mechanical advantages of pulley systems & how it means your whole life has been a lie.

Posted on February 21, 2013 Updated on February 22, 2013

Hey there! Finally put the finishing touches on a report for my strength & conditioning program design class. Last week, we had more of a hands-on lab inside my university’s fitness center. We inspected the pulley systems used in a few different types of isolating machines. What we discovered was jaw-dropping (at least for those who frequently use these machines).

Systems of pulleys can decrease the force required to lift a certain amount of weight.

For example, if I want to lift a 100 lb weight, and I have a pulley system with 3 pulleys, that gives me a mechanical advantage of 2. If I use this pulley system, I only have to exert 50 lbs of force to lift the 100 lb weight. (100/2 = 50). Does that make sense?

Now apply this to the strength training machines you see in weight rooms. Most of these machines have a complex system of pulleys. Meaning that you are never actually lifting the amount of weight you think you are.

MIND BLOWN.

Pulley LAB # 1/ 2-14-12

Measurement ofApplied Resistance From Selected

Strength-Training Apparatus

Koleana Kai McGuire

Discussion

The pulley lab definitely challenged the way we look at strength training machines by critically examining a variety of exercise equipment. Looking at its overall apparatus set up and design, and specifically each of its pulley systems and its impact on the “perceived” load. It made me realize that some manufactures didn’t take into consideration the effects of pulley systems on weight lifting. This is important as a user of these types of machines, because now I have a more accurate understanding of the weight that’s being lifted. As well as a future professional when training clients, I will know what recommendations to make, and how best to design strength training programs based upon the equipment available.

Apparatus # 1: Pulldown

Brand Name: Equalizer

Description of: 3 lines of pull attached to load, 1 line as free end pulling down

Apparatus Commonly used for tricep extensions

# pulleys 1 movable pulley

M.A. 2

Trial  (lbs) 70 lbs

Notes Measured 32.5 lbs on spring scale, which is pretty close to the predicted weight by our formula

P = ½ x (70 lbs) = 35 lbs

This is assuming that the pull is smooth and has no friction.

However, there must be some friction and with P measured at 35 lbs,

Conclusion: The plates actually weigh less than we are lead to believe.

Apparatus # 2 Pulldown

Brand Name Magnum

Description of 2 pulleys

Apparatus Commonly used for bicep curls

# pulleys 2 movable pulley

M.A. 2

Trial  (lbs) 50 lbs

Notes Measured 30 lbs on spring scale, which is greater than the force of pull as predicted by our formula

P = ½ x (50 lbs) = 25 lbs

This is assuming that the pull is smooth and has no friction.

However, there must be some friction making more force required to pull the weight.

Conclusion: The plates may actually weigh less than we are lead to believe.

Apparatus # 3: Pull ups

Brand Name: Life Fitness

Description of: 3 lines of pull attached to load, 1 line as free end pulling down

Apparatus: Can be used for bicep pull ups

# pulleys: 2 pulleys

M.A.: 1

Trial  (lbs): 50 lbs

Notes: Measured 52 lbs on spring scale, which is almost the same force of pull indicated by the weights.

This is assuming that the pull is smooth and has no friction.

Conclusion: The manufacturers of this apparatus designed the machine to accurately measure the force exerted. Meaning that the user is actually lifting (almost exactly) the amount of weight selected with the pins.

Apparatus # 4: Wheelchair accessible

Brand Name: Uppertone

Description of: lever systems

Apparatus: Can be used for upper body workouts, like the chest and arms, for people in wheel chairs

# pulleys: N/A

M.A.: N/A

Trial  (lbs): 30 lbs. The weight could vary depending on how the lever was oriented.

Notes: When the axis of rotation changes, the required force to move the weight changes.

Conclusion: The manufacturers of this apparatus designed it differently from traditional weight machines. The user can increase or decrease the required force by changing the axis for the levers, whereas most other machines will do so by changing the weight plates. By increasing the force arm, we decreased the force required to lift the weight- from 22 lbs to 5 lbs.