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.
Figure 4.5
Figure 4.7
Figure 4.8
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.
Figure 4.10
Section Outline
• Human Strength and Power
– Basic Definitions
– Biomechanical Factors in Human Strength
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•
•
•
•
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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.
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.
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.
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
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.
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.