Biomechanics of Resistance Exercise

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Biomechanics of Resistance Exercise
chapter 4
Biomechanicsof ResistanceExercise
Everett Harman, PhD, CSCS, NSCA-CPT
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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) attach-ment

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Figure 4.1
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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.

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Figure 4.2

• FA force applied to the lever MAF moment
  armof the applied force FR force resisting
  the levers rotation MRF moment arm of the
  resistive force.

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Key Term

• first-class lever A lever for which the muscle
  force and resistive force act on opposite sides
  of the fulcrum.

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Figure 4.3
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 FR
MR . Because MM is much smaller than MR, FM must
be much greater than FR this illustrates the
disadvantageous nature of this arrangement.
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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.

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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.

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Figure 4.4

• 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.

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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.

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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 momentarm
  of the resistive force.
• Because MM is much smaller than MR, FM must be
  much greater than FR.

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Figure 4.5

• 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 momentarm
  of the resistive force.
• Because MM is much smaller than MR, FM must be
  much greater than FR.

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Figure 4.6
The Patella and Mechanical Advantage

• (a) The patella increases the mechanical
  advantage of the quadriceps muscle group by
  maintaining the quadriceps tendons distance from
  the knees axis of rotation.
• (b) Absence of the patella allows the tendon to
  fall closer to the knees center of rotation,
  shortening the moment arm through which the
  muscle force acts and thereby reducing the
  muscles mechanical advantage.

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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 tendons line of action varies
  throughout the range of joint motion.
• When the moment arm (M) is shorter, there is less
  mechanical advantage.

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Figure 4.7
Moment Arm

• 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.

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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 muscles force
  capability during faster movements.

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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
  upper-lower sections.

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Figure 4.10
Planes of the Human Body
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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.

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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
  cross-sectional 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.

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Key Terms

• pennate muscle A muscle with fibers that align
  obliquely with the tendon, creating a featherlike
  arrangement.
• angle of pennation The angle betweenthe muscle
  fibers and an imaginary line between the muscles
  origin and insertion0 corresponds to no
  pennation.

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Figure 4.11
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Key Term

• concentric muscle action A muscle action in
  which the muscle shortens because the
  con-tractile 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.

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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.

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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.

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Human Strength and Power

• Biomechanical Factors in Human Strength
• Strength-to-Mass Ratio
• In sprinting and jumping, the ratio directly
  reflects an athletes 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.
• 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.

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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.

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Figure 4.14
Cam-Based Weight-Stack Machines

• 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.

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Joint BiomechanicsConcerns 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.

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Fluid Ball

• Figure 4.15 (next slide)
• The fluid ball resulting from contraction of
  the deep abdominal muscles and the diaphragm

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Figure 4.15

• The fluid ball resulting from contraction of
  the deep abdominal muscles and the diaphragm

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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.

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Joint BiomechanicsConcerns 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.

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Joint BiomechanicsConcerns 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)

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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.

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Figure 4.16
Reprinted, by permission, from Harman, Johnson,
and Frykman, 1992.
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Figure 4.16 (continued)
Reprinted, by permission, from Harman, Johnson,
and Frykman, 1992.
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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.