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Acute Training Variable Manipulation for Maximal Muscle Growth
Part I - Optimal Exercise Intensity and Rest Period Lengths
By Jacob Malachi Wilson, M.S., CSCS
Introduction to Acute Training Variable Selection
Since the conception of weight training millions upon millions have asked the same questions: "How can I get bigger(?)", "how can I get stronger(?)", "what must I do to become more powerful(?)" Intriguingly enough I entered academia to become more proficiently equipped to answer such questions, and since that time nearly all of my peer reviewed papers, talks at national conferences, experiments, and lab work has been dedicated to enhancing these processes across lifespan, particularly as they concern muscle growth (hypertrophy). What the reader must understand is that during any given training session the desired outcome will be determined by how the exercise session is prescribed. Strength and conditioning specialists base their prescriptions on 8 "acute training variables" which can be divided into (1) muscle action, (2) resistance or intensity used, (3) rest period lengths, (4) volume, (5) exercise selection, (6) exercise order, (7) repetition velocity, and finally (8) frequency of training sessions 1, 2. The way you manipulate these variables changes the signal either within skeletal muscle itself (e.g. an internal signal to grow larger), or outside of the muscle (e.g. an increase in circulating growth hormone).
Changes in Muscle Tissue Size as a Factor of Changing the Local and Surrounding (e.g. hormonal) Environment
Because the focal point of this magazine (and my life) is bodybuilding, the principle focus of this series will be on muscle growth. Strength and power adaptations will be addressed, mainly due to their interaction with hypertrophy. To begin, it is important to realize that muscle growth, in large part is a factor of signaling. In other words the muscle has to be signaled to increase protein synthesis (muscle building), to lower muscle protein breakdown, as well as a host of other factors. Ultimately it is the local and surrounding environment which triggers these changes 3-5.
To elaborate, at rest your muscles are continually bathed in hormone rich blood. Hormones which increase muscle growth are testosterone6, and growth hormone (GH)7, while cortisol promotes muscle loss (Figure 1.0)8. Locally there are literally countless growth factors which increase following exercise. A few of these include insulin like growth factor 1(IGF-1) and mechanogrowth factor (MGF) because of its sensitivity to muscle contraction.
Figure 1.0 The surrounding (blood) and local signaling environment
Training acts to change the surrounding (e.g. blood), and local muscular environment. This was illustrated by Dr McCall et al. 9 who found that heavy resistance training not only led to large increases in GH, but that those individuals who had the highest GH response obtained the greatest increases in muscle growth .Similarly Dr. Kim3-5 from our muscle laboratory and his colleagues found that individuals who had extreme muscle growth responses to training increased local growth factors to a large extent, while those who did not obtained little to no growth. For decades the scientific community has painstakingly looked at all 8 acute training variables and in large part determined how to optimize each for greater increases in both the endocrine (hormonal) and local growth factor response to training. A bodybuilder or strength athlete who is equipped with the science of acute training variable manipulation will therefore empower themselves with the ultimate source of knowledge for obtaining huge freaky size or at least maximizing their genetic potential. Therefore in the next several issues of Natural Muscle Magazine I intend to impart this knowledge to you beginning with how to manipulate exercise intensity and rest period lengths.
Resistance or Intensity Used
Muscle growth and strength
The intensity or resistance used refers to a percentage of your maximal effort when lifting. For example if an individual can bench press 200 pounds once, then 80 % intensity would refer to 160 pounds. Intensity has a large effect on muscle growth. Briefly it is important to know that muscle tissue is comprised of both type I or slow twitch, and type II or fast twitch muscle fibers. Generally the smaller types 1 fibers are recruited at lower intensities, while large intensities are needed to recruit the larger type II fibers. The effects of the selected intensity can be seen by comparing bodybuilders, with weight lifters, and power lifters. Bodybuilders generally lift in a higher repetition range (8-12 repetitions) which corresponds to about 70-85 % 1-RM. Weight lifters and power lifters often perform repetitions at 90 % intensity or 1-5 repetitions. In an extensive analysis Dr. Fry10 found that the majority of the muscle mass in powerlifters and weight lifters was contributed from type II muscle fibers, with a much smaller contribution coming from type I fibers. It is suggested that the extremely heavy loads used by powerlifters and weight lifters mainly stimulate type II fibers to grow, but not type I. In contrast bodybuilders had an equal portion of their muscle occupied by type I and type II fibers, indicating that they had caused growth in both muscle fiber types. In addition when studying individual fiber size by looking at the type II to I fiber ratio, it can be seen that strength athletes had much greater size of type II fibers than type I, while bodybuilders type II fibers were only slightly larger than type I fibers (Figure 3).
Figure 3. Relative (%) fibre type area, and ratio between type II and I fibers for competitive lifters (i.e.weightlifters, powerlifters and body builders). Graph adapted from Fry 10
Dr. Fry also analyzed numerous studies on muscle growth of both type I and II fibers and found that there was a dose dependent response between intensity and muscle growth. However if you look at the graphs below you will note that most of the growth tends to cluster around 75-85 % maximal intensity or about 8-12 repetitions.
Figure 4.0 Dose dependent response between intensity and type I and II fiber growth. Modified with red highlight from Fry 10
Another method for looking at the effects of intensity is to analyze how this variable effects hormone release. For example studies show that while 10 Xs 10 repetition sets were able to stimulate large increases in both growth hormone and testosterone, that 20 Xs 1 repetition sets were not 8. In a similar study 10 repetition sets were able to stimulate GH release, while 5 repetition sets were not7. The reason for differences in GH and testosterone appear to be tied to the fact that moderately high intensity sets (8-12 reps) rely heavily on carbohydrates as fuel while extremely high intensity (1-5 repetition) sets depend on creatine phosphate stores to power contraction. The end result of anaerobic (without oxygen) use of carbohydrates (e.g. glucose) is the production of lactic acid which appears to stimulate the release of GH.
Overall the general consensus is that strength is maximized at 1-5 repetitions, and overall growth at 8-12 repetitions11. This does not however mean that bodybuilders should not train in the 1-5 repetition range, as clearly data from powerlifters and weight lifters indicate that this is a strong stimulus for the larger type II muscle fibers to grow. For this reason bodybuilders should dedicate considerable time lifting in a strength range, but probably not more than 1 out of 3 sessions.
Power training and its relation to muscle growth Power is a function of work (force X distance) over time, and is therefore dependent on how rapidly an individual can increase force output. The optimal combination of force and velocity occurs at 40-60 % of an individual's 1 repetition maximum.12 At this weight an individual can rapidly accelerate the bar. Not only does this increase power, but it also has a number of possible benefits for muscle growth. You see the advantage to power training is that unlike strength routines (1-5 repetitions at 85-100 % 1-RM), power routines actually stimulate similar increases in testosterone as hypertrophy routines (8-12 repetitions at 70-85 % 1-RM), with very little cortisol increases. 13 The reason is that testosterone enhances central nervous system function13. This is critical as the nervous system must function at optimal efficiency in order to rapidly "recruit" their muscle fibers for explosive movements. For this reason after an extreme muscle damaging workout, it may be optimal to perform a power workout a few days later. This technique will stimulate an anabolic hormone release which will increase the muscle growth from the previous training session, while not causing as extreme muscle damage, and thus allowing the athlete to avoid overreaching. Finally power routines have the advantage of stimulating the larger type II muscle fibers which are necessary for high velocity contractions.
Percentage of 1-RM or Repetition Ranges?
One question for the bodybuilder is whether they should base their training on a percentage of their 1-RM, or rather base it on a repetition range. Drs. Fleck and Kraemer11 argue that repetition ranges are more efficient as training experience can have significant effects on the amount of reps you can lift at a given % of your 1-RM. For example in one study trained lifters could get 22 reps at 80 % of their 1-RM in the leg press, while untrained individuals could only lift 12 repetitions. Further through extensive analyses of numerous studies it was found that strength was optimized at 60 %, 80 %, and 85 % in untrained, moderately trained, and advanced athletes respectively, suggesting that a greater stimulus is needed with increased training status. For this reason repetition ranges are likely more efficient than using a % of a 1 RM effort.
Rest Period Lengths
While intensity plays a large role in muscle adaptations, it is critical for Natural Muscle readers to understand that the length of rest between sets can enhance or detract from these adaptations. Below I discuss briefly what scientists have found in terms of varying rest period lengths on the outcomes of muscle growth, strength, and power.
For muscular hypertrophy the goal is to stimulate as anabolic environment as is possible. Studies strongly suggest that this occurs with shorter (30-60 seconds) as opposed to longer rest periods (3-5 minutes). For example in one study7 individuals performed either 5, or 10 repetition sets, with 1 to 3 minutes rest between sets. Only the 10 by 1 minute rest protocol significantly increased Growth Hormone, indicating that this is the optimal rest period range for growth (Figure 5.0). Again, the mechanism is that with shorter rest, individuals have less time to clear lactic acid, and therefore obtain greater GH responses.
Figure 1.0 Growth Hormone Response to 4 different rep / rest schemes. Redrawn from Kraemer et al.7
Strength and Power
As discussed earlier underneath exercise intensity we use primarily two energy systems when we train with weights. The first is the ATP-Creatine Phosphate system (ATP-CP) which lasts about 0 to 10 seconds of all out work, and the second which dominates at 30-90 seconds is the glycolytic system in which we use muscle glycogen or our stored form of carbohydrates to power work. ATP-PC is the most powerful system and it takes about 3-5 minutes to replenish ATP-PC stores following a hard set. For strength and power activities each set should elicit a maximum number of repetitions possible with the given weight lifted. For this to occur, studies suggest that 3-5 minutes rest are optimal, result in lower lactic acid levels, and greater recovery of ATP-PC stores on each set.
Figure 2.0 Increases in squat strength in 30 vs. 3 minutes of rest between sets. Data to construct figure was obtained from Robinson et al.
The figure above is based on a study which showed that training with 3 minutes rest resulted in 7 % increases in squat strength as compared to 30 seconds. Power training requires similar rest periods to strength as it too is reliant on ATP-PC stores, and low levels of lactic acid.
Putting it all Together
In this article I discussed the principle that adaptations to skeletal muscle are triggered in large part by changes in the local and surrounding growth factor environment. I would like to close by describing a fascinating study that was recently conducted on this subject. In this 10 week study Goto and colleagues had 2 groups, one which was a hypertrophy-combination group and one which was a hypertrophy-strength group. For the first 6 weeks trainees performed traditional muscle growth workouts consisting of 10+ repetitions and 30-60 seconds of rest. For weeks 7-10 both groups performed a strength routine with low repetitions and 3-5 minutes rest between sets. However the hypertrophy-combination group on the last set of each exercise only rested 30 seconds and performed a high rep burn out set. The results were just as the principles in this article would predict. The greatest muscle growth was found in both groups during the first 6 weeks of hypertrophy training. However in the last 7-10 weeks only the hypertrophy combo group grew. Intriguingly the growth hormone response was greatest in the hypertrophy workouts, second in the hypertrophy-combo workouts, and least in the strength only workout.
Table 2.0 Take home messages for rest period lengths
and skeletal muscle growth, strength, and power.
? For muscle growth rest periods should last 30-60 seconds
? For Strength rest period lengths should last 3-5 minutes
? For power rest period lengths should also last 3-5 minutes
Table 2.0 Take home messages for rest period lengths and skeletal muscle growth, strength, and power.
As you sip on your protein shake try and imagine all the extra growth you will obtain from the principles outlined over the next few months in this series. As for me, I am headed back to the lab to explore all those questions you and I have regarding optimizing muscle growth. But when I am done I'm headed to Gold's gym, and if you are in Tallahassee I may see you there!
Jacob Wilson is a skeletal muscle physiologist and certified strength and conditioning specialist (CSCS) with degrees in sports nutrition (B.S. Hons), and exercise physiology (M.S.). Jacob is in the final year towards obtaining his PhD. and conducts research in the Florida State University Skeletal Muscle Physiology Laboratory. He has written over 300 articles on topics related to muscle growth and fat loss, and has published in numerous scientific / peer reviewed journals in diverse areas of biochemistry, nutrition and metabolism, physiology, strength and conditioning and sports nutrition. Jacob has also spoken and presented his research regionally, nationally, and internationally and is president and co-founder of abcbodybuilding.com
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Table 1.0 Take Home Messages for Exercise Intensity
-For Muscle Growth A range of 6-12 repetitions or 70-85 % maximal intensity is optimal
-For Strength a range of 1-5 repetitions or 86-100 % intensity is optimal
-For Power a range of 1-5 repetitions at 40-60 % of maximal intensity is optimal
-While the major focus of bodybuilders should be training within the hypertrophy repetition range, a considerable amount of time (at least 1 of 3 workouts) should be dedicated to strength routines as this seems to cause selective growth in the larger, type II muscle fibers
-Finally power type repetitions are excellent tools to use between hardcore workouts as they cause less muscle damage and yet produce an equal amount of testosterone as hypertrophy training, with less of a cortisol response. Thus they serve as a way to maintain a high anabolic environment in the target muscle groups without overtraining. They may also enhance type II muscle fiber growth.