chapter
Biomechanics of Resistance Exercise
2
Biomechanics
of Resistance
Exercise
Jeffrey M. McBride, PhD
Chapter Objectives
• Identify the major components of skeletal
musculature
• Differentiate the various types of levers of
the musculoskeletal system
• Identify primary anatomical movements
during sport activities and exercises
• Calculate linear and rotational work and
power
(continued)
Chapter Objectives (continued)
• Describe the factors contributing to human
strength and power
• Evaluate resistive force and power patterns
of exercise devices
• Identify factors of importance for joint
biomechanics with exercise
Key Term
• biomechanics: The mechanisms through
which components interact to create
movement.
Skeletal Musculature
• Skeletal musculature
– A system of muscles enables the skeleton to move.
– Origin = proximal (toward the center of the body)
attachment.
– Insertion = distal (away from the center of the body)
attachment.
Key Terms
• agonist: The muscle most directly involved in
bringing about a movement; also called the
prime mover.
• antagonist: A muscle that can slow down or
stop the movement.
• synergist: A muscle that assists indirectly in a
movement
Skeletal Musculature
• Levers of the musculoskeletal system
– Many muscles in the body do not act through levers.
– Body movements directly involved in sport and
exercise primarily act through the bony levers of the
skeleton.
– A lever is a rigid or semirigid body that, when
subjected to a force whose line of action does not
pass through its pivot point, exerts force on any
object impeding its tendency to rotate.
A Lever
• Figure 2.1 (next slide)
– The lever can transmit force tangential to the arc of
rotation from one contact point along the object’s
length to another.
– FA = force applied to the lever; MAF = moment arm of
the applied force; FR = force resisting the lever’s
rotation.
– MRF = moment arm of the resistive force. The lever
applies a force on the object equal in magnitude to
but opposite in direction from FR.
Figure 2.1
Key Term
• mechanical advantage: The ratio of the
moment arm through which an applied force
acts to that through which a resistive force acts.
– Greater than 1.0 means a person can apply less
(muscle) force than the resistive force to produce an
equal amount of torque.
– Less than 1.0 means a person must apply greater
(muscle) force than the amount of resistive force
present, creating a disadvantage for the muscle.
Key Term
• first-class lever: A lever for which the muscle
force and resistive force act on opposite sides
of the fulcrum.
A First-Class Lever
• Figure 2.2 (next slide)
–
–
–
–
–
–
O = fulcrum
FM = muscle force
FR = resistive force
MM = moment arm of the muscle force
MR = moment arm of the resistive force
Mechanical advantage = MM /MR = 5 cm/40 cm =
0.125
– Less than 1.0, so is a disadvantage
Figure 2.2
Key Term
• second-class lever: A lever for which the
muscle force and resistive force act on the
same side of the fulcrum, with the muscle force
acting through a moment arm longer than that
through which the resistive force acts. Due to
the muscle’s mechanical advantage, the
required muscle force is smaller than the
resistive force.
A Second-Class Lever
• Figure 2.3 (next slide)
– Plantarflexion against resistance (e.g., a standing
heel raise exercise)
– FM = muscle force
– FR = resistive force
– MM = moment arm of the muscle force
– MR = moment arm of the resistive force
– When the body is raised, the ball of the foot, the
point about which the foot rotates, is the fulcrum (O)
– Because MM is greater than MR, FM is less than FR
Figure 2.3
Key Term
• third-class lever: A lever for which the muscle
force and resistive force act on the same side
of the fulcrum, with the muscle force acting
through a moment arm shorter than that through
which the resistive force acts. The mechanical
advantage is thus less than 1.0, so the muscle
force has to be greater than the resistive force to
produce torque equal to that produced by the
resistive force.
A Third-Class Lever
• Figure 2.4 (next slide)
– Elbow flexion against resistance (e.g., a biceps curl
exercise).
– FM = muscle force
– FR = resistive force
– MM = moment arm of the muscle force
– MR = moment arm of the resistive force
– Because MM is much smaller than MR, FM must be
much greater than FR
Figure 2.4
The Patella
and Mechanical Advantage
• Figure 2.5 (next slides)
– (a) The patella increases the mechanical advantage
of the quadriceps muscle group by maintaining the
quadriceps tendon’s distance from the knee’s axis of
rotation.
– (b) Absence of the patella allows the tendon to fall
closer to the knee’s center of rotation, shortening the
moment arm through which the muscle force acts
and thereby reducing the muscle’s mechanical
advantage.
Figure 2.5(a)
Figure 2.5(b)
Moment Arm
and Mechanical Advantage
• Figure 2.6 (next slide)
– During elbow flexion with the biceps muscle, the
perpendicular distance from the joint axis of rotation
to the tendon’s line of action varies throughout the
range of joint motion.
– When the moment arm (M) is shorter, there is less
mechanical advantage.
Figure 2.6
Moment Arm
• Figure 2.7 (next slide)
– As a weight is lifted, the moment arm (M) through
which the weight acts, and thus the resistive torque,
changes with the horizontal distance from the weight
to the elbow.
Figure 2.7
Key Point
• Most of the skeletal muscles operate at a
considerable mechanical disadvantage.
Thus, during sports and other physical
activities, forces in the muscles and
tendons are much higher than those
exerted by the hands or feet on external
objects or the ground.
Skeletal Musculature
• Variations in tendon insertion
– Tendon insertion: The points at which tendons are
attached to bone.
– Tendon insertion farther from the joint center results
in the ability to lift heavier weights.
• This arrangement results in a loss of maximum speed.
• This arrangement reduces the muscle’s force capability
during faster movements.
Tendon Insertion and Joint Angle
• Figure 2.8 (next slides)
– Changes in joint angle with equal increments of
muscle shortening
– (a) The tendon is inserted closer to the joint center.
– (b) The tendon is inserted farther from the joint
center.
– Configuration b has a larger moment arm and thus
greater torque for a given muscle force, but less
rotation per unit of muscle contraction and thus
slower movement speed.
Figure 2.8(a)
Figure 2.8(b)
Anatomical Planes
and Major Body Movements
– The body is erect, the arms are down at the sides,
and the palms face forward.
– The sagittal plane slices the body into left–right
sections.
– The frontal plane slices the body into front–back
sections.
– The transverse plane slices the body into upper–
lower sections.
(continued)
Anatomical Planes
and Major Body Movements (continued)
• Figure 2.9 (next slide)
– The three planes of movement of the body in the
anatomical position
Figure 2.9
Anatomical Planes
and Major Body Movements
• Figure 2.10 (next slides)
– Major body movements
– Planes of movement are relative to the body in the
anatomical position unless otherwise stated.
– Common exercises that provide resistance to the
movements and related sport activities are listed.
Figure 2.10
(continued)
Figure 2.10 (continued)
Human Strength and Power
• Basic definitions
– Strength: The capacity to exert force at any given
speed.
– Acceleration: The change in velocity per unit of time.
This is associated with resistive force by Newton’s
second law (Force = Mass × Acceleration).
(continued)
Human Strength and Power (continued)
• Positive work and power
– Power: Outside of the scientific realm, power is
loosely defined as “explosive strength.” Also defined
as the time rate of doing work.
• Power = Work / Time
– Work: The product of force exerted on an object and
the distance the object moves in the direction the
force is exerted.
• Work = Force × Displacement
(continued)
Human Strength and Power (continued)
• Negative work
– Work performed on, rather than by, a muscle
– Occurs during eccentric muscle actions
(continued)
Human Strength and Power (continued)
• Angular work and power
– Angular displacement
• The angle through which an object rotates
• Rotational work = Torque × Angular displacement
Key Points
• Although the word strength is often
associated with slow speeds and the word
power with high velocities of movement,
both variables reflect the ability to exert
force at a given velocity.
• The sport of weightlifting (Olympic lifting)
has a much higher power component than
the sport of powerlifting, due to the higher
movement velocities with heavy weights of
the weightlifting movements.
Human Strength and Power
• Biomechanical factors in human strength
– Neural control
• Recruitment affects maximal force output by determining
which and how many motor units are involved in a muscle
contraction.
• Rate coding affects maximal force output by determining
the rate at which the motor units are fired.
– Muscle cross-sectional area
(continued)
Human Strength and Power (continued)
• Biomechanical factors in human strength
– Arrangement of muscle fibers
• Pennate muscle: A muscle with fibers that align obliquely
with the tendon, creating a featherlike arrangement.
• Angle of pennation: The angle between the muscle fibers
and an imaginary line between the muscle’s origin and
insertion; 0°
corresponds to no pennation.
(continued)
Human Strength and Power (continued)
• Figure 2.11 (next slide)
– Muscle fiber arrangements and an example of each
Figure 2.11
Human Strength and Power
• Biomechanical factors in human strength
– Muscle length (at resting length)
• Actin and myosin filaments lie next to each other.
• A maximal number of potential crossbridge sites are
available.
• The muscle can generate the greatest force.
(continued)
Human Strength and Power (continued)
• Biomechanical factors in human strength
– Muscle length (when stretched)
• A smaller proportion of the actin and myosin filaments lie
next to each other.
• Fewer potential crossbridge sites are available.
• The muscle cannot generate as much force.
(continued)
Human Strength and Power (continued)
• Biomechanical factors in human strength
– Muscle length (when contracted)
• The actin filaments overlap.
• The number of crossbridge sites is reduced.
• There is decreased force generation capability.
Muscle Length and Actin
and Myosin Interaction
• Figure 2.12 (next slide)
– The slide shows the interaction between actin and
myosin filaments when the muscle is at its resting
length and when it is contracted or stretched.
– Muscle force capability is greatest when the muscle
is at its resting length because of increased
opportunity for actin–myosin crossbridges.
Figure 2.12
Human Strength and Power
• Biomechanical factors in human strength
– Joint angle
• Amount of torque depends on force versus muscle length,
leverage, type of exercise, the body joint in question, the
muscles used at that joint, and the speed of contraction.
– Muscle contraction velocity
• Nonlinear, but in general, the force capability of muscle
declines as the velocity of contraction increases.
– Joint angular velocity
• There are three types of muscle action.
Key Term
• concentric muscle action: A muscle action
in which the muscle shortens because the
contractile force is greater than the resistive
force. The forces generated within the muscle
and acting to shorten it are greater than the
external forces acting at its tendons to stretch
it.
Key Term
• eccentric muscle action: A muscle action in
which the muscle lengthens because the
contractile force is less than the resistive force.
The forces generated within the muscle and
acting to shorten it are less than the external
forces acting at its tendons to stretch it.
Key Term
• isometric muscle action: A muscle action in
which the muscle length does not change,
because the contractile force is equal to the
resistive force. The forces generated within the
muscle and acting to shorten it are equal to the
external forces acting at its tendons to stretch it.
Force–Velocity Curve
• Figure 2.13 (next slide)
– Force–velocity curve for eccentric and concentric
actions
Figure 2.13
Human Strength and Power
• Biomechanical factors in human strength
– Strength-to-mass ratio
• In sprinting and jumping, the ratio directly reflects an
athlete’s ability to accelerate his or her body.
• In sports involving weight classification, the ratio helps
determine when strength is highest relative to that of other
athletes in the weight class.
(continued)
Human Strength and Power (continued)
• Biomechanical factors in human strength
– Body size
• As body size increases, body mass increases more rapidly
than does muscle strength.
• Given constant body proportions, the smaller athlete has a
higher strength-to-mass ratio than does the larger athlete.
Key Point
• In sport activities such as sprinting and
jumping, the ratio of the strength of the
muscles involved in the movement to the
mass of the body parts being accelerated is
critical. Thus, the strength-to-mass ratio
directly reflects an athlete’s ability to
accelerate his or her body.
Sources of Resistance
to Muscle Contraction
• Gravity
– Applications to resistance training
• When the weight is horizontally closer to the joint, it exerts
less resistive torque.
• When the weight is horizontally farther from a joint, it exerts
more resistive torque.
Key Point
• Exercise technique can affect the resistive
torque pattern during an exercise and can
shift stress among muscle groups.
Sources of Resistance
to Muscle Contraction
• Gravity
– Weight-stack machines
• Gravity is the source of resistance, but machines provide
increased control over the direction and pattern of
resistance.
– Figure 2.14 (next slide)
• In cam-based weight-stack machines, the moment arm (M)
of the weight stack (horizontal distance from the chain to
the cam pivot point) varies during the exercise movement.
• When the cam is rotated in the direction shown from
position 1 to position 2, the moment arm of the weights, and
thus the resistive torque, increases.
Figure 2.14
Sources of Resistance
to Muscle Contraction
• Inertia
– Though the force of gravity acts only downward,
inertial force can act in any direction
– However, upward or lateral acceleration of the
weight requires additional force.
• Friction
– Friction is the resistive force encountered when one
attempts to move an object while it is pressed
against another object.
(continued)
Sources of Resistance
to Muscle Contraction (continued)
• Fluid resistance
– Fluid resistance is the resistive force encountered by an object
moving through a fluid (liquid or gas), or by a fluid moving past
or around an object or through an opening.
• Elasticity
– The more an elastic component is stretched, the greater the
resistance.
Joint Biomechanics:
Concerns in Resistance Training
• Back
– Back injury
• The lower back is particularly vulnerable.
• Resistance training exercises should generally be
performed with the lower back in a moderately arched
position.
Key Point
• The risk of injury from resistance training is
low compared to that with other sport and
physical conditioning activities.
Joint Biomechanics:
Concerns in Resistance Training
• Back
– Intra-abdominal pressure and lifting belts
• The “fluid ball” aids in supporting the vertebral column
during resistance training.
• Weightlifting belts are probably effective in improving
safety. Follow conservative recommendations.
“Fluid Ball”
• Figure 2.15 (next slide)
– The “fluid ball” resulting from contraction of the deep
abdominal muscles and the diaphragm
Figure 2.15
Key Term
• Valsalva maneuver: The glottis is closed, thus
keeping air from escaping the lungs, and the
muscles of the abdomen and rib cage contract,
creating rigid compartments of liquid in the
lower torso and air in the upper torso.
Joint Biomechanics:
Concerns in Resistance Training
• Shoulders
– The shoulder is prone to injury during weight training
because of its structure and the forces to which it is
subjected.
– Warm up with relatively light weights.
– Follow a program that exercises the shoulders in a
balanced way.
– Exercise at a controlled speed.
(continued)
Joint Biomechanics:
Concerns in Resistance Training (continued)
• Knees
– The knee is prone to injury because of its location between two
long levers.
– Minimize the use of wraps.
• Elbows and wrists
– The primary concern involves overhead lifts. However, the
most common source of injury to these areas is from overhead
sports such as throwing events or the tennis serve.