Biomechanics for RT - Lever Systems PDF

1   Skeleton Muscles function by pulling against bones that rotate about joints and transmit force through the skin to the environment. Axial skeleton vs. the appendicular skeleton.  Skeletal Musculature A system of muscles enables the skeleton to...

1   Skeleton Muscles function by pulling against bones that rotate about joints and transmit force through the skin to the environment. Axial skeleton vs. the appendicular skeleton.  Skeletal Musculature A system of muscles enables the skeleton to move. Origin = proximal attachment Insertion = distal attachment 2 3 Fig. 1.2. Haff et al. 2016. Essentials of Strength… Human Kinetics   agonist: The muscle most directly involved in bringing about a movement Also called the prime mover.  antagonist: A muscle that can slow down or stop the movement. 4   Levers of the Musculoskeletal System Many muscles in the body act through levers. Body movements directly involved in sport and exercise primarily act through the bony levers of the skeleton. A lever is a rigid or semi‐rigid body that, when subjected to a force whose line of action does not pass through its pivot point, exerts force on any object impeding its tendency to rotate. 5   Figure 2.1 (next slide) The lever can transmit force tangential to the arc of rotation from one contact point along the object’s length to another. FA = force applied to the lever; MAF = moment arm of the applied force; FR = force resisting the lever’s rotation; MRF = moment arm of the resistive force. The lever applies a force on the object equal in magnitude to but opposite in direction from FR. 6 7Fig. 2.1. Haff et al. 2016. Essentials of Strength… Human Kinetics   mechanical advantage: The ratio of the moment arm through which an applied force acts to that through which a resistive force acts.  A mechanical advantage greater than 1.0 allows the applied (muscle) force to be less than the resistive force to produce an equal amount of torque.  A mechanical advantage of less than 1.0 is a disadvantage. 8   First‐class lever: A lever for which the muscle force and resistive force act on opposite sides of the fulcrum. 9  Figure 2.2  The slide shows elbow extension against resistance (e.g., a triceps extension exercise).  O = fulcrum; FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force.  Mechanical advantage = MM /MR = 5 cm/40 cm = 0.125, which, being less than 1.0, is a disadvantage.  The depiction is of a first‐class lever because muscle force and resistive force act on opposite sides of the fulcrum.  Because MM is much smaller than MR, FM must be much greater than FR; this illustrates the disadvantageous nature of this arrangement. Fig. 2.2. Haff et al. 2016. Essentials of Strength… Human Kinetics 10   second‐class lever: A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm longer than that through which the resistive force acts.  Due to its mechanical advantage, the required muscle force is smaller than the resistive force. 11   Figure 2.3  The slide shows plantar flexion against resistance (e.g., a standing heel raise exercise).  FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force.  When the body is raised, the ball of the foot, the point about which the foot rotates, is the fulcrum (O).  Because MM is greater than MR, FM is less than FR. 12Fig. 2.3. Haff et al. 2016. Essentials of Strength… Human Kinetics   third‐class lever: A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm shorter than that through which the resistive force acts.  The mechanical advantage is thus less than 1.0, so the muscle force has to be greater than the resistive force to produce torque equal to that produced by the resistive force. 13   Figure 2.4  The slide shows elbow flexion against resistance (e.g., a biceps curl exercise).  FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force.  Because MM is much smaller than MR, FM must be much greater than FR. 14 Fig. 2.4. Haff et al. 2016. Essentials of Strength… Human Kinetics   During elbow flexion with the biceps muscle, the perpendicular distance from the joint axis of rotation to the tendon’s line of action varies throughout the range of joint motion.  When the moment arm (M) is shorter, there is less mechanical advantage. 15 Fig. 2.6. Haff et al. 2016. Essentials of Strength… Human Kinetics   Most of the skeletal muscles operate at a considerable mechanical disadvantage. Thus, during sports and other physical activities, forces in the muscles and ten‐ dons are much higher than those exerted by the hands or feet on external objects or the ground. 16  Identify different classes of levers and their components.  Be able to correlate classes of levers with the 3 examples in the human body that were discussed  Be able to calculate the mechanical advantage of a first class lever 17 17   Haff & Triplett. Essentials of Strength Training & Conditioning, 4th edition. Human Kinetics, 2016. 18