Copyright © 2012 American College of Sports Medicine Chapter 2 Biomechanics of Force Production

Copyright © 2012 American College of Sports Medicine Introduction Biomechanics –Science of applying principles of mechanics to biological systems –Applies to: All motor skills performed in sports All training modalities –Kinetics: deals with forces that cause motion –Kinematics: description of motion

Copyright © 2012 American College of Sports Medicine Muscle Actions Concentric (CON): muscle shortening Eccentric (ECC): muscle lengthening Isometric (ISOM): no change in muscle length Isokinetic: velocity-controlled CON & ECC muscle actions

Copyright © 2012 American College of Sports Medicine Active Muscle Length-Tension Relationship

Copyright © 2012 American College of Sports Medicine Passive Muscle Length-Tension Relationship

Copyright © 2012 American College of Sports Medicine Stretch-Shortening Cycle An ECC muscle action that precedes a CON action results in a more forceful CON action. This phenomenon is known as the Stretch- Shortening Cycle (SSC). This allows the athlete to develop large force and power outputs. Depth Jumps… This consists of the Stretch Reflex (SR), allowing the muscle to store elastic energy within its series and parallel elastic components. The SR is initiated by a specific sensory receptor the Muscle Spindle which responds to both the magnitude and rate of muscle length change. The resulting affect is that the SSC can enhance performance by an average of 15-20%. It is most prominent in Type II muscle fibers. Critical to SSC performance is that the CON action follows quickly…as stored elastic energy can be lost as heat energy (which offers little to no effects). Stretching beforehand can reduce this and is not advised.

Copyright © 2012 American College of Sports Medicine Force-Velocity Relationship

Copyright © 2012 American College of Sports Medicine Muscle Architecture Nonpennate: fibers parallel to muscle’s line of pull –Longitudinal (strap): sartorius –Quadrate (quadrilateral): rhomboids –Fan-shaped (radiate, triangular): pectoralis major –Fusiform: biceps brachii –Designed for ROM and contraction velocity Pennate: fibers oblique to line of pull –Unipennate: tibialis posterior –Bipennate: rectus femoris –Multipennate: deltoid –Designed for strength & power

Copyright © 2012 American College of Sports Medicine Muscle Fascicle

Copyright © 2012 American College of Sports Medicine Muscle Architecture (cont’d)

Copyright © 2012 American College of Sports Medicine Muscle Architecture (cont’d) Muscle Fiber Arrangement –Angle of pennation Angle between fibers & central tendon Low (≤5°) High (>30°) –Muscle fascicle length

Copyright © 2012 American College of Sports Medicine Torque and Leverage Linear Motion Angular Motion Torque (aka ‘moment’) –Rotation caused by a force about a specific axis –Product of force & moment arm length Lever –Used to overcome large resistance & enhance speed & ROM –Components: fulcrum (pivot point), resistance, & force –First-, second-, & third-class levers

Copyright © 2012 American College of Sports Medicine Figure 4.2

Copyright © 2012 American College of Sports Medicine

Key Term Mechanical Advantage: The ratio of the moment arm through which an applied force acts to that through which a resistive force acts. A mechanical advantage greater than 1.0 allows the applied (muscle) force to be less than the resistive force to produce an equal amount of torque. A mechanical advantage of less than 1.0 is a disadvantage in the common sense of the term.

Copyright © 2012 American College of Sports Medicine Torque Generation at Two Angles of Force Application

Copyright © 2012 American College of Sports Medicine Three Classes of Levers

Copyright © 2012 American College of Sports Medicine Based on lever systems, it appears that the human body was designed to produce motion at higher speeds at the expense of the large force application. In other words, people are made more for speed of movement than for strength.

Copyright © 2012 American College of Sports Medicine Tendon Insertion Can favor speed (tendon close to axis) or force (tendon farther from axis). This is a GENETIC factor which does NOT change with training. Moment arms and bodily proportions.

Copyright © 2012 American College of Sports Medicine Effort Arm Changes During Elbow Flexion

Copyright © 2012 American College of Sports Medicine Ascending-Descending Strength (Torque) Curve

Copyright © 2012 American College of Sports Medicine Ascending Strength (Force) Curve

Copyright © 2012 American College of Sports Medicine Descending Strength (Force) Curve

Copyright © 2012 American College of Sports Medicine Action/Reaction Forces and Friction Action Force –Force applied to an object with the intent to accelerate, decelerate, stop, maintain, or change direction Reaction Force –Equal & opposite force in response to action force (Newton’s 3 rd law of motion) Friction –Force parallel to action & reaction forces that acts to oppose relative motion of these two surfaces –Static friction-between 2 objects not moving relative to each other and Dynamic or sliding friction-between 2 surfaces moving relative to each other resulting in sliding.

Copyright © 2012 American College of Sports Medicine Ground-Reaction Force Curves

Copyright © 2012 American College of Sports Medicine Stability Ability of an object to resist changes in equilibrium Principles of stability –Greater stability is seen when: Center of gravity (COG) is lower Line of gravity is aligned equidistantly within base support Base support is wide Objects with larger mass Level of friction is greater –Stability decreases when external loading is applied to upper body

Copyright © 2012 American College of Sports Medicine Mass and Inertia Mass –The amount of matter an object takes up Inertia –Resistance of an object to changing its motion In Linear Motion: –Greater mass & greater inertia = greater stability In Angular Motion: –Distribution of mass is critical –Moment of inertia: property of an object to resist changes in angular motion; a product of object’s mass & mass distribution

Copyright © 2012 American College of Sports Medicine Momentum and Impulse Linear impulse (F × T) = linear momentum (m × ∆v) –So, F × T = m × ∆v –Increasing force and/or time increases impulse –Increasing mass and/or velocity increases momentum Angular impulse = torque × time Angular momentum = joint angular velocity × moment of inertia –Maximizing angular momentum necessitates optimal combination of angular velocity & moment of inertia

Copyright © 2012 American College of Sports Medicine Body Size Larger the body size, the larger the force potential –Relative to muscle mass –Positive relationship between muscle mass & absolute force production As body size increases, body mass increases to a greater extent than muscle strength

Copyright © 2012 American College of Sports Medicine Other Kinetic Factors in S&C Intra-Abdominal Pressure (IAP) –Pressure developed within abdominal cavity during contraction –Pushes against spine & helps keep torso upright –Prevents lower-back injuries –Increased by: Abdominal contraction & subsequent trunk muscle training Breath holding Lifting belts Wraps Bench press shirts, Lifting suits

Copyright © 2012 American College of Sports Medicine Intra-Abdominal Pressure

Copyright © 2012 American College of Sports Medicine Other Kinetic Factors in S&C (cont’d) Lifting Accessories –Lifting belts –Wraps –Bench press shirts –Lifting suits