The Biomechanics of Resistance Training

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The Biomechanics of Resistance Training

• Strength Conditioning
• University of Massachusetts Boston

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The Musculoskeletal System

• The combined system comprised of bones, joints,
  muscles and tendons that function together to
  allow human movement and locomotion
• Muscles are the Force producers which act on
  bones to create movement at the joints
• The musculoskeletal system is a series of rigid
  levers (bones) linked together (joints)
• Muscle fibers themselves only know how to
  contract, or Pull
• Based on the design of the joint and the lever
  system, Pulling or Pushing action is attained.
• A Pull is the action of moving an object closer
  to the core of the body whereas a Push is the
  action of moving an object further away from the
  core of the body
• A muscle has an Origin (proximal attachment) and
  an Insertion (distal attachment)

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Muscle Action Terminology

• Agonist muscle or muscle group that is most
  directly related to the initiation and execution
  of a particular movement
• Antagonist muscle or muscle group that is
  considered to have the opposite action of a
  particular agonist
• Synergist two or more muscle that cooperate to
  execute a particular movement
• Stabilizer a muscle or muscle group that
  functions to support or stabilize while the
  agonist executes a movement
• Neutralizer a muscle or muscle group that
  counteracts unwanted motion at the joint on which
  the agonist or synergists are attempting to
  execute a movement
• EXAMPLE
• Gluteus Maximus Agonist of hip extension
• Hamstrings Synergists
• Hip Flexors - Antagonists
• Core Stabilizers
• Gluteus Medius Hip Adductors - Neutralizers

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Biomechanical Terminology

• Muscle Force force generated by a muscle
• Resistive Force force generated by a source
  outside of the body, i.e gravity or collision
  with another object
• Moment Arm shortest distance between a F and
  the A of R
• Torque (T) the product of the Force times the
  Moment Arm (F x M.arm) T is the rotary
  equivalent of F the vast majority of human
  movement is rotational
• Lever a rigid body (bone that when subjected to
  a force (muscle force), exerts a force and
  creates movement at a joint (fulcrum or pivot
  point)

• Muscle Force Moment Arm (MM) product of the
  internal F (muscle) and the M.arm.
• Resistive Force Moment Arm (MF)-product of the
  ext. F and the M.arm.

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Mechanical Advantage

• Mechanical Advantage (MA) is also referred to as
  leverage
• MA the ratio of the moment arm of the muscle
  force (MM) to the moment arm of the resistive
  force (MR)
• Because most muscle/joint systems in the body are
  3rd class levers, the MR gt MF , therefore MF
  MR is much less than 1
• MF only varies a small amount at a given joint,
  however the MR can vary greatly depending on the
  lever and external resistive force applied

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Mechanical Advantage (cont.)

• The human musculoskeletal system is designed
  inefficiently for movement as most joints
  operates at a mechanical disadvantage
• As a joint moves through its ROM, the angle of
  insertion of a muscles tendon changes this
  results in changes in mechanical advantage
• A person whose tendons insert at a greater
  distance from the joint Axis of Rotation has
  greater inherent mechanical advantage and will be
  able to lift heavier weights however this same
  mechanical advantage results in a loss of maximum
  speed of movement because the muscle has to
  contract to more to move the joint through a
  given ROM
• Understanding the principle and application of
  mechanical advantage is of more importance to the
  strength and conditioning professional than is
  the classification of lever systems

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Musculoskeletal Levers

• First Class Lever A of R is positioned between
  two opposing levers Internal and External F
  typically act in the same direction, however
  producing opposing rotary torques
• Second Class Lever A of R is located at one end
  of a bone the muscle possesses greater leverage
  than the External F (IMA gt EMA) This is the
  least common musculoskeletal lever system.
• Third Class Lever A of R is located at one end
  of a bone External F possesses greater advantage
  than the Internal F (muscle force) This is the
  least efficient, but most common lever system.

F
R
F
R
F
R
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1st Class
2nd Class
3rd Class
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Anatomical Planes of the Human Body
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Human Strength Power

• Strength the ability to exert a force the
  maximal force that a muscle or muscle group can
  generate at a specified velocity the ability to
  do Work (W)
• Work the product of Force (F) exerted on an
  object and the distance that the object moves
  (D) W FD
• Velocity (V) the change in position of an
  object over time D/t
• Power (P) the rate of work W / time (t) P
  W/t
• P W/t
• W/t F x D/t or F x Velocity
  (WFV)
• Training power in an athlete requires doing work
  quickly, or explosively
• How is Power measured and trained in sport and
  exercise?

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Strength Power (cont.)

• The sport of Powerlifting (squat, bench press,
  deadlift) involves high forces but relatively low
  movement speeds
• In comparison to many other types of sports, i.e.
  Olympic Lifting (snatch, clean jerk),
  Powerlifting is more of a Strength sport
• For a sports movement made relatively slow by
  high resistance, low-speed strength is critical
  (i.e. football lineman)
• For a sports movement made very fast due to low
  resistance, high-speed strength is important
  (i.e. racquet sports)

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Measuring and Training Power in the Athlete
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Power in Sport