chapter
Biomechanics of Resistance Exercise
4
Biomechanics
of Resistance
Exercise
Everett Harman, PhD, CSCS, NSCA-CPT
Chapter Objectives
• Identify the major bones and muscles of the
human body.
• Differentiate among the types of levers of the
musculoskeletal system.
• Calculate linear and rotational work and power.
• Describe the factors contributing to human
strength and power.
• Evaluate resistive force and power patterns of
exercise devices.
(continued)
Chapter Objectives (continued)
• Recommend ways to minimize injury risk during
resistance training.
• Analyze sport movements and design movementoriented exercise prescriptions.
Section Outline
• Musculoskeletal System
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Skeleton
Skeletal Musculature
Levers of the Musculoskeletal System
Variations in Tendon Insertion
Anatomical Planes of the Human Body
Key Terms
• anatomy: The study of components that make
up the musculoskeletal “machine.”
• biomechanics: The mechanisms through
which these components interact to create
movement.
Musculoskeletal System
• Skeleton
– Muscles function by pulling against bones that rotate about
joints and transmit force through the skin to the environment.
– The skeleton can be divided into the axial skeleton and the
appendicular skeleton.
• 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
Human Skeletal Musculature
• Figure 4.1 (next slide)
– (a) Front view of adult male human skeletal
musculature
– (b) Rear view of adult male human skeletal
musculature
Figure 4.1
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.
Musculoskeletal System
• 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 4.2 (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 4.2
Key Term
• mechanical advantage: The ratio of the
moment arm through which an applied force
acts to that through which a resistive force acts.
A mechanical advantage greater than 1.0
allows the applied (muscle) force to be less
than the resistive force to produce an equal
amount of torque. A mechanical advantage of
less than 1.0 is a disadvantage in the common
sense of the term.
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 (the Forearm)
• Figure 4.3 (next slide)
– The slide shows elbow extension against resistance (e.g., a triceps
extension exercise).
– 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, which, being
less than 1.0, is a disadvantage.
– The depiction is of a first-class lever because muscle force and
resistive force act on opposite sides of the fulcrum.
– During isometric exertion or constant-speed joint rotation, FM · MM =
F R · MR .
– Because MM is much smaller than MR, FM must be much greater than
FR; this illustrates the disadvantageous nature of this arrangement.
Figure 4.3
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
its mechanical advantage, the required muscle
force is smaller than the resistive force.
A Second-Class Lever (the Foot)
• Figure 4.4 (next slide)
– The slide shows 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 4.4
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 (the Forearm)
• Figure 4.5 (next slide)
– The slide shows 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 4.5
The Patella and Mechanical
Advantage
• Figure 4.6 (next slide)
– (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 4.6
Reprinted, by permission, from Gowitzke and Milner, 1988.
Moment Arm and Mechanical
Advantage
• Figure 4.7 (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 4.7
Moment Arm
• Figure 4.8 (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 4.8
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.
Musculoskeletal System
• 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 4.9 (next slide)
– The slide shows changes in joint angle with equal
increments of muscle shortening when the tendon is
inserted (a) closer to and (b) 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 4.9
Reprinted, by permission, from Gowitzke and Milner, 1988.
Musculoskeletal System
• Anatomical Planes of the Human Body
– 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 upperlower sections.
Planes of the Human Body
• Figure 4.10 (next slide)
– The three planes of the human body in the
anatomical position
Figure 4.10
Section Outline
• Human Strength and Power
– Basic Definitions
– Biomechanical Factors in Human Strength
•
•
•
•
•
•
•
•
•
Neural Control
Muscle Cross-Sectional Area
Arrangement of Muscle Fibers
Muscle Length
Joint Angle
Muscle Contraction Velocity
Joint Angular Velocity
Strength-to-Mass Ratio
Body Size
Human Strength and Power
• Basic Definitions
– strength: The capacity to exert force at any given
speed.
– power: The mathematical product of force and
velocity at whatever speed.
Human Strength and Power
• Biomechanical Factors in Human Strength
– Neural Control
• Muscle force is greater when: (a) more motor units are
involved in a contraction, (b) the motor units are greater
in size, or (c) the rate of firing is faster.
– Muscle Cross-Sectional Area
• The force a muscle can exert is related to its crosssectional area rather than to its volume.
– Arrangement of Muscle Fibers
• Variation exists in the arrangement and alignment of
sarcomeres in relation to the long axis of the muscle.
Key Terms
• 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.
Muscle Fiber Arrangements
• Figure 4.11 (next slide)
– Muscle fiber arrangements and an example of each
Figure 4.11
Human Strength and Power
• Biomechanical Factors in Human Strength
– Muscle Length
• At resting length: actin and myosin filaments lie next to
each other; maximal number of potential cross-bridge sites
are available; the muscle can generate the greatest force.
• When stretched: a smaller proportion of the actin and
myosin filaments lie next to each other; fewer potential
cross-bridge sites are available; the muscle cannot
generate as much force.
• When contracted: the actin filaments overlap; the number
of cross-bridge sites is reduced; there is decreased force
generation capability.
Muscle Length and Actin
and Myosin Interaction
• Figure 4.12 (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 cross-bridges.
Figure 4.12
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 4.13 (next slide)
– Force–velocity curve for eccentric and concentric
actions
Figure 4.13
Reprinted, by permission, from Jorgensen, 1976.
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.
Human Strength and Power
• 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.
Section Outline
• Sources of Resistance to Muscle
Contraction
– Gravity
• Applications to Resistance Training
• Weight-Stack Machines
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–
–
–
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Inertia
Friction
Fluid Resistance
Elasticity
Negative Work and Power
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.
– Weight-Stack Machines
• Gravity is the source of resistance, but machines provide
increased control over the direction and pattern of
resistance.
Cam-Based Weight-Stack Machines
• Figure 4.14 (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 4.14
Sources of Resistance
to Muscle Contraction
• Inertia
– When a weight is held in a static position or when
it is moved at a constant velocity, it exerts constant
resistance only in the downward 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.
Sources of Resistance
to Muscle Contraction
• 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 orifice.
• Elasticity
– The more an elastic component is stretched, the greater the
resistance.
• Negative Work and Power
– Negative work refers to work performed on, rather than by, a
muscle.
– The rate at which the repetitions are performed determines the
power output.
Section Outline
• Joint Biomechanics: Concerns in
Resistance Training
– Back
• Back Injury
• Intra-Abdominal Pressure and Lifting Belts
– Shoulders
– Knees
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.
– 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 4.15 (next slide)
– The “fluid ball” resulting from contraction of the deep
abdominal muscles and the diaphragm
Figure 4.15
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.
• Knees
– The knee is prone to injury because of its location between two
long levers.
– Minimize the use of wraps.
Joint Biomechanics:
Concerns in Resistance Training
• How Can Athletes Reduce the Risk of
Resistance Training Injuries?
– Perform one or more warm-up sets with relatively
light weights, particularly for exercises that involve
extensive use of the shoulder or knee.
– Perform basic exercises through a full ROM.
– Use relatively light weights when introducing new
exercises or resuming training after a layoff of two or
more weeks.
– Do not ignore pain in or around the joints.
(continued)
Joint Biomechanics:
Concerns in Resistance Training
• How Can Athletes Reduce the Risk of
Resistance Training Injuries? (continued)
– Never attempt lifting maximal loads without proper
preparation, which includes technique instruction in
the exercise movement and practice with lighter
weights.
– Performing several variations of an exercise results
in more complete muscle development and joint
stability.
– Take care when incorporating plyometric drills into a
training program.
Section Outline
• Movement Analysis and Exercise
Prescription
Major Body Movements
• Figure 4.16 (next two slides)
– 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 4.16
Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.
Figure 4.16 (continued)
Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.
Key Point
• Specificity is a major consideration when
one is designing an exercise program to
improve performance in a particular sport
activity. The sport movement must be
analyzed qualitatively or quantitatively to
determine the specific joint movements that
contribute to the whole-body movement.
Exercises that use similar joint movements
are then emphasized in the resistance
training program.