Intensity or Effort – Part 2
Activating all motor units
This brings us to the heart of the matter- does the size principle mean that:
a) maximum or near maximum intensity (i.e. maximum or near maximum weight) is required in order to activate all motor units, or
b) maximum effort (regardless of the amount of weight being lifted) is required to activate all motor units.
Conventional wisdom holds that maximum or near maximum weight is required for maximal motor unit activation. That’s why the majority of strength programs recommend work sets of 3-5 reps. Conversely, some researchers, the most vocal being Dr. Ralph Carpinelli, suggest that the size principle means that “The level of effort in voluntary muscle actions determines the degree of motor unit activity. …if maximal motor unit activation is desired, a maximal or near maximal effort at the end of a set of repetitions – regardless of the amount of external resistance – will elicit maximal motor unit activity. Effective resistance training does not require the use of a maximal or near maximal force to stimulate the available motor units and produce significant increases in muscular strength.”(Carpinelli 2008).
Interpolated Twitch Technique
In support of his contention that maximal effort will activate all the motor units regardless of the amount of weight being lifted Dr. Carpinelli cites 12 interpolated twitch technique (ITT) studies (Klass et al 2005: Behm et al 2002: Jakobi & Rice 2002: Klein et al 2001: Knight & Kamen 2001: Connelly et al 1999: Kent-Braun & Ng 1999: Roos et al 1999: De Seres & Enoka 1998: Hurley et al 1998: Kent-Braun & Le Blanc 1996: Vandervoort & McComas 1986) in his review article (Carpinelli 2008). ITT is a method for measuring the level of motor unit activation during a maximum voluntary contraction (MVC).
A MVC is a test designed to elicit the peak force a muscle can produce during a single all-out voluntary contraction. Typically a MVC is done as a brief, static contraction against an immovable object. The subject is instructed to exert themselves maximally during the test while the level of force produced is measured.
With an ITT test a supramaximal electrical stimulus is superimposed onto a muscle or its nerve during a MVC test. The supramaximal stimulus causes all the motor units in the target muscle to fire and a maximum amount of force to be produced. If all the available motor units were already active during the MVC then the ITT will not produce any additional force. If some fibers were inactive during the MVC then additional force will be produced because the ITT will cause the inactive fibers to fire. The difference between the amount of force produced during the MVC and the ITT is an indirect measure of the level of motor unit activation. The magnitude of the increase represents the portion of muscle mass not activated during the MVC.
Note that the ITT is performed during a MVC, not during a submaximal level of force production, so the ITT does not measure the amount of muscle mass activated during a submaximal exercise, such as an 8 rep set.
Eleven of the studies cited by Carpinelli were ITT tests conducted during a MVC on various populations (young, old, male, female, healthy, unhealthy, etc) to determine the amount of muscle activated during the MVC test. The eleven studies reported that all healthy populations were capable of activating all or nearly all the motor units in the tested muscles; age, sex, or other status did not influence how much muscle mass could be activated during a MVC. For example Klass and colleagues (2005) studied younger (average age 26 years) and older (average age 77 years) males and females and reported that all of the groups were able to activate about 100% of their motor units during the MVC, with no significant difference detected in the amount of muscle mass activated during a MVC in any of the groups.
The twelfth study cited by Dr. Carpinelli was an ITT test on subjects 30 seconds to 4 minutes after performing a sub-maximal exercise to exhaustion.
Dr. Carpinelli’s interpretation of these studies was that since a MVC requires the subject to exert maximum effort that these studies support his claims that it is maximal effort that causes all motor units to fire. “These motor unit activation studies…showed that the differences in the ability to generate force did not affect the ability to voluntarily activate specific motor units. The studies strongly support the previously discussed neurophysiological concept… It is the intensity of the effort that determines the activation level of motor units and the resultant force output.”
There are two challenges with Dr. Carpinelli’s interpretation. First, ITT studies are equivocal as to whether all motor units are indeed active and firing during any MVC. Todd et al (2007), Hunter et al (2006), Goodall et al (2009), Sidhu et al. (2009), Todd et al. (2003) and Lee et al (2008) have all reported a decrease in motor unit recruitment in fatigued muscles. The decrease in motor unit activation with fatigue appears to be significant. The 2006 study by Hunter and colleagues reported that activation levels declined 23.3% as muscle fibers fatigued. “Voluntary activation calculated using the estimated resting twitch was… at the end of the fatiguing task (76.7%)…”
The research indicates that the level of motor unit activation during a MVC is likely not 100% in many cases. Factors known to affect the level of motor unit activation include:
- Fatigue: Fatigue causes a decrease in motor unit activation.
- Activity: Different exercises for the same muscle activate different amounts of motor units within that muscle. For example, the leg extension exercise activates more motor units in the quadriceps than the squat exercise.
- Measurement method: Different methods of measuring motor unit activation produce different results. The CAR method of measuring motor unit activation indicates a lower level of activation than the ITT method.
- Selected muscle: The muscle selected for testing has an influence on activation. Higher levels of activation are easier to achieve in some muscles, like the biceps, than in other muscles, like the quadriceps.
The bottom line is that it is possible but extremely difficult to maximally activate all the motor units within a whole muscle.
The second major challenge to Dr. Caprinelli’s interpretation is that a MVC is the equivalent of a 1RM. In other words, a MVC is a one time, all-out attempt to produce as much force as humanly possible, just like a 1RM test. In both an MVC and a 1RM the subjects are exerting brief, maximal effort during an event designed to elicit peak muscle force. The MVC is not a test to see how long you can produce a sub-maximal force nor is it a test to see what amount of muscle mass is activated at failure during a sub-maximal exercise. The MVC does not measure motor unit activity during a sub-maximal exercise so MVC tests do not support the claim that maximal effort during a sub-maximal exercise carried to the point of failure activates all available motor units.
Studies that tell us that maximum or near maximum motor unit activation occurred during a MVC (and presumably a 1RM due to the similarities between a 1RM and a MVC) do not also tell us the level of motor unit activation during sub-maximal exercise performed to exhaustion. In fact, in his review article Dr. Carpinelli did not cite any studies that examined muscle activation levels during sub-maximal exercise carried to the point of failure. The research I have cited in this paper shows that there are many instances where muscle is not maximally activated despite an all-out effort. Is there any research that examines motor unit activation levels during multiple reps carried to the point of failure? Let’s see.