NSCA CPT Chapter 4 - Biomechanics
Chapter 4 - Biomechanics

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    Chapter Objectives:

    • Know the human movements with the appropriate anatomical and mechanical terminology.
    • Use mechanical concepts for human movement problems.
    • Know the factors that contribute to human strength and power.
    • Define muscle actions involved in movement tasks.
    • Give analyses for biomechanical aspects of resistance exercises.

    Mechanical Foundations

    Functional anatomy is the study of the body and how its systems cooperate to do particular tasks.

    Biomechanics is the field of study applying to mechanical principles in order to know the functions of living organisms and systems.

    Mechanics is a branch of physics dealing with the effects that forces and energies have on bodies. 

    Mechanical terminology and principles:

    • Each “body” refers to a collection of matter. This could be the whole human body or even just a limb.
    • Linear motion and angular motion are the two main types of movement.
    • Linear motion can be rectilinear, which is in a straight line, or it can be curvilinear, which will follow a curved path.
    • Angular motion will rotate around a fixed line that we will call the axis of rotation. 
    • Many of the movements that we do will follow a combination of these movement types. We call this general motion.

    There are three planes that movements o through. These are:

    • Frontal: A vertical plane that divides the body or organs into the front and back portions.
    • Sagittal: A vertical plane that divides the body or organs into their left and right portions.
    • Transverse: This horizontal plane divides the body or organs into upper and lower portions.

    Kinematics is the study of movement from a descriptive perspective without regard to underlying forces.

    There are five different primary variables to kinematics:

    • Timing, or temporal, measures.
    • Position or location.
    • Displacement.
    • Velocity.
    • Acceleration.

    Kinetics is the movement assessment with respect to forces involved.


    This is the fundamental element in human movement mechanics. It is the mechanical action or effect applied to a body of some kind that will produce acceleration typically. Internal forces are things like muscles, tendons, and ligaments. External forces are things like gravity, friction, and air resistance.

    Force effects depend on the combination of these seven force related factors:

    • Magnitude is how much force is made or applied.
    • Location is where the force is being applied.
    • Direction is where the force is directed.
    • Duration is how long a single force application is applied.
    • Frequency is how many times the force is applied within a time.
    • Variability is how much the force changes over time.
    • Rate is how quickly the force is going to apply.

    Newton’s laws of motion

    The First law of motion states that a body at rest or in motion will remain in that state unless an outside force acts on it.

    The Second law of motion states that a net force that is acting on a body makes an acceleration that is proportional to the force according to this equation: SF = m X a This essentially says that force is equal to mass times acceleration.

    The Third law of motion states that for every action, there is an equal and opposite reaction.

    Momentum and Impulse

    Momentum characterizes a body’s quantity of motion. The larger a body is and the faster a boy is moving, the more momentum it has.

    Linear momentum is calculated as mass times acceleration.

    Angular momentum is inertia times angular velocity.

    Transfer of momentum is the mechanism of momentum being transferred from one body to another. Like when a pitcher transfers momentum from the legs and lower body into the upper body to throw.

    Impulse is force multiplied by time.


    Angular motion uses the mechanical agent of torque, or moment of force.

    Torque is similar to force creating a linear acceleration, but instead it is with angular acceleration.

    Magnitude of torque is calculated by T = F X d

    The moment arm is the perpendicular distance from the fulcrum to the line of force action. 

    Torque is measured using newtons and meters.

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    For you to increase torque, you should increase the force, the moment arm, or both. 

    Lever Systems

    A lever is a rigid structure that is fixed to a single point. We call this point a fulcrum or an axis.

    There are three levers that we have in the body:

    First class lever: This is a lever where the fulcrum is located between the two forces.

    Second class lever: A second class lever has the resistive force located between the fulcrum and the muscle force.

    Third class lever: There are more third class levers in the body than there of any of the other lever types. These levers have the muscle action between the fulcrum and the resistive force.

    Mechanical advantage is a concept we run into when looking at levers. The distance of the muscles and the fulcrums and forces all play a role in affecting how much force is required to move. So, we have mechanical advantages we can create to make movements easier.


    Work in the mechanical sense is defined as the product of both force and distance of movement for an object. W = F X d

    The standard unit for work that we use is the joule (J)


    Power is the amount of work divided by the amount of time that the work was done in. So, P = w / t

    It can also be calculated by the product of force and velocity. So, P = F X v


    Mechanical energy is the typical term we use, as there are many definitions for the word “energy”.

    Mechanical energy is either kinetic or potential energy and it is defined as the ability to perform mechanical work.

    There is both linear and angular kinetic energy.

    There is both gravitational and deformational potential energy.

    Mechanical and Movement Efficiency

    Efficiency is known as how much mechanical work can be done with a given amount of energy, or metabolic input.

    In reality we really only use one quarter of the energy produced to perform the work, with the other three quarters going to heat energy or the energy recovery process.

    There are a few actions and conditions that contribute the most to inefficiencies.

    Muscle coactivation is when an antagonistic muscle is working against an agonist muscle.

    Jerky movements are when alternating changes in direction take away metabolic energy for acceleration and deceleration of limb segments.

    Extraneous movements are excessive arm movements during running that are outside of balance.

    Isometric actions are when there is no work technically done, as isometric movements don’t change where you are.

    Excessive center of gravity excursions are when metabolic energy is needed for raising and lowering the center of gravity beyond he minimum for the task.

    Biomechanics of Human Movement


    Skeletal muscles consist of 40 – 45% of the body weight for people and they perform many of the necessary functions of the body.

    Muscle tissue has four distinguishing characteristics.

    • Excitability is the ability to respond to a stimulus.
    • Contractibility is the ability to generate a pulling force known as tension also.
    • Extensibility is the ability to lengthen or stretch.
    • Elasticity is the ability to return to original lengths and shape with the removal of a force.

    Muscle Architecture

    Muscle tissue has contractile components and other noncontractile components. 

    Some muscle fibers are arranged at an angle to the line of pull. We call this angle the angle of pennation.

    Some have multiple pennation angles.

    Types of Muscle Action

    Concentric Muscle Action: This occurs when the force produced in the muscle overcomes the external resistance in the opposing direction. This is termed as positive work.

    Eccentric Muscle Action: This is the lengthening action. When the force produced by the muscle is less than the force opposing it. This is termed as negative work.

    Isometric (static) Muscle Action: This is when the muscle force is equal to the opposing force. These actions lead to no movement. There is technically no work being done in this type of muscle action as there is no displacement in location.

    The muscle action type is an important factor in producing and controlling joint motion and transferring energy between body segments during movements.

    Length – Tension

    The length – tension relationship says that force produced by the musculotendinous unit is determined by the length of the muscle. 

    Because of this, we develop an active range of motion, or a functional range of motion. 

    Force – Velocity 

    A muscle’s ability for force generation also depends on the velocity of contraction. 


    There are both intramuscular and intermuscular factors playing a role in the max force a muscle can produce.

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    Intramuscularly we can increase force three ways:

    • Increasing the firing frequency of the motor unit.
    • Increasing the number of motor units that will be recruited.
    • Recruiting progressively larger motor units.

    Intermuscularly we can increase force by increasing activation of the synergist and agonist muscle in a movement and decreasing the antagonist muscle activations.

    Strength and Sticking Points

    Strength refers to the ability for exerting force.

    Strength is expressed as the amount that can be lifted through the weakest point in the range of motion. That point is known as the sticking point.

    Muscular Control of Movement 

    Understanding the muscles that are controlling a movement and how modifying exercises will affect the muscles is key when you are selecting the appropriate exercises for a specific goal.

    Biomechanics of Resistance Exercise

    Constant Resistance Devices

    Constant resistive forces do not change throughout the whole range of motion. This is provided by free weight and some machines.

    Free weights

    This is an object that has a fixed mass and no constraints on where it can go in motion. These are commonly dumbbells, barbells, medicine balls, and a person’s own body. Free weights have the force of gravity acting on it, and that is basically all.

    To move a free weight, you must produce a force that is slightly greater than the weight.

    Forces may be constant on the weight, but not on the muscle, as the forces change as distance, length and the many other variables change throughout movement.

    We have multijoint movements, like the squat, where the path seems linear due to the bar going up and down and staying in the same line, but the forces are all angular on the joints.

    Typically, we use single and multijoint movements to complement each other.


    Machines constrain the motion of resistance in some way. The resistance path is linear or angular.

    Variable Resistance Devices

    The forces for these devices will increase, decrease, or both throughout the range of motion. 

    We see this with some machines, elastic bands, and chains.


    This machine type changes the resistance throughout the motion.

    The greatest resistance is available when the lever is parallel to the floor.

    Elastic Resistance

    Bands and tubing can be used to provide changing resistance. This happens as the band is lengthened it will add more or take off more weight.


    Chains serve a similar purpose as elastic items. The chains add weight to the typical harder part of the exercise and decrease if they are in an exercise that gets closer to the ground and some of the chains are taken out as a factor.

    Accommodating Resistance Devices

    These devices vary depending on how much force someone will be applying to it. Here we have isokinetic dynamometers, flywheels, and fluid resistance.

    Performing Exercises in the Water

    When exercising in the water, buoyant forces act upon the person in an upward direction, so opposite of gravity.

    There is also a resistive force known as ‘drag’ that is acting on the swimmers. 

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