By: Charles Ridgely
Found on Bodybuilding.com
Mechanical loading of your muscles is one of the key
principles on which your training should be based. For a long
time, however, muscle fatigue has been relied upon as a gauge
for the effectiveness of one's training. According to tradition,
one must work the muscles to momentary muscular failure so as to
cause as many muscle fibers as possible to receive a growth
Often it is suggested that the fast twitch, or white, muscle
fibers are not even called into action until the last few
repetitions of a set. As discussed in support of
Hypertrophy-Specific Training TM (HST) [1-3],
however, a great deal of research suggests that all types of
muscle fibers are called into action when the muscles are
exposed to heavy enough loads. Because of this, HST emphasizes
heavy mechanical loading of the muscles rather than staying with
the lighter weights proposed by many other training programs.
According to HST convention, you should spend at least a
portion of your training with heavy loads approaching your 5
rep-max (RM) weight [1-3]. For those readers that are new to the
iron game, a RM specifies the maximum number of times you can
lift a weight before hitting muscular failure. For example, once
you know how much weight you can bench press five times, that
weight is called your 5RM for the Bench Press.
One way to increase the mechanical loading of your muscles
beyond your 5RM is by performing eccentric repetitions, or
negatives, with your 3RM or 2RM weights. With negatives, you
typically have a partner help you lift the weight and then you
lower the weight under control. You may choose to perform five
negative repetitions of each exercise or you may choose to
perform two concentric, or positive, repetitions followed by
three negative repetitions where your partner helps you lift the
Negatives are frequently touted as by far the most effective
approach to muscle growth after having worked up to your 5RM
with positives. Herein, a simple model is provided to
demonstrate the effectiveness of negative repetitions.
Nerves & Muscle Fibers
Muscles are comprised of many muscle fibers. When the muscles
are needed for exerting forces against objects, such as a
barbell, the brain sends signals to the muscles via nerves.
These signals cause the muscle fibers to shorten, or contract.
The precise science underlying muscle physiology is very complex
and goes far beyond our present needs. For this reason, we'll
rely on the simpler concept of "motor units."
A motor unit is defined as all the muscle fibers that are
innervated by a single nerve cell, called a neuron . Figure 1
illustrates an ultra-simplistic representation of such a motor
unit. When the motor unit is turned "OFF" (i.e., no
signal is transmitted by the neuron), the motor unit is not
contracted and remains relaxed.
When the motor unit is turned "ON," it contracts
fully, creating a mechanical force F. For a more detailed
discussion of muscle function, the reader is referred to "How Muscles
Work," located at the HowStuffWorks website
Motor unit deactivated and activated
An important feature of motor units is that each motor unit
is either turned on and is fully contracted or turned off and
fully relaxed. There is no way for the motor unit to be
partially turned on or partially contracted. Activation and
contraction of the motor unit is an all or none affair .
Another important feature is that not all of the motor units
making up a given muscle are activated at the same time when the
muscle contracts. Rather, a sufficient number of motor units are
turned on so that the muscle exerts a required level of force.
For instance, fewer motor units are required to lift a spoon to
your mouth than to curl your 2RM weight.
Moreover, when you lift a heavy weight some of the motor
units will begin to fail and turn off. To maintain the force
output of the muscle, other motor units are turned on to take
the place of the failed motor units. Thus, during muscular
contraction, there is a continual turnover of motor units being
activated, failing, and being replaced by other motor units and
Recruiting Motor Units
The act of turning on motor units to exert a muscular force
is referred to as "recruitment" of the motor units.
When you contract a muscle to lift a weight, you recruit a large
number of motor units which exerts a combined, total force that
is greater than the downward force of the weight. The weight
moves upward simply because the muscular force is greater than
the downward force of the weight.
Fewer motor units are recruited to hold the weight in a
stationary position, called an isometric contraction. For an
isometric contraction, you recruit just enough motor units to
produce a total muscular force which is essentially equal to the
downward force of the weight. Because the upward muscular force
and the downward force of the weight are equal, the weight
remains in the stationary position.
You lower the weight by turning off some motor units. This
decreases the force exerted by your muscle on the weight. Once
the downward force of the weight is a bit greater than the
muscular force, the weight moves downward under control. Thus,
fewer motor units are recruited to lower the weight under
control than to hold the weight stationary.
A motor unit performs work when it contracts against an
external load, such as a barbell. The load also exposes the
motor unit to tension, which damages the motor unit. The motor
unit receives a consequent growth stimulus that causes
hypertrophy of the motor unit when it gets repaired. The more
tension placed on the motor unit, the greater is the growth
stimulus experienced by the motor unit (within reason, of
course). This is one reason why lifting heavy weights is more
productive for muscle growth than staying with lighter weights.
The key to understanding negatives lies in the recruitment of
motor units, discussed above. When motor units are turned off to
lower a weight, a decreasing number of motor units are left
holding the weight. These remaining motor units are thus exposed
to a greater level of tension, which provides an opportunity for
a greater growth stimulus. Let's consider a simple example to
see how this works.
Turning on motor units lifts the weight
Suppose you're going to use a muscle comprised of ten motor
units to lift a weight of 100 lbs. When you lift the weight, all
of the motor units participate, as shown in Figure 2. Because
all ten motor units participate, the weight exposes each motor
unit to 10 lbs of tension. Once you've lifted the weight, you
lower it by turning off some motor units. Let's suppose that you
turn off three motor units, as shown in Figure 3.
Turning off motor units lowers the weight
This leaves seven motor units to lower the weight under
control. Because there are now fewer motor units supporting the
weight, each motor unit is exposed to more tension. In this
example, each motor unit experiences about 14.3 lbs of tension
while lowering the weight. This is an increase in tension of
about 43% over that experienced during the concentric portion of
Since each motor unit is exposed to greater tension during
the eccentric portion of the lift, the eccentric portion is a
bit more inclined to produce a growth stimulus.
Clearly, performing negatives can lead to muscular growth
more quickly than concentric or isometric repetitions. Of
course, the model described above is purely fiction; you're not
guaranteed to increase your muscle growth by 43% just by doing
The primary advantage to doing negatives, however, is that
not only do your muscles experience greater tension during the
eccentric portion of a lift, but also you can generally lower
more weight under control than you can lift. Thus, doing
negatives is one way you can expose your muscles to greater
levels of tension than experienced during concentric lifts.
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