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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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)

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Chapter Objectives (continued)

• Recommend ways to minimize injury risk during
  resistance training.
• Analyze sport movements and design
  movement-oriented exercise prescriptions.

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Section Outline

• Musculoskeletal System
• Skeleton
• Skeletal Musculature
• Levers of the Musculoskeletal System
• Variations in Tendon Insertion
• Anatomical Planes of the Human Body

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Key Terms

• anatomy The study of components that make up the
  musculoskeletal machine.
• biomechanics The mechanisms through which these
  components interact to create movement.

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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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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

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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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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.

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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
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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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Figure 4.4
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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
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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 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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Figure 4.6
Reprinted, by permission, from Gowitzke and
Milner, 1988.
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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
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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.

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Figure 4.8
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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 ten-dons are much
  higher than those exerted by the hands or feet on
  external objects or the ground.

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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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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.

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Figure 4.9
Reprinted, by permission, from Gowitzke and
Milner, 1988.
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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
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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

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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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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.

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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.

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Figure 4.12
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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.

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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.

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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.

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Section Outline

• Sources of Resistance to Muscle Contraction
• Gravity
• Applications to Resistance Training
• Weight-Stack Machines
• Inertia
• Friction
• Fluid Resistance
• Elasticity
• Negative Work and Power

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Sources of Resistanceto 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.

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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
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Sources of Resistance to Muscle Contraction

• Inertia
• When a weight is held in a static position or
  whenit 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.

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Sources of Resistanceto 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.

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Section Outline

• Joint Biomechanics Concerns in Resistance
  Training
• Back
• Back Injury
• Intra-Abdominal Pressure and Lifting Belts
• Shoulders
• Knees

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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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Figure 4.15
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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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Section Outline

• Movement Analysis and Exercise Prescription

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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.

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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.