Mechanics In Exercise

Angle of Pull

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Gravity Vectors Load offers varying degrees of resistive force against muscles Very little force is required of agonist muscles when load moves perpendicular to gravity (signified by orange arrow). Perpendicular to gravity = almost 0 effort Except for forces required to overcome inertial and maintain posture for supporting musculature. Moderate motive forces are required to overcome resistive forces when load moves diagonal to to gravity. Examples: 30° = effort is half load, 45° = effort is 71% load Also see Incline Plane Greatest resistive forces are offered to agonist muscles when load moves parallel to gravity. Parellel to gravity = 100% load Incidentally, rotary forces from working muscle acting upon load are greatest in Components of Force Diagram below. Orange arrow can also signify resistive force vector of pulley cable with relative positioning of motive force angles of pull. Articulations in isolation follow a curvilinear path Load is moved in and out of line of gravity. Load tends to be shifted from muscles to skeletal frame and joints, and vice versa Compound movement seemingly move in a linear motion (line of push or line of pull) Compound movements can be seen as a coordinated combination of two or more isolated movements Beginning posture: primarily tension or compression forces on bones and joints Execution Pushing movements: muscles begin to contract eccentrically Pulling movements muscles begins to contract concentrically Analysis of Arm Curl Arm straight weight in hand pulls arms (joint supporting bone) down Initiation of flexion with arm straight. arm flexors overcome inertia (see Newton's first law) smaller brachialis has slightly better angle of pull as compared to biceps at this wide angle dumbbell moves nearly perpendicular to gravity offering relatively low resistive forces. with this angle of pull, rotary force of biceps is weakest Approaching 90 degrees resistive force (R) progressively increases at 30 degrees, approximately 50% of weight * lever arm ratio at 45 degrees, approximately 71% of weight * lever arm ratio 90 degrees resistive force is greatest when path of weight is parallel to gravity. 100% of weight * lever arm ratio rotary force of biceps is strongest [see angle of pull above (2nd diagram above)] Travelling beyond 90 degrees resistive force progressively decreases at 45 degrees, approximately 71% of weight * lever arm ratio at 30 degrees, approximately 50% of weight * lever arm ratio rotary force of brachialis and then biceps diminishes [see angle of pull above (3rd diagram above)] End of movement or change to eccentric contraction antagonist muscles may be activated to overcome inertia biceps torque force is only relieved at the flexed position if slight shoulder flexion positions forearm perpendicular. Also see Tension Potential and its impact on force production. Components of Force Definitions: Angle of Pull: angle between muscle insertion and bone on which it inserts. Components of Force Rotary component: force of a muscle contributing to bone's movement around a joint axis; greatest when muscles angle of pull is perpendicular to bone (i.e. 90 degrees). Stabilizing component: degree of parallel forces generated on the lever (bone and joint) when the muscles angle of pull is less than 90 degrees. Dislocating component: degree of parallel forces generated on the lever (bone and joint) when the muscle's angle of pull is greater than 90 degrees. Components of force due to angle of pull >90 degrees includes stabilizing component =90 degrees 100% rotary force <90 degrees includes dislocating component

Shoulder abduction force vector diagram E.g.: Dumbbell Lateral Raise and Lying Lateral Raise. Angle of Pull Upper: Supraspinatus Lower: Lateral Deltoid Rotary component perpendicular to lever arm Stabilizing component parallel to lever arm from insertion through fulcrum Also see Supraspinatus Weakness Knee flexion abduction force vector diagram E.g.: Lever Lying Leg Curl. Color codes on diagram are same as Components of Force above. Hamstring Agonist Active insufficient position Rectangle force vector above knee knee flexion Quadriceps (Rectus Femoris) Antagonist Stabilizer Passive insufficient position Rectangle force vector through knee Counters posterior forces of hamstring See pulley-like arrangement of Patella at knee. Vertical arrow by hip Flexes hip Attempts to position hamstring back in active sufficiency Sartorius Synergist Remains actively sufficient with hip extended Arrow not illustrated Flexes knee Diagonal arrow by hip Flexes hip Attempts to position hamstring back in active sufficiency Gracilis Synergist Somewhat actively insufficient since knee is flexed and hip is adducted and not externally rotated Arrow not illustrated flexes knee Popliteus Synergist Arrow not illustrated Flexes knee Gastrocnemius Synergist Arrow through calf flexes knee Tibialis Anterior Antagonist Stabilizers Dorsal flexes ankle Positions Gastrocnemius in active sufficiency so it can flex at knee Orange circle marks approximate location of virtual fulcrum within the distal condyles of the femur. When knee is extended, virtual fulcrum is located more anteriorly, due to gliding action of knee joint.