KIN 425,  Concepts of Strength & Conditioning

Chapter 3, Basic Biomechanics

The Importance Of Specificity

-          Specificity is a major consideration when 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.

Human Musculature – Terminology

-          Origin- attachment closer to the center of the body, also known as the proximal attachment

-          Insertion- attachment is farthest from the center of the body, also known as the distal attachment

-          Fleshy attachments- muscle fibers attaching directly to the bone, often found at the proximal end of the muscle

-          Fibrous attachments- continuous with muscle sheaths and bone connective tissue, additional fibers extend into the bone itself for additional strength       example: tendons

-          Agonist- muscle most responsible for action, “prime mover”

-          Antagonist- muscle that acts to slow down or stop a movement, important for joint stability and limb braking (protects ligaments and cartilaginous joints structures from damage)

-          Synergist- muscle that assists directly in the movement

Lever Systems

-          Lever- A rigid or semi-rigid body that, when subjected to a force whose line of action does not pass directly through its pivot point, exerts a force on any object impeding its tendency to rotate

-          Lever Elements

o        Fulcrum- pivot point 

o        Moment arm- perpendicular distance from the line of action of the force to the fulcrum

o        Torque (moment)- the degree to which a force tends to rotate about the fulcrum, torque=f*moment arm 

o        Muscle force- force generated by the muscle as it contracts 

o        Resistive force- force generated by the source of resistance 

o        Mechanical advantage- ratio of moment arm of muscle force and moment arm of resistive force, MA> 1 is advantageous, MA<1 is not

-          First Class Lever

o        Lever in Which Muscle Force and Resistive Force Act on Opposite Sides of the Fulcrum

-          Second Class Lever

o        Lever in Which Muscle Force and Resistive Force Act on the Same Side of the Fulcrum With the Moment Arm Length of the Muscle Force Being Longer Than That of the Resistance Force

-          Third Class Lever

o        Lever in Which Muscular Force and Resistive Force Work on the Same Side of the Fulcrum With the Moment Arm Length of the Resistive Force Being Longer Than That of the Muscular Force

Important Points

-          Skeletal muscles operate at a considerable mechanical disadvantage. Thus, during sports and other physical activities, forces in the muscles and tendons are much higher than those exerted by the hands or feet on external objects or the ground

-          Fulcrums and mechanical advantages often vary through out the course of movements

How Does Mechanical Advantage Work?

-          When the moment arm is shorter there is less mechanical advantage; Thus if a person has tendon insertions farther from the join should be able to lift greater amounts of weight

Mechanical Advantage Application in Weight Training

-          During weightlifting, the moment arm is equal to horizontal distance from a line through the center of mass of the resistance to the body joint about which rotation occurs

-          Thus the resistive torque changes or can change throughout the movement

-          The longer the moment arm the greater the MA

Increased moment arm length and MA does have its             disadvantages

-          As the MA of a movement is increased, the speed of the movement decreases

-          This is caused by the inverse relationship between force and velocity in muscle contraction

o        This relationship states the faster a muscle contracts the less force it can produce

-          In depth explanation found on pp. 31-33

Planes of the Human Body

-          Anatomical position- body is erect with arms at the sides and palms facing out

-          Sagittal plane- divides body into left and right (standing barbell curl)

-          Frontal plane- divides body into front and back (lateral raises)

-          Transverse plane- divides body into upper and lower (DB flyes)

Strength & Power

-          Strength is the maximal force that a muscle or muscle group can generate at a specified velocity

-          Power is the measurement of the ability to exert force at higher speeds

-          Power = force x velocity and/or work/time

-          Work = force x distance

-          Manipulation of any of these factors will affect power production; (i.e., When lifting the same amount of weight over the same distance, if the speed of the movement is increased, power output will be greater)

-          Small changes in theses factors can have significant effect on power output

Biomechanical Factors in Human Strength

-          Neural control- affects the maximal force output of a muscle by determining which and how many motor units and therefore muscle fibers are recruited (contraction) and fired (rate coding)

-          More fibers recruited and faster firing = greater force production!

-          Cross sectional area- muscle force exertion is related to cross sectional area, force production increases with an increase in fiber cross sectional area, larger athletes can have the same muscular cross sectional area as smaller athletes but may appear “weaker” in movements requiring body lift and acceleration-due to the effect of bodyweight.

-          Muscle fiber arrangement- level of force production in muscles can vary according to pennation of the muscle, pennation of muscle may also have varying effects on different movement types

-          Muscle length- muscles generate maximum force at resting length, beyond or below resting length there is less chance for actin/myosin cross-bridging to occur

-          Joint angle- affects amount of torque to be overcome in movement by muscular force

-          Muscle contraction velocity- muscular force declines as contraction speed increases, more significant over the lower range of movement velocities

-          Joint angular velocity – greatest muscle force happens during eccentric contractions; Pp. 40 in book

-          Strength to mass ratio- ratio of strength of muscles involved versus the mass of the limbs being accelerated, variation of either factor will affect acceleration capability, athletes should strive to maximize this ratio

-          Body size- muscle strength is related to cross sectional area and not volume, as body size increases muscle volume grows faster than muscle strength. Therefore smaller athletes are often stronger pound for pound than larger athletes

Resistance to Muscle Contraction

-          Gravity

-          Gravitational force varies during movements which causes torque values to vary

-          Inertia- occurs when mass is accelerated-muscles, connective tissue, and joint structures need to adapt to inertia throughout movements as well, more force is required in muscle to cause or offset accelerations (inertia)

-          Friction- can affect movements and activity in which two surfaces are touching, friction is harder to overcome at rest as compared to when movement is already initiated

-          Fluid resistance- sometimes used in resistance equipment, force varies according fluid viscosity,  fluid orifice size, speed of movement

-          Elasticity-sometimes used in resistance equipment, force varies according to distance stretched, the more it is stretched the greater the resistance, material composition also is a factor

Prepared by Matt Green, Kinesiology TA