Today we bring you a great review conducted by several experts with article development by fitness guru Charles Poliquin. Understanding more about the science of exercise allows us to practice it more efficiently. The topic of focus for today is rest intervals (how long you rest during exercise). Whether you’re catching your breath running or resting in between sets during weight training, the following should give you a better idea of what is going on with your body.

Article Reviewed: “Research Note: Effect of Load Reductions over Consecutive Sets on Repetition Performance” by Jeffrey M. Willardson, Mark S. Kattenbraker, Maureen Khairallah, and Fabio E. Fontana.Journal of Strength and Conditioning Research, March 2010, Volume 24, Issue 3, pages 879-884.

Purpose
This study examined the effects on fatigue that relatively short rest intervals (one minute) had in three exercises: the back squat, the leg extension and the leg curl. Research was conducted at the Human Performance Laboratory at Eastern Illinois University (EIU).

Background
Achieving results as quickly as possible requires the precise control of all the loading parameters of training, such as reps, sets, tempo and rest.  This is a complex subject, as all the loading parameters are interrelated. Let me provide an example.

There is an inverse relationship between sets and reps: The more reps you perform, the fewer sets you need to achieve the appropriate training response. An understanding of this concept explains the dramatic training differences between a bodybuilder and a competitive weightlifter or powerlifter. Whereas a common protocol for a bodybuilder might be to perform the bench press for four sets of 12-15 reps, a powerlifter might perform the same exercise for 8-10 sets of 2-4 reps.

In my workouts I’ve always promoted the concept of Let the reps determine the load. Attempting to use specific percentages may result in not being able to stay within the optimal rep range for the results you are seeking. On any particular day 90 percent of a 1-repetiton maximum may be 180 pounds, but on another day it may be 190 pounds. Insisting on following these percentages may result in you failing to perform within the repetition range needed to achieve the appropriate training stimulus. And if the intensity range is too great, the body does not recognize what it is supposed to adapt to. Even if you reduce the weight to stay within the repetition bracket, the quality of the workout will decrease. The authors of the EIU study agreed: “When performing consecutive sets of a resistance exercise, maintaining performance within a specified repetition range has been shown to be essential for achieving muscular adaptations conductive to different training goals.”

Let’s say your goal is to build maximal strength, and you decide, after warm-up, to squat five reps for five sets. If you rest only 30 seconds between sets, you may find that such incomplete recovery will not allow you to use as much weight on subsequent sets. If you squat 200 pounds for five reps on your first set, for example, and you rest only 15 seconds between sets, then by your final set you may be using only 150 pounds due to accumulative fatigue. And all you would be doing is taxing your recovery ability. Whereas if you rested four minutes, by your final set you may be able to handle as much as 195 pounds. A greater load translates into higher activation of the higher motor units.

Although the difference between 15 seconds and four minutes is extreme, the effects serve to demonstrate the importance of studies such as this one. Now let’s get into some details.

Procedures
For each of the three exercises, only one minute of rest was allowed between subsequent sets of each exercise. This variable (rest between sets) was kept constant. The variable that was manipulated was the weight used. Reps were performed with resistance that was varied four ways: kept constant, reduced by 5 percent, reduced by 10 percent, and reduced by 15 percent.

I was especially impressed by the experimental approach to this study. Strength coaches supervised the workouts “to ensure proper technique and provide spotting and verbal encouragement,” recreationally trained subjects (11 men) were used, and the subjects participated in a three-week preparatory period to help establish baseline strength levels. If the study subjects had been an untrained group or a group that had not trained these lifts recently – or was detrained in general – an acceptable baseline performance would not have been achieved.

Results
In examining the data, the authors found that “when averaged across sets, significantly fewer repetitions were accomplished for the back squat and the leg curl within the constant condition vs. the 15% condition (p<0.05). Conversely, for the leg extension, there were no significant differences in the repetitions accomplished between conditions (p > 0.05).”

Discussion
It is understandable that it would be difficult to recover from an exercise as difficult as the back squat, due to the fact that so many major muscle groups are involved in the exercise and it is more neurologically complex compared to the two machine exercises used in this study. What is puzzling is why the leg extension did not suffer from a significant decrease in performance when compared to the leg curl. Both are lower body exercises, both are considered isolation-type movements, and both involve antagonistic muscle pairs with the same movement pattern. The answer, I believe, lies in looking more closely at the muscle fibers involved.

The hamstrings are composed primarily of Type IIb fibers, which are built for powerful muscular contractions but not endurance. If you look at hamstring development of elite sprinters, you’ll see that these athletes often have such tremendous upper hamstring development that there is virtually no appearance of a gluteal fold. And the training of sprinters is focused on high intensity (i.e., fast sprints) for short time periods (often sets lasting only 10 seconds). Not so with the quads.

The quadriceps contain more of the Type IIa fibers, which cannot produce the same powerful contractions as the Type IIb fibers but can sustain their contractions for longer periods – that is, they are genetically gifted for a higher level of muscular endurance than the quadriceps fibers are. Speed skaters and cyclists, even those who compete in the longer sprint races, often have tremendous quadriceps development.

These fiber type differences would explain why the short rest intervals did not have as much of an effect on the performance of subsequent sets of leg extensions as compared to the leg curl. In other words, you could conclude that the hamstrings are built for strength and the quads are built for endurance.

Practical Applications
This study confirmed my belief that you have to consider several variables when determining the appropriate rest intervals between sets. As a general guideline, for example, you could conclude that exercises that work the quads do not require as much rest time as those that work the hamstrings. And you could apply this same logic to other muscle groups.

As I explained in the first edition (1997) of The Poliquin Principles, muscle biopsies and autopsies have shown that the soleus is approximately 88 percent slow-twitch muscle fibers and the gastrocnemius is approximately 60 percent fast twitch. This fact, along with the implied results of this study, suggests that optimal rest intervals for a seated calf raise (a soleus exercise) would be less than for a standing calf raise (a gastrocnemius exercise).

For those who are interested in learning more about fiber types, here is an excellent reference: Bosco, Carmelo.La Forza Muscolare: Aspetti Fisiologici Ed Applicazioni Pratiche. Societé Stampa Sportiva Roma, 1997. Also, consider that the subject of rest intervals is discussed in great detail in PICP courses.

At the end of the EIU study, the authors recommended that the topic of rest intervals should be examined with subjects who have used short rest intervals for longer periods.

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