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
- 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
- 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
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
- Pushing movements:
- muscles begin to contract eccentrically
- Pulling movements
- muscles begin to contract concentrically
- 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
- 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.
Also see Tension
Potential and its impact on force production.