CSCS Chapter 2: Biomechanics of Resistance Exercise

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Chapter Goals

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

The Musculoskeletal System

Skeletal Musculature

The skeletal muscles cause movements to generate force against objects.

There is always an origin and insertion attachment for the muscle.

Agonists’ 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 a 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 the 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 the 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 acting on opposite sides of the fulcrum.

Second class levers: A lever like the Calves, where muscle 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 the increased mechanical advantage of the quads by keeping the quad tendon farther from the knee’s axis of rotation. 

If the paella were absent, the tendon would be closer to the knee’s rotation, reducing 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 the arm changes the horizontal distance from the weight to the elbow.

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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, and 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 performed on a muscle, not by a muscle. 

Angular Work and Power

Work and power are required for an object to rotate around an axis or to change the velocity of rotation. 

Torque is the degree to 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 often associated with slower speeds, and power is associated with higher movement velocity. Both are actually a reflection of the ability to exert force and velocity.

Olympic weightlifting has a higher component of power than that powerlifting. This is because of the higher velocities of movement.

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Biomechanical Factors in Human Strength

Neural control: The muscle force is greater when more motor units are used in contractions when the motor units are larger, and 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 varies about the muscles’ long axes. Pennate muscles are muscles with fibers aligning obliquely with tendons, creating arrangements looking like feathers. The angle of pennation is where 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 can 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 overlaps each other; there are fewer cross bridges and less ability to generate 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, and the body joint. 

Muscle Contraction Velocity: This is nonlinear. The ability to produce force decreases as the velocity we contact increases.

Joint angular Velocity:

Concentric actions are muscle actions that shorten the muscle that is working. The muscle is stronger than the resistive force.

Eccentric actions involve 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 of an athlete to accelerate his body. In sports with rate classifications, it determines the highest strength relative to the person’s weight.

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 ratio 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 to move an object that is against another.

Fluid resistance

This resistive force is found when objects move through fluid or pass/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 arched to perform movements properly.

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 air compartment 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.

You must 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. 

A full range of motion is required for optimal muscle development.

Don’t ignore the 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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