Fundamental Biomechanical Concepts to Understand and Enhance Performance

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Victoria Harris
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1 Fundamental Biomechanical Concepts to Understand and Enhance Performance Loren Z.F. Chiu, PhD, CSCS Neuromusculoskeletal Mechanics Research Program University of Alberta Work & Energy Examine the causative role of performing mechanical work Identify strategies to increase work performed and subsequently enhance movement Improve understanding of the mechanical determinants of performance

2 Vertical Jump Point mass idealization Object has mass, but no geometry Jumper s mass represented at their center of mass (COM) VJ displacement COM Apex COM Standing Thermodynamics First law of thermodynamics Energy within a system is constant Types of energy (possessed by the human body) Mechanical Chemical (ATP) Thermal (heat) Mechanical Energies Gravitational Potential ( E P ) As an object moves away from the ground, it gains gravitational potential energy As an object moves towards the ground, it loses gravitational potential energy E P = mg h Kinetic ( E K ) As an object s velocity increases, it gains kinetic energy As an object s velocity decreases, it loses kinetic energy E K = ½m v 2 v = change in velocity h = change in height

3 Vertical Jump Gravitational Potential Energy t 3 = Apex Possess more gravitational potential energy at apex (t 3 ) than at standing (t 0 ) h E P = mg h t 0 = Standing Vertical jump displacement can be explained by the gain in gravitational potential energy Where did this mechanical energy come from? Temporal Events Time: temporal distance between events At a given event a closed system has an energetic state Types and amount of energies possessed Changes in types & amount of mechanical energies directly corresponds to the movement that occurs between events Vertical Jump Events Vertical Jump Displacement = 0.53 m (~21 inches) h 3 h 0 h 2 h 1 t 0 h = 0.94 m t 1 h = 0.58 m t 2 h = 1.03 m v = 2.92 m/s t 3 h = 1.47 m

4 Vertical Jump: Countermovement h 3 h 0 h 2 h 1 t 0 h = 0.94 m t 1 h = 0.58 m t 2 h = 1.03 m v = 2.92 m/s t 3 h = 1.47 m t 0 to t 1 : COM height decreases; lose gravitational potential energy t 0 to t 1 : COM velocity is the same; no change in kinetic energy Vertical Jump: Propulsion h 3 h 0 h 2 h 1 t 0 h = 0.94 m t 1 h = 0.58 m t 2 h = 1.03 m v = 2.92 m/s t 3 h = 1.47 m t 1 to t 2 : COM height increases; gain gravitational potential energy t 1 to t 2 : COM velocity increases; gain kinetic energy Vertical Jump: Flight h 3 h 0 h 2 h 1 t 0 h = 0.94 m t 1 h = 0.58 m t 2 h = 1.03 m v = 2.92 m/s t 3 h = 1.47 m t 2 to t 3 : COM height increases; gain gravitational potential energy t 2 to t 3 : COM velocity decreases; lose kinetic energy

5 Vertical Jump Changes in Energy Phase Changes in Energy Comments t 0 t 1 Decrease gravitational potential System loses mechanical energy t 1 t 2 Increase gravitational potential Increase kinetic t 2 t 3 Lose kinetic Increase gravitational potential System gains mechanical energy No gain/loss mechanical energy Adding/Removing Energy to a System How do we increase (or decrease) the amount of mechanical energy a system possesses? Perform mechanical work W = E P + E K Performing work can increase/decrease the amount of gravitational potential energy Performing work can increase/decrease the amount of kinetic energy How is Work Performed? Work = Force Displacement Displacement Vertical Ground Reaction Force Positive Work Add Energy Negative Work Remove Energy

6 Vertical Jump Work Performed Phase Changes in Energy Work Performed t 0 t 1 Decrease gravitational potential Negative work performed t 1 t 2 Increase gravitational potential Increase kinetic t 2 t 3 Lose kinetic Increase gravitational potential Positive work performed None Work = Power Time t 0 t 1 Positive t 2 Work Negative Work Performing Work Muscles exert force on segments resulting in force exerted on the ground Ground exerts reaction force on the body Muscles exert moments of force to rotate joints, moving the body; therefore, muscles perform work Ground exerts force on a moving body, therefore the GRF performs work proportionate to the work performed by muscles W Muscles = W GRF

7 Performing More Work W GRF = W Hip Extensors + W Knee Extensors + W Ankle Plantar Flexors + W Spine Extensors W Muscle = M θ M = Moment of force θ = Change in joint angle How can the work performed by a muscle be maximized? Exert large moments of force through an optimal range of motion What About Power? Power = Work / Time Work [t 0 t 2 ] Gain in Gravitational Potential Energy VJ Displacement Direct causative relation between work performed and VJ displacement Time required to perform work is irrelevant Two individuals (with the same body mass) who perform the same work over different time intervals will have the same VJ displacement Projectile Motion

8 Determinants of Projectile Motion Work Performed by Vertical Force v Y W = E P + E K W = mg s Y + ½m v Y 2 v X h = s Y Work Performed by Horizontal Force W = E K W = ½m v X 2 What is a Projectile? A projectile is an object in flight The only vertical force acting on a projectile is gravity A projectile has vertical velocity at release (take off) A projectile may have horizontal velocity at release (take off) A projectile is created by performing work that imparts kinetic energy to an object The Human Body as a Projectile Activity Vertical Velocity Horizontal Velocity Vertical Jump Yes No Long Jump Yes Yes Running Stride Yes Yes

9 Running Velocity Running Velocity = Stride Length * Stride Frequency Elite sprinters have a lower stride frequency and longer stride length Usain Bolt runs 100 m with 41 strides Other world class sprinters require ~44 strides Lower caliber sprinters can achieve the same or greater stride frequency as elite sprinters Stride Length Takeoff Displacement Flight Displacement Landing Displacement Stride Length = Takeoff Displacement + Flight Displacement + Landing Displacement Maximizing Stride Length Flight Displacement = Horizontal Velocity * Flight Time Horizontal velocity Increase work performed by horizontal force to increase horizontal velocity Flight time Dependent on vertical velocity Increase work performed by vertical force to increase flight time

10 Work Performed Causes Movement All movement can be described as changes in mechanical energy Work is performed to increase/decrease the mechanical energy an object possesses Citius, altius, fortius (Olympic motto) Citius Faster v [kinetic energy] Altius Higher h [gravitational potential energy] Fortius Stronger mass li ed [gravita onal poten al and kine c energy] Training Misconceptions Speed training Speed (velocity) is the result of performing sufficient work to impart kinetic energy Power training Power is the result of performing sufficient work in a relatively small time If sufficient work cannot be performed, power is irrelevant Citius, Altius, Fortius All training to improve movement performance should be directed at optimizing the ability to perform work If a training program is effective, it is effective because it optimized the work performed

11 How to Optimize Work Performed Utilize the correct muscles to perform work Utilize the optimal joint range of motion Increase joint range of motion Increase the force a muscle can generate in the desired range of motion Increase the ability for muscles to perform work repetitively What To Take Home All movement can be explained by changes in mechanical energy e.g. Vertical jumping is explained by the gain in gravitational potential energy Muscles perform work to cause changes in mechanical energy To improve performance, training should be directed at optimizing the work performed by muscles

Energy and Power in (Sports) Biomechanics. Objectives. Contents

Energy and Power in (Sports) Biomechanics Department Center of for Sport Sensory-Motor and Exercise Interaction Science SPORTSCI 306 Technique Anvendt Assessment Biomekanik Uwe Uwe Kersting Kersting MiniModule