Body Balance and Stability Control Balance and Equilibrium used synonomously "System in BALANCE" "System in EQUILIBRIUM" Equilibrium System not changing speed or direction STATIC Equilibrium vs. DYNAMIC Equilibrium BALANCE Some control over movement (i.e. equilibrium is controlled for a given purpose) ex. control equilibrium = ability to figure skate no control = awkward/clumsy person Balance implies the concept of STABILITY STABILITY refers to the amount of disturbance a system can accept and still remain in equilibrium ex. take much disturbance = stable, not = not Linear and Rotary Equilibrium Bodies move in LINEAR and ROTARY paths Experiences equilibrium in each of these two paths For linear motion: occurs when no NET external force exists For rotary motion: occurs when no NET external TORQUE exists Otherwords, both the torques and the forces EQUAL ZERO Text examples: Total equilibrium student sleeping in class Downward force vs. reaction force, no motion Rotary equilibrium but not linear equilibrium diver in a pike position rotation is constant but accelerating downward Linear equilibrium but not rotary equilibrium Ice dancer performing "axle?" (rotate about longitudal axis with foot on ground) ice slows foot so no rotary eq., no linear motion so eq (no change in linear motion) Linear and rotary stability Stability = resistance to influence of outside forces/torques to change equilibrium Both linear and rotary stability exist Body can be linearly stable and not rotationally, or visa versa Linear Stability Some activities is desireable for linear stability rowing, canoeing, swimming, cycling Some activities it is not surfing, skateboarding, skiing * Concepts * The greater the mass (or inertia), the greater the stability i.e. takes a larger force to disrupt the path The greater the friction, the greater the stability works independant of mass/inertia Rotary Stability Is the more common mode of stability in HUMAN MOVEMENT Rotary balance/stability balance beam, handstands, stepping into a canoe Refers to the ability of the body to be rotated about some point (axis) To determine stability: if the CG passes through the base, is stable Example: 2*4 Block, On its face is stable On the side is not WHY ?? 1) center of gravity is higher 2) base of support is lower Thus, exists a relationsip between - height of CG and width of base Types of stability Stable, unstable, and neutral Stable is the block on the table (large force to move it) Unstable is the block in a tipped postion (any force will move it) Neutral is a ball on the table (any force will move it, but no tipping occurs) * Overhead on Slipping * - show forces and no motion - increase weight and no increase in friction motion occurs downhill How are Humans different ? Are not rigid bodies. If CG moves out of Base of Support, can move body parts to stabilize Base of support includes any STABLE objects we come into contact with ground, wall, etc. Walking: throw weight forward then change base of support Begining walkers, must have enough neuromuscular control over balance Standing (or any other static activities) are not "truly" static instead, require minute muscular adjustments See many toddlers tipping due to overcompensation Alcohol (effect neuromuscular control - maintaining CG) (And standing is hard enough, but throwing the weight forward and then dynamically catching it?) Many times see them go faster and faster then crash Legs can't keep up with changes in CG Where do we then want CG Walking - slightly forward of hips Running - further forward Sliding into a base - behind How about when APPLYING or RECIEVING Forces Consideration of the objects reaction force must be considered Generally: The body's base of support should be enlarged horizontally in the direction of the horizontal force to be given or received. This applies when force has a horizontal component if not - adjust your CG over the object and lift with your legs (if at all possible) Closer look at general rule: forward push = front to back stride CG stays within base of support longer can then step forward sideways push = feet placed laterally same rationale Works for THROWING, STRIKING, and CATCHING as CG leads the way and the foot creates a new base of support Pushing or Pulling of Objects The main concern is the amount of friction If friction is low: should be able to push or pull If friction is high: pull is more efficient 1) friction directly proportional to Weight/Reaction Forces can decrease the weight by upward pull and, change the angle of pull to further decrease friction 2) * overhead on friction * push results in decrease of friction on foot pull actually increases friction Point of application high induces tipping lower induces slipping Line of action low point of application and high line of action = tip opposite = tip Lifting or carrying any object lifted becomes part of the mass may therefore inhibit or increase the liklihood of tipping/slipping Changing speed or direction - skills analysis includes both linear and rotary stability Changing speed- Start: push CG beyond base of support move legs forward to avoid falling Running: must now consider momentum (mv) Stopping: retract CG behind weight muscle resistance overcomes momentum in several steps (same thing seen in long jump when body falls forward with legs in front) Changing direction More common than starting or stopping Occurs with or without forward motion In forward motion: small changes are made by lateral forces by the lower extremities When speeds increase: Centrifugal force increases, causing the need to lean into the turn Without forward motion: Side to side motions occur, or rolling/tipping occurs