CSCS Chapter 2: Biomechanics of Resistance Exercise

CSCS Study Guide Chapter 2

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Chapter Goals

  1. Major components of skeletal musculature will be identified
  2. Describe the various types of levers in the musculoskeletal system.
  3. Recognize the main anatomical movements that occur in sports and exercise.
  4. Determine linear/rotational work and power.
  5. Explain factors of strength and power.
  6. Find the important factors of biomechanics for joints.

The Musculoskeletal System

  • Skeletal Musculature
    • The skeletal muscles cause movements in order to generate force against objects
    • There is always an origin and insertion attachment for the muscle.
    • Agonist muscles are responsible for bringing about movement. They are the prime movers.
    • Antagonist muscles slow down or stop the movement. They are the opposing muscle.
    • Synergist muscles indirectly assist the prime movers.
  • The Levers of the Musculoskeletal System
    • Levers are rigid or semirigid bodies. When they are given a force that doesn’t pass its pivot point, it exerts force on objects stopping rotation.
    • 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. 
      • Mechanical advantage is a ratio of the moment arm that an applied force acts compared to one through which resistive force acts. Any mechanical advantage that is greater than 1.0 will allow the muscle force to be less than resistive force. This produces an equal amount of torque. Any mechanical advantage smaller than 1.0 is a disadvantage.
      • First class levers: A lever like the Triceps, where muscle force and resistive forces act on opposite sides of the fulcrum.
      • Second class levers: A lever like the Calves, where the muscle forces and resistive forces act on the same side of the fulcrum. The muscle force acts through a longer moment arm than the resistance force is acting. The mechanical advantage means the muscle force required is less than the resistance.
      • Third class lever: A lever like the biceps, where the muscle and resistive forces act on the same sides as the fulcrum. The muscle force that acts through the moment arm is shorter than the resistive force. The mechanical advantage is smaller than one, thus it is a disadvantage.

The Patella and its Mechanical Advantage

  • The patella is responsible for increasing mechanical advantage of the quads by keeping the quad tendon father from the knee’s axis of rotation. 
  • If the paella was absent, the tendon would be closer to the knee’s rotation, and that would reduce the advantage.

Moment Arm and Mechanical Advantage

  • During flexion of the elbow with the biceps, the distance from the joint’s rotational axis varies throughout the movement.
  • There is a mechanical advantage when the moment arm is shorter.
  • As weights are lifted, the moment arm changes horizontal distance from the weight to the elbow.

It Is important to know that most of the skeletal muscles in our body operate at a mechanical disadvantage. 

Variations in Tendon Insertion

  • The tendon insertion is the point at which tendons are inserted into the bones.
  • An insertion that is a greater distance from the center of the joint results in a better lifting ability.
    • Max speed is reduced.
    • Force capability during faster movements is reduced.

The Anatomical Planes of the Human Body

  • The standard position has the body erect, arms to the side with the palms facing forward. 
  • Sagittal plane: this places the body in left and right sections.
  • Frontal plane: this puts the body into front and back sections.
  • Transverse plane: this puts the body in upper and lower halves.

Human Strength and Power

  • Strength is the capacity to exert force at a certain speed.
  • Force is equal to Mass times Acceleration. It is also measured in Newtons.
  • Power is defined as the rate of work. 
    • Power is equal to work divided by time. Time is measured in seconds. Power is measured in watts.
  • Work is defined as the product of force put on an object and the distance at which it moves in the direction of the force.
    • Work is equal to Force times Displacement. Distance is measured in meters. Work is measured in Joules.
  • Negative Work and Power
    • This occurs during the eccentric portions of moves. Lowering a weight during a bicep curl is an eccentric bicep contraction. 
    • Negative work is referring to work performed on a muscle, not by a muscle. 
  • Angular Work and Power
    • Work and power is required in order for an object to rotate around an axis or to change the velocity of rotation. 
    • Torque is the degree at which a force will rotate an object around a fulcrum. This is called a moment.
    • Rotational work is equal to torque multiplied by angular displacement.
  • Strength vs. Power
    • Strength is oftentimes associated with slower speeds and power is associated with higher velocities of movement. Both are actually a reflection of the ability to exert force and velocity.
    • Olympic weightlifting has a higher component of power than that of powerlifting. This is because of the higher velocities of movement.
  • Biomechanical Factors in Human Strength
    • Neural control: The muscle force is greater when there are more motor units used in contractions, when the motor units are larger, and also when the firing rate of contractions is greater.
    • Muscle cross sectional area: The force of a muscle is related to this instead of volume.
    • Arrangement of the muscle fibers: Sarcomere arrangements vary in relation to the long axes of muscles. Pennate muscles are muscles with fibers aligning obliquely with tendons, creating arrangements looking like feathers. Angle of pennation is where the the muscle fibers and the imaginary line between insertion and origin are measured. A zero degree shows no pennation. 
    • Muscle Length: When resting, actin and myosin lie next to each other. The maximum number of possible cross bridges are available. The muscle is able to generate the greatest force during the length.
    • When the muscle is stretched, there are less myosin and actin next to each other, there are fewer available cross bridges, and the muscle generates significantly less force.
    • When the muscle is contracted, the myofilament overlap each other, there are fewer cross bridges, and there is less ability to generated force.
    • Join Angle: Torque depends on force vs length of the muscle, leverage, the types of exercise, the muscles used in that joint, the speed the muscle contracts at, and the body joint. 
    • Muscle Contraction Velocity: This is nonlinear. The ability to produce force is less as the velocity we are contacting increases.
    • Joint angular Velocity:
      • Concentric actions are muscle actions that shortens the muscle that is working. The muscle is stronger than the resistive force.
      • Eccentric actions are actions that involve the active muscle lengthening because the force contracting the muscle is less than that of the resisting force.
      • Isometric actions are muscle actions that have the muscle length remaining constant. The contracting force is equal to that of the resistive force. No movement occurs.
    • Strength to Mass Ratio: In sports like sprinting, this ratio is used to reflect the ability for an athlete to accelerate his body. In sports with rate classifications, it is used to determine the highest strength relative to the weight of the person.
    • Body Size: With an increase in body size, we see an increase in body mass unequal to the increase in strength. Smaller athletes have a higher strength to mass ration than larger ones.

Sources of Resistance to Muscle Contraction

  • Gravity
    • Applications to resistance training.
      • Less resistive torque is exerted when weights are closer to the joints horizontally.
      • More resistive torque is exerted when weights are farther from the joints.
    • Weight stack machines
      • The resistance source is gravity, but machines increase the control of the direction and resistance pattern.
  • Inertia
    • When weights are held in static positions or moved in unchanging velocities, constant resistance is exerted downward.
  • Friction
    • This resistive force is found when someone attempts moving an object that is against another.
  • Fluid resistance
    • This resistive force is found when objects move through some type of fluid, or if they past/around objects.
  • Elasticity
    • The more stretched some elastic thing is, the more resistance it has.

Joint Biomechanics: Concerns in Resistance Training

  • Injury risk is low for resistance training.
  • The Back
    • Back injury: The lower back is more vulnerable to injury. The lower back needs to remain in an arched position to properly perform movements.
    • Intra-abdominal pressure and lifting belts: The Valsalva maneuver is when the glottis is closed. This keeps the air from expiring and contracts the abdominal and rib cage muscles. We create a rigid compartment of air in the upper torso and fluid within the lower torso. Weightlifting belts are used to create more pressure in the abdomen. 
  • The Shoulders
    • The shoulders are particularly prone to injuries during weight training because of their structure and the forces they are subjected to.
    • It is very important that you warm up with light weights first. 
    • You must exercise the shoulders through a balanced program.
    • Control the speed of movement well.
  • The Knees
    • The knee’s location between two long levers of the body makes it prone to injury. We must minimize how much we rely on wraps for the knees.
  • The Elbows and the Wrists
    • Overhead lifting is the biggest concern relating to these two places. 
  • Reducing Resistance training Injury Risk
    • Performing warm-up sets with lighter weights is always recommended. 
    • Full range of motion is required for optimal muscle development.
    • Don’t ignore pain you feel in your joints.
    • Max load lifting without proper prep should never happen. 
    • Plyometric drills require extra care in training programs.

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