Brawn behind performance John Milton BIO-39
Brain or brawn?
Newton was a sports fan I remember that I had often seen a tennis ball struck with an oblique racket describe such a curved line. For a circular as well as progressive motion being communicated to it by that stroke, its parts on that side where the motion conspire must press and beat the contiguous air more violently, and there execute reluctancy and reaction of the air progressively greater. Sir Isaac Newton (1671)
What is a force? If something is able to keep a spring deformed, this something is called a force in the static sense. If something is able to accelerate (or decelerate) a mass, this something is called a force in a dynamic sense.
Newton s laws of motion The law of inertia: body at rest stays at rest, body moving stays moving The law of acceleration: F=ma Body weight is a force The law of impact and reaction forces: for every action there is an equal and opposite reaction
First and second laws: Overstretch injury Must be able to stop momentum once started Cheat : Initiate lift using momentum from different part of body than are training
Over-stretch injury (cont.) Most often occurs in weight lifters who are in lowering stage of lift Symptom: Acute pain Sign: injured limb cannot be maintained against gravity Very serious; long time to recover Emphasis on prevention Technique; spotters, etc
Third law: The Darkside of Golf Prevalence of injuries is 50-60% For one golfer playing, two have left the game The two hurts: hurts their feelings, hurts their bodies
Ground reaction force: locomotion How much force impacts the ground while walking and running? One body weight (BW)? NO Walking 1-2 BW Running Up to 7 BW Why? Muscular forces contribute
Convention: F net = F muscle -F load F muscle -F load > 0 Then the muscle shortens
Exercise terminology Concentric (shortening) contraction Muscle > load Eccentric (lengthening) contraction Muscle < Load Isometric contraction Muscle = load
Generators of muscle force The myofibril The whole muscle including its associated connective tissues The force generated by muscles acting across a joint
Contractile proteins Actin = thin filaments Myosin = thick filaments
Muscle contraction
MF-1: Muscles can only pull For every flexor there is an extensor, etc Push-ups: Triceps contracts, not because biceps elongates
educated elbows Co-activation Both antagonist and agonist tend to contract Controls stiffness of movement Trade-off between speed and accuracy
MF-2: Muscle-joint complex functions like a third class lever
Mechanical advantage MA = effort arm / moment arm Effort arm = E-F Moment arm = R-F First and second class levers have MA > 1, but for third class MA < 1 Why?
Advantages of third class lever Speed Distance
MF-3: Force generated by a muscle is proportional to its cross-sectional area
Muscle size changes hypertrophy Atrophy
Effect of 8-week weight training on muscle size
Effect of training of one limb on the strength of the other
Effect of imagined exercise
Motor force principles MF1: Muscles can only pull MF-2: A muscle joint complex functions like a third-class lever MF-3: The force generated by muscles is proportional to its cross-sectional area