CSCS Study Guide Chapter 2: Biomechanics of Resistance Exercise
Chapter 2 of the free CSCS exam study guide I'm writing to help myself and others become a better personal fitness trainer.
This chapter focuses on the mechanical laws that movements follow. It builds on Chapter 1: Structure and Function of Body Systems, and provides perspective into how the body moves placing different stresses on those structures. The flow is from an overview of skeletal musculature, to primary movement patterns, to biomechanical principles, to forms of resistance and finally joint mechanics.
Italic terms are Key Terms that the book has highlighted.
- Figure 2.1
- Figure 2.2
- Figure 2.3
- Figure 2.4
- Figure 2.6
- Figure 2.7
- Figure 2.8
- Figure 2.9
Biomechanics of Resistance Exercise
- Biomechanics-the study of how the body exerts force to produce movement.
- Origin-the point where muscle is attached. Also, traditionally the proximal end.
- Proximal-closer to the trunk (body).
- Insertion-the distal end of muscle traditionally.
- Distal-further from the trunk (body).
- Fleshy Attachments-direct attachment of muscle to bone. Wide area so force is distributed.
- Fibrous Attachments-blended attachment to bone with connective tissue.
- Tendons-strong flexible tissue that attaches muscle to bone.
- Agonist-the main muscle involved in a movement.
- Antagonist-the muscle that can slow down or stop the movement.
- Cartilaginous Joint-joints connected by cartilage, allow movement between bones.
- Synergist-muscles that assist indirectly in a movement.
- First-Class Lever-a lever where muscle force and resistive force act on opposite sides of a fulcrum.
- Fulcrum-a levers pivot point.
- Lever-a rigid structure (like bone) that is used to move a load when pressure is applied to the other end.
- Mechanical Advantage-when muscle force must be less than a ratio of 1.0 to move a load.
- Most muscles in the body operate at a mechanical disadvantage. That’s why force inside the body is higher than forces exerted outside.
- Moment Arm-also called a lever arm, the shortest distance between the joint axis and the line of force acting on that joint.
- Muscle Force-force generated by muscle.
- Resistive Force-the force that’s being applied to the body (ex. a weight, gravity, friction).
- Second-Class Lever-a lever where muscle force and resistive force act on the same side of the fulcrum. Mechanical advantage. Longer moment arm than force arm.
- Third-Class Lever-a lever where muscle force and resistive force act on the same side of a fulcrum. Mechanical disadvantage. Longer force arm than moment arm.
- Torque-twisting force that tends to cause rotation.
- Classifying levers depends on an arbitrary decision around where the fulcrum is so it’s more important to understand the principle than to classify all the levers.
- Mechanical advantages change during movements.
- There are variations in tendon insertion amongst individuals, this changes mechanical advantages amongst individuals. This explains why some people can naturally lift heavier weights than others with the same relative size and muscle.
- The closer a tendon insertion, the stronger the person but there is a tradeoff in speed because the muscle contracts over a longer distance.
Anatomical Planes and Major Body Movements
- Anatomical Position-a standard body position. Standing, feet forward, legs slightly apart, arms slightly spread from sides with palms facing forward.
- Sagittal Plane-divides the body into left-right. Movements in front and behind the mid-line like bicep curls or triceps extensions.
- Frontal Plane-divides the body into front-back. Movements directly to the sides or at the mid-line like lateral raises or hip abductions.
- Transverse Plane-divides the body into upper-lower or top-bottom. Movements that involve rotation like rotating your head or bending over and performing reverse flyes occur here.
Human Strength and Power
- Many authors use different language to define power and strength. One that I like in particular was shown to me in one of my favorite books, Periodization: Theory and Methodology of Training by Tudor Bompa and Gregory Haff, that was low-speed strength and high-speed strength. I find it less confusing. To think of strength and power this way. Both are exerting force at different velocities.
- Strength-the ability to exert force.
- Acceleration-change in velocity per unit of time.
- Power-the time rate of doing work or exerting force.
- Work-the result of force exerted on an object and the distance it moves in the direction which force is exerted.
- Know the definition of work. Know the definition of velocity.
- Negative work is work performed on a muscle rather than by a muscle ex. Eccentric part of exercises.
- Weight-mass times local acceleration (gravity).
- Angular Displacement-the angle through which an object rotates.
- Angular Velocity-rotational speed.
- Rotational Work-work done in rotating an object.
- Rotational Power-time rate of doing rotational work.
- Because they’re directly related if you know two out of three on power, force or velocity, you can calculate the third.
- Recruitment-how many muscle fibers are used in a movement.
- Rate Coding-the rate that the muscle fiber neurons fire at.
- Force muscle can exert is related to its cross-sectional area.
- Pennate Muscle-fibers that align obliquely with the tendon creating a feather like appearance.
- Angle of Pennation-the angle between the muscle fibers and an imaginary line between origin and insertion ends of a muscle.
- Muscle action is the same thing as contraction but, contraction isn’t used because it also means shortening.
- Concentric Muscle Action-muscle shortens because the contractile force to shorten the muscle is greater than resistive force to lengthen it.
- Eccentric Muscle Action-muscle lengthens because the contractile force to shorten the muscle is lesser than the resistive force to lengthen it.
- Isometric Muscle Action-muscle length does not change because the contractile force to lengthen the muscle is the same as the force to lengthen it.
- Classic Formula-load lifted divided by body weight to the two-thirds power. Know this formula. Used to compare strength between athletes.
Sources of Resistance to Muscle Contraction
- Gravity-downward force from the pull of the earth
- Electronic or calibrated spring loaded scales show actual weight.
- Exercise technique can shift stress to other muscle groups when altered.
- Inertial Force-a tendency to remain moving in the same place or a given direction.
- Bracketing Technique-performing a sport movement with less than and greater than normal resistance.
- Friction-resistive force against an objects movement when pressed against another object.
- Fluid Resistance-resistive force encountered when an object moves through fluid.
- Surface Drag-one of the two sources of fluid resistance. Friction from passing along the surface of an object.
- Form Drag-the other source of fluid resistance. Friction from the way in which fluid passes over an object due to its shape.
- Fluid resistance machines do not provide constant resistance. It is easier at first and then movement is slowed by the amount of liquid that can pass through.
- Elastic resistance is not typically best because it provides the least resistance at the beginning of a movement and the most at the end. This is typically when most movements at the end range of motion and their weakest.
Joint Biomechanics: Concerns in Resistance Training
- Exercise is risky, like any activity but, substantially less risky compared to other sports.
- Back injuries are more prone to happen because the back operates at an extremely low mechanical advantage.
- Lordotic-arched back, opposite of normal.
- Vertebral Column-the spine.
- Ventral-towards the front. Abdominal.
- Dorsal-towards the back.
- Valsava Maneuver-holding the breath and contracting the ribs and abdominals to attempt to create more stability.
- Weight lifting belts may also be used to create more internal pressure but, you’re cautioned to avoid using them regularly as they will ultimately limit the stimulus the muscles that create that pressure naturally receive.
- A belt is not needed for exercises that do not directly stress the lower back.
- You may use a belt on only your heavy sets. You may choose to never use a belt.
- The shoulder has the greatest range of motion of any joint in the body. That also contributes to it vulnerability.
- Tendinitis is a result of too much volume and intensity without appropriate progression.