1 BIOMECHANICS Biomechanics - the application of mechanical laws to living structures, specifically to the locomotor system of the human body. I. Uses of Biomechanical Analyses Improvement of sports skill techniques Design of sports equipment Prevention of injuries Clinical analysis of movement pathologies Design of prostheses Design of rehabilitation devices Qualitative analysis - a non-numerical description of a movement based on direct observation. Conducted primarily by teachers and coaches. Quantitative analysis - a movement is analyzed numerically based on measurements from data collected during the performance of the movement. Conducted by researchers. II. Levers of the Human Body Refer to Unit 13 in Lab Manual. Lever a rigid bar that turns about an axis in the body, the bones represent the bars and the joints are the axes. Force point the exact point where the effort is applied Resistance point the exact point on which the resistance acts Fulcrum the axis of motion
2 Force arm the perpendicular distance from the fulcrum to the line of action of the force acting on the force point Resistance arm the perpendicular distance from the fulcrum to the line of action of the resistance acting on the resistance point First-class its fulcrum at some location between the force point and the resistance point [ex. Is a teeter-totter] Second-class have their resistance point at some location between the force point and the fulcrum [ex. Wheel barrow, swinging door] Third-class levers has its force point at some location between the resistance point and the fulcrum [Most common in the body since it permits the muscle to be inserted near the joint and to produce distance and speed of movement although at a sacrifice of force. Ex. Shoveling, swinging a golf club] Mechanical advantage - Force level Speed lever Mass the quantity of matter contained in an object. Units = kilograms [kg] Force Mass X acceleration. Units = Newtons (N) 1 N = (1 kg) (1 m/s 2 ) Weight the amount of gravitational force exerted on a body Weight = mass X acceleration of gravity = ma g Acceleration of gravity = 9.81 m/s 2 Units of weight _Newtons (N) If a person has a mass of 80 kg, his weight = (80 kg) (9.81 m/s 2 ) = 785 N Torque the product of force and the perpendicular distance from the force s line of action to the axis of rotation. It may be thought of as rotary force.
3 Torque = Force (N) x Moment arm (m) Units = Newton-meters (N m) Moment arm the perpendicular distance between the force s line of action and the axis of rotation. The Principle of Levers -This principle states that a lever of any class will balance when the produce of the force times the distance from the point of application of the force to the fulcrum is equal to the resistance times the distance from the point of application of the resistance to the fulcrum or axis. Force x force arm = Resistance x Resistance Arm F x FA = R x RA Example: Given Elbow flexed at 90 degrees 10-kg mass held in the hand Assume that: Angle of muscle pull is 90 degrees [not true] Then: FA = 5cm [i.e biceps muscle inserts 5cm from the joint axis] RA =35cm Question: How much force does the biceps muscle have to exert to hold the weight? Solution: [TIP Remember to convert the units of measure!!!!] F x FA = R x RA [You can use 10 instead of 9.81] F x 5cm = (10kg x 9.81) x 0.35m F = 70 x 9.81N F = 687 N Levers of the human body Conversely, when the resistance arm is longer than the force arm, the lever gfavours speed and range of motion at the sacrifice of force nad is called a speed lever this is the most common lever in the human body
4 Mechanical advantage = force arm/resistance arm -Is the mechanical advantage of a first class lever greater than, less than or equal to one? IT COULD BE ANY!! This is because of the axis or rotation. RA and FA could be exactly the same or it could be closer to either the FA or RA. -For a second class lever The 2 nd class lever is always MA > 1 because it s always a force lever 3 rd class MA<1 and is ALWAYS a speed lever because the mechanical advantage is always less than one. Mechanical advantage of a lever the ratio of force arm length to reisistance arm length Volume the amount of space that a body occupies Pressure force distributed over a given area. Units = N/cm 2 Pressure = F/A Compression pressing or squeezing force directly _axially through a body Tension pulling or stretching force directly axially through a body Shear force directed parallel to a _surface_ Mechanical stress = F/A Similar to pressure. Is it better to be stepped on by a woman wearing a spike heel or a woman
5 wearing a smooth-soled athletic shoe? Given: -Woman s mass = 50kg -the spike heel area (As) =5cm^2 -the athletic shoe area (Aa)=100cm^2 Solution: Formulas -Weight (fg) =mag (assume ag=10m/s^2) -Pressure (P) =Force/Area Woman weight = 50kg x 10m/s^2 = 500N The spile heel pressure = Fg/As = 500N/5CM^2 = 100N/cm^2 The athletic shoe pressure = Fg/Aa = 500M/100cm^2 = 5N/cm^2 Comparison = 20 times more pressure with high heel Lifting A Heavy Object From the Floor 1. If the object is very heavy, get someone to _help you. Don't be a "hero". Use techniques that minimize the actual weight of the load being handled. 2. Stand facing the object with your feet _flat_ on the floor, at shoulder _width, and pointing straight _ahead_. Ensure that you have a stable base of support so that you don t slip as you are lifting the load. 3. Face the object in the direction which you intend to move with it, so that you don't have to turn while holding the object. Avoid twisting and the simultaneous generation of high twisting torques. 4. Keep the object as _close to your body as is convenient to minimize the reaction torque on the low back. When 40cm away, Tobject = 0.4m X 100N = 40NM [Newton meters] When 20cm away, Tobject = 0.2 X 100N =10NM 5. Get a good _grip_ on the object so that you don t lose control of it as you are lifting it.
6 6. Bend at the knees and hips, and keep your back as straight_ as possible. Avoid a fully flexed or bent spine. 7. Lift the object using the knee and hip extensor muscles, not by pulling upwards with the arms and back. 8. Carry the object close to your center of gravity. The flat back lifting posture has been found to be better overall than a rounded back in minimizing _L5/S1 disc compressive force and ligament strain. Avoiding full flexion of the trunk ensures a lower shear load on the vertebrae and significantly lowers the probability of ligament damage. The probability of disc herniation is increased by repeated or prolonged full flexion of the trunk. One should never move immediately to a heavy lifting task from a stooped posture or after prolonged sitting. _Standing for a brief period, and even consciously _extending the trunk, will prepare the disc and posterior passive tissues to reduce risk of injury. Contraction of the abdominal muscles aids in supporting the vertebral column during lifting. Such support may significantly reduce both the forces required by the erector spinae muscles to perform a lift and the associated disc compressive forces. Avoid lifting or spine bending shortly after rising from _bed_. Forward bending stresses on the disc and ligaments are higher after rising from _bed_, compared with one to two hours later, causing discs to become injured at lower levels of load and degree of bending. III. Center of Gravity (CG) The center of gravity in human body is an imaginary point in the center of the body where the weight of the body is balanced Definition of center of gravity refer to Lab Manual, page 14-1.
7 For the human body in anatomical position, the CG is approximately 5 cm anterior to the second _sacral vertebra, or 6 cm below the _belly button. On average, it is slightly higher in males than in females 57% versus 55% of height. The exact location of the center of gravity varies from person to person depending on body _proportions. The CG is influenced by changing body position or limb positions. The addition of external weight, such as a backpack, will relocate the CG. Segmentally each body area contains its own CG. A. Why Is It Useful to Determine CG? 1. Used to describe the movement of the body through _space 2. Important for stability 3. It is an important factor in calculation of amount of work done. B. Location of Center of Gravity 1. Reaction board method - used for a _static_ position of the human body. Assume that the center of gravity is the fulcrum or balance point and then apply the Principle of Levers. To measure in the sagittal plane it will consist of the subject standing on a board facing forward In frontal plane position the subject on the board facing to either side. ---This method is good because it is easy, inexpensive but the subject must be motionless. 2. _Segmental method - can be used for locating CG of a body in motion. [Expensive, complicated and difficult to do]
8 C. Balance and Stability For balance to be maintained in any stationary position, the CG must remain over the base_ of support. Whenever the CG passes outside the base of support, the body is off balance in that direction. If a heavy object is carried close to the body's CG, there will be _less likelihood of a loss of balance. Stability - firmness of balance - can be increased by: 1. Increasing body mass 2. Increasing the size of the base of support in the direction of the line of action of an external force 3. Vertically positioning the CG as _low as possible 4. Increasing friction between the body and the surface contacted 5. Horizontally positioning the CG near the edge of the base of support towards the oncoming external force IV. Newton s Laws of Motion First Law - Law of Inertia - A body will maintain a state of rest or constant velocity unless acted on by an external force that changes the state. The amount of inertia a body possesses is directly proportional to its mass. Second Law - Law of Acceleration - force equals mass X acceleration force in N [newton], mass in KG and acceleration in m/s^2 Third Law - Law of _Reaction - When one body exerts a force on a second body, the second body exerts a reaction force that is equal in magnitude and _opposite in direction on the first body. Sample question? How much force must be applied by a hgolf clkub to give a stationaly 0.10 kg ball an acceleration of 40m/s^2?
9 Given: Mass = 0.10KG Acceleration = 40.0 m/s^2 Looking for force Formula: F = ma = (.10kg)(40m/s^2)=4.0 A high jumper with a body weight of 750 N exerts a force of 3500 N against the ground during take off. How much force is exerted by the ground on the high jumper? Given : Jumper weight = 750 N Force exerted = 3500 N Because the jumper weights 750 and exerts 3500 N then according to Newton s law of motion, the ground exerts the same amount in the opposite direction Therefore, 4250 N is exerted by the ground on the high jumper. Momentum A mechanical quantity that is important in situations involving collisions. Momentum = mass X velocity If two football players are running at each other with player A being m=90, v=6m/s^2 and player B is m=80kg and v = 7m/s As player B has more momentum than player A, player B will push player A backwards when they collide. V. Work and PowerRelationships Work = force X distance Units of work - 1.0 Nm = 1.0 J (joule) Power = work per unit of time = Fd t = force X velocity Units of power = watts - 1.0 W = 1.0 J/sec. A force of 20N pushing an object 5m in the direction of the force. How much work is done?
10 Given: F = 20N, D=5m Work = Fxd = 20N X5M = 100nm or 100J Given: work 100 J, time = 2s Power = work/time=100j /2s=50W If you do 100 joules of work in two seconds, how much power is used? VI. Walking Versus Running Differences between walking and running: 1. In running there is a period when both feet are off the ground. Consider running as a series of jumps. 2. In running, there is no period when both feet are in contact with the ground at the same time 3. In running, the stance phase is a much smaller portion of the total gait cycle than in walking. Running speed = stride length X stride rate Length of stride is dependent primarily upon leg _length and the power of the stride. Leg speed (frequency) is mostly dependent on speed of muscle contraction and neuromuscular coordination (skill) in running. Running mechanics vary from person to person and they vary in the same person running at different speeds. At slow running speeds, complete foot contact is used. As running speed increases, the amount of foot contact becomes less. At slower running speeds, runners tend to run more erectly, whereas at full speed, the typical sprinter leans forward at about 15 degrees from the perpendicular.