Kinetics: Levers and Mechanical Advantage

Define the 3 basic classes of levers and provide examples in the human body.
Differentiate between classes of levers using figures and drawings.
Differentiate a high and low mechanical advantage as it impacts force, effort, and resistance in the human body.

The lever system consists of:
- Effort: The force applied by muscle contractions.
- Resistance: The external load that must be moved.
- Fulcrum: The pivot point around which the lever rotates.
There are 3 classes of levers:
- First Class Lever
- Second Class Lever
- Third Class Lever

The axis of rotation (AoR) is located between the opposing forces (effort and resistance).
Positioning can vary:
- Fulcrum in the middle, e.g., seesaw.
- Fulcrum closer to the effort, e.g., elbow extension overhead.
- Fulcrum closer to the resistance, e.g., cranial extensors holding head up.
Internal (effort) and external (resistance) forces typically act in similar directions while producing rotary motion in opposite directions.

Rare in the human body but noteworthy in function.
The AoR is at one end of the bone.
The resistance is found between the AoR and effort.
The internal force (effort) generates more leverage than the external force (resistance).
- Leverage can be calculated as the length of the moment arm.
- Mechanical advantage is always greater than 1.
Example: Gastrocnemius during a heel raise.
Data Calculation:
- Body weight (BW) = 667 N (150 lbs)
- Internal moment arm (IMA) = 12 cm
- External moment arm (EMA) = 3 cm
- Mechanical Advantage = 4
- Formula: MF x IMA = BW x EMA
- Rearranging gives us
  MF = \frac{BW \times EMA}{IMA}
  MF = \frac{667N \times 3cm}{12cm} = 166.8N (37.5 lbs)

Most common lever type in the human body.
The AoR is located at one end of the bone, and effort is situated between the AoR and the resistance.
The external weight (resistance) always has greater leverage than the muscle force (effort).
Mechanical advantage is always less than 1.
Examples include biceps (elbow flexion) and hamstrings (knee flexion).
Data Calculation:
- External weight (EW) = 66.7 N (15 lbs)
- Internal moment arm (IMA) = 5 cm
- External moment arm (EMA) = 35 cm
- Mechanical Advantage = 0.143
- Formula: MF x IMA = EW x EMA
- Rearranging gives us
  MF = \frac{EW \times EMA}{IMA}
  MF = \frac{66.7N \times 35cm}{5cm} = 467N (105 lbs)

Mechanical Advantage (MA) can be described as:
- High Mechanical Advantage (> 1):
- Acts as a force amplifier.
- Known as “slow but powerful”.
- Prioritizes strength at the expense of speed and range of motion.
- Effort is applied over a long lever arm, requiring a small force to move it a long distance.
- Example Calculation:
  Torque required = 5 lbs \times 5 ft = 25 lb-ft
- Low Mechanical Advantage (< 1):
- Acts as a motion amplifier.
- Known as “fast but weak”.
- Prioritizes speed and range of motion at the expense of force.
- Greater effort is required to produce less output.
- Example Calculation:
  Torque required = 10 lbs \times 2.5 ft = 25 lb-ft

Most muscles in the musculoskeletal system function with a mechanical advantage much less than 1.
- This environment necessitates that internal effort (muscle force) is greater than the external resistance.
- Despite not having to move far, they must produce larger forces.
Example of Specific Muscle Function:
- Supraspinatus and Deltoid have a mechanical advantage of 1/20.
- To maintain the shoulder at 90° abduction with a weight:
- Requires muscle force of 20 times the weight of the external load.
- Despite the muscle contracting only 5% of the distance, this small contraction results in significant movement of the arm:
  - 1 cm contraction yields 20 cm movement of the arm.
Muscles functioning under a mechanical advantage < 1 often generate large internal forces, which can create compression and shear stresses.
- Periarticular tissues (e.g., articular cartilage, fat pads, bursae) are integral to absorbing or dissipating these forces.

The majority of joints in the human body act as third-class levers, resulting in substantial muscle forces and joint reaction forces.

Understanding the mechanics of levers is essential for studying human movement and the forces involved in various physical activities. Leverage aspects contribute to our understanding of sports, rehabilitation, and biomechanics in everyday actions.