Biomechanics of Resistance Exercise

1. Biomechanics of Resistance Exercise

2. Biomechanics • Biomechanics is the study of body movement • The body's mechanics are muscles, bones, tendons and ligaments • Makes use of the laws of physics and engineering concepts

3. Force • Defined as any interaction, a push or pull, between two objects that can cause an object to accelerate or decelerate • Forces are characterized by magnitude (how much) and direction (which way are they moving) • For example a push on the ground generated by knee extension and hip extension may cause the body to accelerate upward-jump

4. Characteristics of a Force • Point of Application: • Specific point at which the force is applied to an object • Insertion of the muscle • Line of Action: • Direction of the force • A straight line of infinite length

5. Types of Forces • Gravity • Inertia • Friction • Tension • Compression • Shear • Torsion • Torque • Muscle Force • Ground Reaction Force

6. Gravity • Downward force on an object • Equal to the object’s mass multiplied by motion • Mass is the amount of matter that constitutes an object and expressed in kilograms • Mass is constant, regardless of where it is measured (earth or on the moon)

7. Gravity • The pound is a unit of force not mass • The mass of a BB stays constant while its weight varies according to motion • The kilogram on a weight refers to its mass • It is incorrect to say that an object weighs a certain number of kilograms (weight relates to force not mass) • The mass of the BB is 45kg

8. Center of Gravity • The CG (center of mass) of a body refers to its balance point   • The CG is that point where all of the weight of the body is concentrated  (CG over BOS) • Anterior to S2 when standing

9. Center of Gravity • Changes when body is in motion • Crouching, kneeling, or a sitting position will lower the center of gravity and increase stability.  • A wrestler and defensive lineman will increase stability by lowering their CG

10. Center of Gravity • Narrow base of support is unstable • When lifting or carrying, keep objects COG close to yours • Lifting objects far from COG may result in fatigue, sprain, strain, joint damage…

11. Center of Gravity and Exercise • Move limbs and weight away from COG to increase resistance • Keep limbs and weight close to COG to decrease resistance

12. Inertia • Resistance to movement • Related to mass of the object • The greater the mass of an object, the greater its inertia and harder to move it • “Cheat lifting movements” • A baseball player can train with a heavy bat that provides greater inertial resistance, which slows down the swing speed

13. Friction • A force acting parallel (sandwiched) between two objects during motion • When two surfaces pressed together rub against each other. • Belt or brake pad, slide board, football sled • Fluid Resistance: • Pushing an object through a liquid or gas. • Swimming, cycling, skydiving, baseball, rowing, kayaking, golf

14. Tension • A stretching force pulling at both ends • Pull of a contracting muscle tendon

15. Compression • A force that tends to flatten or squeeze an object • Lifting a BB presses the ends of the bones together and is produced by muscles and gravity along the length of the bones • Necessary for the development of bone growth • If a large compressive force is applied, and the load surpasses the stress limits, a fracture or break will occur

16. Shear Force • Applied parallel to the surface of the object • Across the surface of a bone • A good example of shear force is seen with a simple scissors. • The two handles put force in different directions on the pin that holds the two parts together.

17. Torsion • Twisting force • Foot is planted and the body changes direction (ankle sprain)

18. Torque • The force which causes rotation • Rotation occurs about a pivot point • Line of action of force must act at a distance from the pivot point

19. Muscle Force • A muscle can only generate a pulling or tensile force • Elbow extension: the triceps brachii pulls on the olecranon process of the ulna • Elbow flexion: biceps brachii pulls on the radial tuberosity of the radius • Movement at any joint occurs by opposing pairs of muscles and gravity assists in the motion

20. Ground Reaction Force (GRF) • Force provided by the surface an individual is moving on (i.e., sandy beach, gym floor, grass lawn, concrete sidewalk) and the reaction to the force the body exerts on the ground • If the body is pushing down and forwards, the ground reaction force is up and backward; if the body pushes down and backward, the ground reaction force is up and forwards

21. Levers of the Musculoskeletal System • Involve bones, joints, and skeletal muscles • Bring about most movements of the limbs and the whole body • Body movements characteristic of sport and exercise act primarily through levers of the skeleton in order to exert forces on the ground, object, and other people • Muscles not acting through bony levers include those of the face, tongue, heart, arteries, and sphincters

22. Parts of a Lever • Lever: a rigid or partially rigid structure that can rotate about a pivot point or fulcrum. • Fulcrum (pivot point): axis of rotation or the point about which the lever pivots • The axis is the point of rotation about which the lever moves

23. Levers • Bones of the body act as levers which create a mechanical advantage of speed or strength • The fulcrum is formed by the joint • Effort is any force applied to the lever • Load or resistance is a force that resists the motion of the lever

24. Muscle Force • Effort Arm/Force Arm • The force exerted by a muscle at either of its ends when its electrochemically stimulated to shorten

25. Moment Arm (Lever Arm or Force Arm) • Moment arm of the muscle force • Moment arm of the resistive force • Distance between the point where the force acts and the point of rotation

26. Application • Levers = humerus, radius, and ulna • Effort= contraction of biceps • Load= weight of the arm, gravity… • Fulcrum or axis of rotation = humeroulnar joint

27. Mechanical Advantage (MA) • Mechanical Advantage: created by a machine that enables people to do work while using less force • Used to allow a small effort to move a large load. • It is calculated by dividing the load by the effort

28. Mechanical Advantage (MA) • A normal human can't lift a 1,000-pound vehicle off the ground, so you need the mechanical advantage of a jack. • If the weight of the vehicle (the load) is 1,000 pounds and the weight of the jack handle (the effort) is seven pounds, then the mechanical advantage provided by the jack is 1,000/7, or "one thousand to seven."

29. A MA greater than 1.0 means that muscle force exerted is less than resistive force A MA less than 1.0 means that muscle force exerted is greater than the resistive force A MA less than 1.0 is a disadvantage Mechanical Advantage (MA)

30. Musculoskeletal System • Most of the skeletal muscles operate at a considerable mechanical disadvantage. • Variations in Tendon Insertion • Tendon insertion farther from the joint center results in the ability to lift heavier weights. • This arrangement results in a loss of maximum speed. • This arrangement reduces the muscle’s force capability during faster movements

31. Three Classes of Levers • Each providing different levels of mechanical advantage • The levers are referred to as class 1, class 2 and class 3. • Differs by the force you apply (effort), load (opposing force), and fulcrum (pivot point)

32. First Class Lever • Fulcrum is placed between the load and the effort • The force you apply is on the opposite side of the fulcrum to the force you produce

33. First Class Lever • If the two arms of the lever are of equal length, the effort must be equal to the load • If the effort arm is longer than the load arm, as in a crowbar, the effort travels farther than the load and is less than the load • The mechanical advantage is calculated by measuring the length of the lever on either side of the fulcrum.

34. First Class Lever • Triceps Extension: • Muscle force and resistive force act on opposite sides of the fulcrum

35. First Class Lever • The head is raised off the chest • As the head is raised, the facial portion of the skull is the resistance, the fulcrum is between the atlas and occipital bone, and the effort is the contraction of the muscles of the back

36. First Class Levers • Balanced Movements • Fulcrum near the middle • Seesaw • Erector spinae extending the head • Speed and Range of Motion • Fulcrum is near the effort • Scissors • Triceps extending the elbow • Force motion • Fulcrum is near load • Crow bar

37. Second Class Levers • Load between the effort and the fulcrum • A wheelbarrow is a second-class lever. The wheel’s axle is the fulcrum, the handles take the effort, and the load is placed between them. • The effort always travels a greater distance and is less than the load.

38. Second Class Lever • Gastrocnemius and soleus in plantarflexing the foot to raise the body on the toes • The ball of the foot is the fulcrum, gastrocnemius and soleus are the muscle force, and the body is the resistance

39. Second Class Levers

40. Third Class Levers • Effort placed between the load and the fulcrum • Increase distance and speed not force • A hammer acts as a third-class lever when it is used to drive in a nail: the fulcrum is the wrist, the effort is applied through the hand, and the load is the resistance of the wood.

41. Third Class Lever • DB Biceps Curl • The fulcrum is the elbow, the effort is applied by the biceps muscle, and the load is the weight in the hand. • Most movements of the body are produced by third class levers

42. Third Class Lever • Pitchers use their arms as third-class levers. • During a pitch, the pitcher’s forearm pivots at the elbow, which is the fulcrum. • The muscles of his forearm supply the force, and the ball provides the resistance. • The lever action of a pitch magnifies the speed a pitcher can give to the ball.

43. Summary of Levers • Most body movements produced are third class levers • We are more adapted to speed than strength (short force arm/long weight arm) • First class levers give the advantage of strength or speed depending on where the fulcrum is located • Second class levers give the advantage of strength

45. Mnemonic • First Class Lever : E F L or L F E • Second Class Lever: F L E or E L F • Third Class Lever: F E L or L E F • ETHEL (EF L) the FLEA (FLE) FELL (FE L)

46. Questions 1. Identify the 3 parts of a lever. 2. Draw and label a diagram of each of the 3 types of levers

47. Questions 3. In lifting an object, the biceps represents the _____________ force; the elbow represents the _____________;  the object lifted represents the ______________ force. This is an example of a ______________ class lever.

48. Identify the Classes of Levers

49. Questions 5. In a first class lever, the _________________ is between the effort and the load. 6. In an off-center first class lever (like a pliers), the load is larger than the effort, but is moved through a smaller_____________. 7. Identify two examples of first class levers

50. Questions 8. In a second class lever, the ________________ is between the fulcrum and the effort. 9.Identify two examples of second class levers.