Acute Training Variable Manipulation for Maximal Muscle Growth
Part II Repetition Speed and Contraction Type
By Jacob Malachi Wilson, M.S., C.S.C.S.
In part I of this series (June’s issue of Natural Muscle) we discussed the theory that
muscle growth is a factor of signaling events which occur from growth factors within the muscle
such as IGF-1 and surrounding the muscle, such as increases in growth hormone and testosterone
following exercise. The goal of this series is to analyze how we can manipulate variables such as
exercise intensity, and volume to maximize both the hormone and intramuscular growth factor
response. In brief we discussed that muscle growth was greatest in a range of 6-12 repetitions or
70-85 % maximal intensity. The main reason was that lifting in this repetition range relies on
energy systems which produce large amounts of lactic acid (responsible for the burning sensation
you get while training). Lactic acid is important as it has been shown to stimulate growth
hormone and testosterone. We also discussed that shorter rest periods (e.g. 30-60 seconds vs. 2-
5 minutes) resulted in larger hormone responses compared to long rest periods. Today we will
discuss how you can manipulate the speed of contraction of a given lift as well as the type of
contraction within a given lift. Again our main focus will be muscle tissue growth.
Literally every month a new article comes out regarding super slow training. In fact at
my old gym there was this one guy who worked out with small weights each week, and
performed 1 long, slow repetition per set. Was it just an isolated incident that this guy was not
growing in the gym, or is there evidence to support my observation of the ineffectiveness of this
training style? As a background, there are generally two ways an individual can perform a slow
repetition. The first is non-intentional and usually occurs near the end of the set as the muscles
begin to fail and can no longer generate the same force output. For example one study found that
during a 5 repetition maximum set of bench presses during the up / press phase the average speed
was 1.2 seconds, while the last two reps were 2.5 and 3.3 seconds1. This is a natural part of
The second way is to intentionally slow the repetition speed. This is a problem because
force is equal to mass Xs velocity (speed with direction). Thus if you lower velocity, you lower
force output, which means you will not need to recruit as many muscle fibers2. One study found
that super slow lifting (10 s concentric and 5 s eccentric), compared to a faster speed (2 s
concentric, and 4 s eccentric) not only lowered force output during the exercise session, but also
resulted in lower strength gains following 10 weeks of exercise2 Overall both moderate (1 to 2
seconds for concentric and eccentric), and fast (1 second each) velocities compared to slow
velocities (> 5 seconds) have increased total work/volume done in a given workout3, enhanced
strength4, and most importantly resulted in greater muscular remodeling and hypertrophy
(growth) 5, 6
To many this may come as a surprise – for decades so called “experts” have preached the
importance of performing slow, succinct reps to achieve maximal strength and hypertrophy
gains. But once again, this is where I, as a scientist take the opportunity to step in, and separate
fantasy from reality.
There are several rationales to explain these results. First, as discussed, from a
biomechanics point of view you will exert greater force (mass X velocity), and perform more
work (force X distance), which will also result in a greater hormonal response. Second, from a
neurological perspective high speed repetitions have been shown to selectively recruit fast twitch
muscle fibers, which are largely responsible for muscle size and strength. Additional
advantageous neural adaptations to high velocity movements may also be expected, such as the
ability to recruit and therefore stimulate more muscle fibers during any given training session. A
third possible mechanism is actually related to muscle damage. Skeletal muscle is composed of a
cytoskeleton which holds the tissues together (non-contractile components), as well as contractile
tissue (actin and myosin). What is important to understand is that muscle fibers are most
susceptible to damage in the non-contractile elements. Now, the greater time you have to
contract, the greater the opportunity for tension to be built up in the contractile component of
muscle; consequently, distributing the resistance across contractile tissues, thereby alleviating the
non-contractile elements. However, with higher velocity contractions damage is higher for the
reason that the contractile elements of muscle do not have time to build up maximal force and so
the non contractile components of muscle have to bear a greater load resulting in more muscle
damage and a greater local growth factor response (e.g. greater local IGF production).
Table 1.0 Take home messages for repetition speed.
Super slow training hinders force output, muscle fiber recruitment and ultimately
strength and muscle gains
It appears that strength and hypertrophy are optimized at moderate (1-2 seconds
each for concentric and eccentric) and fast (1 second each) speeds
For power the individual should attempt to explode on each repetition with as much
acceleration as possible.
When we lift weights our muscles contract in three different ways. These include concentric,
eccentric, and isometric contractions.
Figure 1A. A concentric contraction occurs during the up phase of a lift. During this
process the muscle shortens.
Figure 1B. During an eccentric contraction the individuals lowers the weight, and in the process
lengthens the muscle
Figure 1C. In an isometric contraction the individuals holds the weight in one place, contracts,
but does not move the muscle. For example squeezing at the top portion of a biceps curl.
The following paragraphs will analyze the effects of each contraction in terms of muscle
growth, strength, and power.
Concentric vs. Eccentric contractions
Concentric contractions occur when you shorten a muscle such as during the up phase on
a squat, while eccentric actions occur on the lowering phase of a movement when the muscle is
lengthening such as during the lowering phase of a biceps curl. Studies are very clear, eccentric
contractions produce more strength and muscle growth than concentric contractions 1. Eccentric
contractions basically change the local environment to favor muscle growth over concentric
contractions. For example, studies show that eccentric contractions produce more IGF-1 within
muscle1, raise protein synthesis to a greater extent2, lower protein degradation3, and increase the
machinery (ribosomal proteins) responsible for protein synthesis to a greater extent than
concentric contractions3. Further bodybuilding is dependent on actually remodeling muscle
tissue (e.g. making it more dense and giving it a greater ability to generate force), and evidence
suggests that eccentric contractions actually trigger the remodeling process, possibly due to their
greater ability to induce muscle fiber damage4, 5 However, most exercise programs include both
lowering and shortening contractions, and so eccentric contractions are generally emphasized as
much as concentric contractions. The data presented in this article however provides support for
the use of incorporating forced negatives following a set as a means to increase muscle growth.
It also demonstrates why people who fling weights around using momentum, and who allow
gravity to take the weight down instead of resisting the weight generally experience poor gains in
muscle size and strength.
Isometric actions occur when an individual contracts without actually shortening the
muscle. Gripping a weight and flexing in a muscle in a stationary position are examples of
isometrics. Isometric contractions by themselves do very little for muscle growth, and generally
only cause increases in strength or power at the angle used6. However isometric contractions are
effective at occluding blood flow. Recently my colleagues Layne Nortan, Gabriel Wilson and I
wrote an article for Ironman Magazine on blood flow occlusion, in which you wrap a muscle
group with a compression band in order to shut down blood flow to the muscle.
A. Rest B. Isometric Contraction
Figure 2.0 Effects of isometric contraction on blood vessel compression.
Lowered blood flow decreases oxygen in the muscle, crippling its ability to clear lactic
acid (a byproduct of muscle contraction). As stated lactic acid is a powerful stimulator of growth
hormone7, and therefore studies using blood occlusion demonstrate large increases in both
growth hormone, and muscle growth 8-11. Isometrics completely occlude blood flow at around
65 % of maximal contraction (moderately hard contraction)12. This is because the increased
girth of the muscle acts to compress the vascular network.
Set 1 Biceps Curls Set 2 Biceps Curls Set 3 Biceps Curls
Flex Without Rest Flex Without Rest
Figure 3.0 Example of an isometric, hypertrophy oriented protocol.
Thus, following an intense exercise set, instead of allowing muscles to relax, a
bodybuilder might flex and hold the muscles they trained, preventing lactic acid from being
cleared, and raising the growth hormone response. For example, standing barbell curls
supersetted with static flexing of the arms, followed immediately without rest by another set of
Table 2.0 Take Home Messages for Muscle Action’s and Velocities Effects on Hypertrophy,
Strength, and Power.
Generally a combination of both concentric and eccentric contractions, as in a normal upward
and downward lift is recommended. However, because eccentric contractions appear to
promote greater muscle growth, protein synthesis, and skeletal muscle remodeling it is
recommended that eccentric only training be incorporated into an individual’s routine. This
can be used at the end of a set, or as an eccentric only set using greater than average loads.
Isometric contractions should not be used to increase overall strength or muscle size by
themselves. However, if you flex (a form of isometric contraction) between normal weight
lifting sets you will occlude blood flow to the target muscle, thereby lowering the oxygen
availability in that muscle and trapping lactic acid in the area. Increased acidity is a powerful
stimulator of protein synthesis and hormones and thus can serve as an effective stimulus for
My philosophy on bodybuilding is that we have to maximize each and every workout in the gym.
Clearly this begins with an understanding of each component of a single repetition, be it the
concentric, eccentric, isometric phase, or even the speed of the lift. With the knowledge in this
article I have no doubt you will get closer to your goals of becoming absolute monsters in the
gym. Finally I would like to give a shout out to my close friend Dave Bogan. Dave trains here
at Tallahassee University Gold’s gym and is one of the sickest, most shredded and symmetrical
natural bodybuilders I know. No question about it the man is set to do damage in this year’s
light heavy weight division in Muscle Mania!
Yours in Sport
Jcob Wilson is a skeletal muscle physiologist and certified strength and conditioning
specialist (CSCS) with degrees in sports nutrition (B.S. Hons), exercise physiology and
sports psychology/motor learning (M.S). Jacob is in the final year towards obtaining his
PhD. In muscle physiology 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
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