Mechanics In Exercise: Angle of Pull

Gravity Vectors | Arm Curl Analysis | Force Components | Shoulder Adduction | Knee Flexion

Gravity / Force 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 / force vector = 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 gravity / force vector.
    • 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 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 begin 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 30º, approximately 50% of weight * lever arm ratio
    • At 45º, 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)]
• Traveling beyond 90º
  • resistive force progressively decreases
    • At 135º (or 180º-45º), approximately 71% of weight * lever arm ratio
    • At of 150º (or 180º-30º), 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.

See question regarding elbow position during arm curl. 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