Mechanics in Exercise: Part I Levers

Mechanics in Exercise

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Levers

Movement in the body is produced by a system of levers. These series of levers work together to produce coordinated action, some by actual movement (dynamic) and others by stabilization (static).

Definitions
- Lever: Rigid bar that turns about an axis of rotation or a fulcrum (A)
- Motive Force (F): effort or exertion applied to cause movement against resistance or weight
- Resistive Force (R): opposes motive force

First Class Lever

axis is placed between force and resistance
examples: crowbar, seesaw, scissors
examples in body:
- elbow extention
  - triceps applying force to olecranon (F) in extending the nonsupported forearm (R) at the elbow (A)
- flexing muscle
  - agonist (F) and antagonist (R) muscle groups are simultaneously contracting on either side of a joint axis (A).

lever characteristics
- balanced movement
  - axis is midway between force and resistance
  - eg: seesaw
- speed and range of motion
  - axis is close to force
  - eg: elbow extension
- force
  - axis is close to resistance

Second Class Lever

resistance is between axis and force
classic examples: wheel barrow, nut cracker
complex example: rowing
- paddle in water acts as slipping axis (A)
- boat resistance is resistive force (R)
- rower is motive force (F)
relatively few examples in body
- planter flexion of foot to raise body up on toes
  - ball of foot (A) serves fulcrum as ankle plantar flexors apply force to calcaneous (F) to lift resistance of body at tibial articulation (R) with foot.
- entire body during push-up
  - foot is axis of rotation (A) when reaction force of ground pushing against hands (F) lifts weight of body's center of gravity (R).
lever characteristics
- produces force: large resistance can be moved by a relatively small force
- weight machines: more resistance needed, lower inertia, smoother feel.

Third Class Lever

force is placed between the axis and resistance
examples:
- tongs: food (R) is supported by grip on handles (F) while axis is on opposite end.
- shovelling: dirt on shovel (R) is lifted by force to handle by hand (F) while upper hand on end of shovel handle serves as axis (A)
- rowing: oar is moved through water (R) by pulling on middle of oar (F) while holding end of oar with opposite hand (A).
  - Note: shovelling and rowing actions can also be first class lever systems if the hand closes to the force remains stationary (A) and the hand on the far end of the shovel or oar is moved (F).
- batting: ball is hit (R) by moving bat toward ball with hand of far arm (F) while supporting lower portion of bat with hand of near arm (A).

example in body
- most levers in body are third class
- elbow flexion
  - Biceps and brachialis pull ulna (F) lifting the forearm, hand, and any load (R) at the elbow (A).
- knee flexion
  - hamstring contract (F) to flex the lower leg (R) at the knee (A).
lever characteristics
- produces speed and range of motion
- requires relatively great force to move even small resistances
- weight machines: less resistance required, greater inertia
  - harder to start and stop movement

Lever Arm Length

Definitions
- Resistance Arm: distance between axis and point of resistance application.
- Force Arm: distance between axis and point of force.
Formula

F x FA = R x RA

Force x Force Arm = Resistance x Resistance Arm

Example:

Initial Example Change insertion 1 cm Change point of resistance 1 cm

F x 2 cm = 10 kg x 9 cm

2 F = 90 kg

F = 45 kg

F x 3 cm = 10 kg x 9 cm

3 F = 90 kg

F = 30 kg

F x 2 cm = 10 kg x 8 cm

2 F = 80 kg

F = 40 kg

Lever characteristics
- Long resistance arm: speed and range of movement
- Short resistance arm: force
Mechanical advantage
- Motive force arm length / Resistive force arm length
- No mechanical advantage if quotient = 1 (same length):
- If quotient >1: mechanical advantage in force
- If quotient <1: mechanical advantage in speed and range of motion

Variable Resistance Levers

Example
- Universal gym equipment

Resistive force (R) is initially relatively short [close to fulcrum (A)]. As motive force (F) acts on lever, resistive arm becomes longer requiring progressively greater motive forces throughout movement.

Angle of Pull

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 bones 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 muscles 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

**Components of force due to angle of pull**

>90 degrees includes stabilizing component	=90 degrees 100% rotary force	<90 degrees includes dislocating component

Gravity Vectors

Load offers a resistive force against targeted muscles when it moves against gravity.
- Practically no significant forces are offered on target muscles when load moves perpendicular to gravity.
  - except for forces required to overcome inertial and maintain posture for supporting musculature.
- Greatest resistive forces are offered when load moves parallel to gravity.
- Movements are curvilinear
  - load is moved in and out of path of gravity.
  - load tends to be shifted from muscles to skeletal frame and joints and visa versa
  - compound movements can be seen as a coordinated combination of two or more isolated movements
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 brachilas 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 increase
    - at 30 degrees, approximately 50% of weight x lever arm ratio
    - at 45 degrees, approximately 71% of weight x lever arm ratio
- 90 degrees
  - resistive force is greatest when path of weight is parallel to gravity.
    - 100% of weight x lever arm ratio
  - rotary force of biceps is strongest [see angle of pull above (2nd diagram above)]
- Travelling beyond 90 degrees
  - resistive force progressively decrease
    - at 45 degrees, approximately 71% of weight x lever arm ratio
    - at 30 degrees, approximately 50% of weight x lever arm ratio
  - rotary force of brachilais 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.

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Resistive force (R) is initially relatively short [close to fulcrum (A)].	As motive force (F) acts on lever, resistive arm becomes longer requiring progressively greater motive forces throughout movement.