount of weight lifted.  Based on the “size principle” of muscle fiber recruitment Carpinelli and Jungblut believe that if you train to failure then it doesn’t matter how many reps you do (as long as you are doing less than about 20).  They claim that as long as the trainee exerts maximal effort during a set then any rep scheme between 3 and 20 repetitions will increase muscle strength and size equivalent amounts.  Essentially they claim that “maximum effort” is the stimulus for increased strength and size, not the amount of weight being lifted.

Dr. Carpinelli took direct aim at those espousing the necessity for heavy weights/low reps for strength training in a review article titled “The Size Principle and a Critical Analysis of the Unsubstantiated Heavier-Is-Better Recommendation for Resistance Training” published in 2008 in the Journal of Exercise Science and Fitness.  Mincing no words Dr. Carpinelli claimed that those who believed in the necessity of heavy weights were making an error in reasoning and misapplying the size principle.  Dr. Carpinelli called the heavier-is-better philosophy “grossly distorted” and questioned the motives of those espousing this belief. “The question is whether the incorrect application was unintentionally created and perpetuated because of a misunderstanding, or whether it was intentionally created and perpetuated in order to support a preconceived opinion.”

Not surprising his article created some interesting debate and controversy in the strength training community.  Is Dr. Carpinelli correct that effort is really the primary ingredient for building strength and size?  Or is the old adage true; if you want to get strong you have to lift heavy.  Is it intensity or effort?  In this series we will attempt to answer these questions with an in-depth review of the research data.

Intensity and Effort Defined

For resistance training research physiologists have very specific definitions for the terms “intensity” and “effort”.  The term intensity is used to quantify the amount of weight being lifted and effort to quantify the level of exertion during exercise.

The maximum amount of weight you can lift for one repetition in any specific exercise is called your one rep max (1RM) for that exercise.  For example, you have a 1RM in the bench press and a different 1RM in the bicep curl.  Intensity is used to quantify the amount of weight you are lifting in relation to your 1RM.  Once your 1RM is known for a specific exercise then intensity is prescribe as a percentage of your 1RM such as 90% 1RM or 70% 1RM.  If your 1RM in the bench press is 200 lbs and your training program prescribed a set at 90% 1RM in the bench press then you would put 180 lbs on the bar (200 x .9 = 180).

Effort is the term used to quantify the level of exertion achieved during exercise.  The most common level of effort prescribed during strength training is “maximum”, also known as “failure”.  When training with maximum effort you lift the weight until you are unable to complete another repetition, i.e. you reach the point of failure.

While training to failure is a very intense level of effort, don’t mistake the use of the term “intensity” as a description of the level of effort being exerted.  The term “intensity” is reserved for quantifying the amount of weight being lifted as a percentage of 1RM only; it says nothing about the level of effort being produced.  So when you see the term “intensity” being used know that it is referring to the amount of weight being lifted.

For our purposes in this article I use the terms “maximum” and “failure” interchangeably to describe the level of effort reached while performing a set to the point of repetition failure.

The Size Principle

The debate about intensity versus effort centers on the “size principle of muscle recruitment”.  To understand the size principle you first have to understand what motor units are and how they work.

Within any whole muscle, such as the bicep, tricep, or quadriceps, you have thousands of individual muscle fibers.  Muscle fibers don’t contract individually though; they work together in groups known as a motor unit.  A motor unit is composed of a single nerve and all the muscle fibers it innervates.  A single nerve may innervate anywhere from a few to several hundred muscle fibers and a whole muscle may contain only a few or as many as several hundred motor units.

All the muscle fibers in a specific motor unit are the same types of fibers.  Some motor units consist of slow twitch fibers, others consist of intermediate (type IIA) fibers, and others consist of fast twitch fibers (type IIB).  The main point is that all the fibers in a motor unit are the same type; they are not mixed.  You don’t have motor units that have a mix of slow and fast fibers, for example.

The amount of force a muscle produces during a contraction is determined by the number of motor units that are simultaneously activated.  If you need to produce a small amount of force then only a few motor units are recruited.  When more force is required then more motor units are recruited.  When maximum force is required all the motor units (or as many as you can recruit) in that muscle are recruited and firing.

Any time a sufficiently strong electrical signal is sent from the brain to a motor unit nerve it causes all the fibers in the motor unit innervated by that nerve to contract simultaneously, as one.  It is an all-or-none contraction; either all the muscle fibers in the motor unit contract or they don’t contract.  It is not the case that some individual fibers within the motor unit contract while others remain relaxed.

The amount of electrical stimulus required to cause a motor unit to fire is varied.  Smaller motor units require less electrical stimulus to fire while larger motor units require a greater electrical stimulus to fire.  Motor units with slow twitch fibers require the least amount of electrical charge to fire and fast twitch motor units require the most.  Intermediate fiber motor units fall in between.

This brings us to the size principle, which states that motor units are recruited based on size.  The smallest motor units are the most easily activated so they are recruited first.  Increasingly larger and more difficult to activate motor units are progressively recruited as more force is required.