NASM essentials of sports performance training
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NASM essentials of sports performance training

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dynamically stabilize the leg from exces-
sive movements in the frontal and transverse planes (4,9,15) (Fig. 2.6A). During jumping, the
transversus abdominis and multifidus muscles stabilize the lumbar spine (Fig. 2.6B). During
throwing, the rotator cuff dynamically stabilizes the shoulder joint (Fig. 2.6C). 
A concentric muscle action occurs when the contractile force is greater than the resistive force, re-
sulting in shortening of the muscle and visible joint movement. This is referred to as the \u201cposi-
tive\u201d during integrated resistance training (5,11). In athletic activities, all explosive movements
require concentric muscle actions including sprinting, jumping, cutting, and throwing.
A force is defined as the interaction between two entities or bodies that result in either the accel-
eration or deceleration of an object (1,4,5,7). Forces are characterized by both magnitude (how
strong) and direction (which way they are moving) (1,5). The HMS manipulates variable forces
from a multitude of directions to effectively produce movement. As such, the Sports Performance
Professional must gain an understanding of some of the more pertinent mechanical factors that
affect force development that the HMS must deal with and how motion is affected.
Length-tension relationship refers to the resting length of a muscle and the tension the muscle
can produce at this resting length (1,6,16,17). There is an optimal muscle length where the actin
and myosin filaments in the sarcomere have the greatest degree of overlap (Fig. 2.7). The thick
myosin filament is able to make the maximal amount of connections with active sites on the thin
actin filament, leading to maximal tension development of that muscle. When the muscle is stim-
ulated at lengths greater than or less than this optimal length, the resulting tension is less because
there are fewer interactions of the myosin cross bridges and actin active sites. (1,5,6,16\u201318).
This concept is important to the Sports Performance Professional and coincides with the
previously discussed concept of joint alignment. The starting point for a lift, proper posture, the
ability (or inability) to develop tension when reacting or correcting a movement are all impacted
by the length of the muscle when stimulated. Just as the position of one joint can drastically
An influence applied by
one object to another, which re-
sults in an acceleration or de-
celeration of the second object.
Sarcomere length
Resting length
FIGURE 2.7 Length-Tension Relationships.
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affect other joints, a change in joint angle can affect the tension produced by muscles that sur-
round the joint. If muscle length is altered as a result of misalignment (i.e., poor posture), then
tension development will be reduced and the muscle will be unable to generate proper force for
efficient movement. With movement at one joint being interdependent on movement or prepa-
ration for movement of other joints, any dysfunction in the chain of events producing movement
will have direct effects elsewhere (2,10).
The force-velocity curve refers to the relationship of muscle\u2019s ability to produce tension at differ-
ing shortening velocities. This hyperbolic relationship (Fig. 2.8) shows that as the velocity of a
concentric contraction increases, the developed tension decreases. The velocity of shortening ap-
pears to be related to the maximum rate at which the cross bridges can cycle and can be influenced
by the external load (17). Conversely, with eccentric muscle action, as the velocity of muscle ac-
tion increases the ability to develop force increases. This is believed to be the result of the use of
the elastic component of the connective tissue surrounding and within the muscle (1,4\u20136,16\u201318).
Muscles produce a force that is transmitted to bones through elastic and connective tissues
(tendons). Because muscles are recruited as groups, many muscles will transmit force onto their re-
spective bones, creating movement at the joints (1,5,8). This synergistic action of muscles to pro-
duce movement around a joint is also known as a force-couple (1,5,8) (Fig. 2.9). Muscles in a force-
couple provide divergent tension to the bone or bones to which they attach. Because each muscle
has different attachment sites and lever systems, the tension at different angles creates a different
force on that joint. The motion that results from these forces is dependent upon the structure of the
joint, intrinsic properties of each fiber, and the collective pull of each muscle involved.
Velocity of contraction
FIGURE 2.8 Force-Velocity Curves.
Teres major
Teres minor
FIGURE 2.9 Force-Couple Relationships.
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neuromuscular control
Normal length-
tension relationships
Normal force-
couple relationships
Optimal sensorimotor 
Optimal neuromuscular
Optimal tissue
Normal joint
FIGURE 2.10 Efficient Human Movement System.
Altered length-
tension relationships
Altered force-
couple relationships
Altered sensorimotor 
Altered neuromuscular
Tissue fatigue 
and breakdown
Altered joint 
FIGURE 2.11 Human Movement System Dysfunction.
In reality, however, every movement we produce must involve all muscle actions (eccentric,
isometric, concentric) and functions (agonists, synergists, stabilizers, and antagonists) to ensure
proper joint motion as well as minimize unwanted motion. Therefore, all muscles that work to-
gether for the production of proper movement are working in a force-couple (1,5,8). Proper
force-couple relationships are needed so that the HMS moves in the desired manner. This can
only happen if the muscles are at the optimal length-tension relationships and the joints have
proper arthrokinematics (or joint motion). Collectively, optimal length-tension relationships,
force-couple relationships and arthrokinematics produce ideal sensorimotor integration and ul-
timately proper and efficient movement (2,3) (Figs. 2.10 and 2.11).
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The amount of force that the HMS can produce is not only dependent upon motor unit re-
cruitment and muscle size, but also on the lever system of the joint (1,4). A level system is com-
posed of some force (muscles), a resistance (load to be moved), lever arms (bones), and a ful-
crum (the pivot point). Three classes of levers are present in the body (Table 2.4). A class I lever
has the fulcrum between the force and the load. A class II lever has the load between the force
and the fulcrum. Class III levers, the most common in the body, has the pull between the load
and the fulcrum. See Table 2.4 for common examples and examples in the body.
In the HMS, the bones act as lever arms that move a load from the force applied by the mus-
cles. This movement around an axis can be termed rotary motion and implies that the levers
(bones) rotate around the axis (joints) (4,5,9). This \u201cturning\u201d effect of the joint is often referred
to as torque (Fig. 2.12 A,B,C) (10,19).
In resistance training, torque (distance from the load to the center of the axis of rotation
\ufffd the force) is applied so we can move our joints. Because the neuromuscular system is ulti-
mately responsible for manipulating force, the amount of leverage the HMS will have (for any
given movement) depends on the leverage of