168 Introduction to Anatomy and Physiology
Muscular Strength
It may be tempting to think that muscular
strength is the amount of force a given muscle
can produce. It is impossible, however,
to measure muscle force directly without
penetrating the body. So to avoid invasive
procedures, we use external measures (such as
the amount of resistance a person can move) to
establish an indirect measure of muscle strength.
Remember that most joints in the human
body are crossed by more than just one muscle.
Additionally, many exercises involve more than
one joint. This means that an index-of-strength
measure such as maximum bench press actually
assesses the collective work of several muscles
that cross the shoulder and elbow (Figure 5.12).
The main muscles that work during execution
of a bench press include the pectoralis major,
pectoralis minor, anterior deltoid, and triceps
brachii.
A more precise assessment of the strength of
a muscle group at a given joint is the amount of
torque (TORK), or rotary force, that the muscles
can generate. Torque is the product of the size
of a force and the perpendicular distance of
that force from an axis of rotation. For the joint
shown in Figure 5.13, the torque produced by
a muscle is the product of muscle force and
the perpendicular distance from the muscle
attachment to the center of rotation at the joint.
The more torque a muscle generates at a joint,
the greater the tendency for movement of the
bones at the joint. Machines called dynamometers
measure joint torques. Measures of joint torque,
which we use as a measure of strength, are based
solely on the resistance moved or matched. The
speed with which a resistance is moved is not
relevant to the strength measurement.
Muscular Power
The variable that does involve speed is
muscular power. Mechanical power is definedfi
as force multiplied by velocity (force × velocity).
Muscular power, then, has been definedfi
specifically as muscle force multiplied by muscle- fi
shortening velocity during contraction. Notice,
however, that neither muscle force nor shortening
velocity can be measured from outside the body.
Research dynamometers have the ability to
generate estimates of muscular power based on
the resistance moved and movement speed.
Like muscular strength, muscular power is
typically generated by several different muscles
working collectively. Sprinting, along with the
jumping and throwing events in track and field, fi
are good examples of activities that require
muscular power. Because force production and
movement speed contribute equally to muscular
power, the sprinter with the greatest leg strength
may not necessarily be the fastest.
Muscular Endurance
Muscular endurance is the ability of a
muscle to produce tension over a period of
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Figure 5.12 The amount of weight being lifted is an
indirect measure of this person’s muscular strength.
What would be a more precise, direct method of
measuring his strength?