CSCS Muscle Levers Explained

mechanical advantage moment arm torque Jul 07, 2022

1st class, 2nd class, and 3rd class levers, how the heck do you keep them all straight?

The 3rd class lever is the most common type, so let's start there. 

  • Bicep curl 

The bicep tendon inserts on the front of the forearm, so we have muscle force that is pointed upward towards the shoulder as the bicep muscle shortens. The resistance is applied close to the hand by the dumbbell, barbell, cable machine, or band, which is referred to as the force resistance. 

Moment Arm

We also need to understand the concept of the moment arm:

  • The distance between the end of the fulcrum and the attachment site of the muscle is referred to as the moment arm of the muscle (MAm)
  • The distance between the end of the fulcrum and the site of the resistance is referred to as the moment arm of resistance (MAr)

Torque

The torque of the muscle refers to the force x the moment arm of the muscle and refers to the point at which rotation occurs.

  • T = F x MAm

Mechanical Advantage

If you've ever heard of mechanical advantage, it's just a ratio that refers to the moment arm of the muscle (MAm) in relationship to the moment arm of resistance (MAr).

  • MAm:MAr
  • ex: 1:4
    • Meaning the muscle (the moment arm of the muscle) has to work 4 times as hard as the resistance does to overcome the mechanical advantage
    • I nice analogy is shutting a door, when you push the door on the opposite side from the hinge, it will be much easier than if you are trying to push the door shut closer to the hinge

Joint Angles

Training muscles from different angles (concentric, isometric, & eccentric) will produce different types of stimuli due to the force being applied at different moment arms of the muscle. 

  • A very acute angle (the concentric portion of the bicep curl) will result in a shorter moment arm of the muscle because it is situated a short distance away from the fulcrum, meaning the horizontal distance between the fulcrum and the resistance is being applied is shortened 
  • A 90 degree angle (the isometric portion of the bicep curl) will result in the greatest moment arm of the muscle because it results in the greatest distance away from the fulcrum
  • A very obtuse angle (the eccentric portion of the bicep curl) will also result in a shorter moment arm of the muscle because it is situated a short distance away from the fulcrum again

This is why it's easier to finish the concentric & eccentric portions of the bicep curl after you break parallel. 

The Differences Between a 1st, 2nd, and 3rd class Lever

  • 1st class: Ex: Tricep extension.
    • The fulcrum of the tricep extension is still the elbow, with the muscle force on the back of the arm (behind the elbow)
      • The moment arm of the muscle is on one side of the fulcrum (still a shorter distance like in the bicep curl)
      • The moment arm of the resistance is on the opposite side of the fulcrum (still a longer distance like in the bicep curl)
      • So the moment arm of the muscle (MAm) is shorter < than the moment arm of the resistance (MAr)
  • 2nd class: Ex: Calf raise
    • The fulcrum of the calf raise is the toe
      • The moment arm of the muscle is on the back side of the lower leg in this case the achilles tendon
      • The moment arm of the resistance is all the way from the achilles tendon to the toes where you would raise up from in the calf raise 
      • So the moment arm of the muscle (MAm) is longer > than the moment arm of the resistance (MAr)
      • BONUS: It's common to think of the ankle as the fulcrum, not that is not the case, since the movement is being initiated from the toes, the toes are the fulcrum 
  • 3rd class: Bicep curl
    • The fulcrum of the bicep curl is the elbow, with the muscle force being the upper arm
      • The moment arm of the resistance (MAm) is shorter < than the moment arm of the resistance (MAr) 

Practical Application

In a second-class lever, is the mechanical advantage in favor of the muscle or of the resistance?

  • The mechanical advantage is to the muscle because there is a longer moment arm of the muscle (MAm), meaning the muscle has to apply less force than the resistance (ex: 4:1 instead of 1:4 like in the bicep curl)

While doing a bent-over row with a slow concentric and slow eccentric, what muscle levels are acting at the elbow?

  • The bicep is pulling up during the concentric portion of the movement and the triceps are pushing downward on the eccentric portion of the movement
    • 1st class: The tricep
    • 3rd class: The bicep

When lowering a bicep curl from a 90-degree angle down, are you increasing, decreasing, or keeping the moment arm of the muscle the same?

  • The obtuse angle created when lowering the arm down during the bicep curl would be decreasing or shortening the moment arm of the muscle. 

The key to identifying if a lever is a 1st class, 2nd class, or 3rd class level is identifying the fulcrum first and then identifying the direction and force being applied to the fulcrum by the surrounding musculature and resistance. 


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Edited by Danielle Abel, MSN, FNMS(c), CSCS(c)

https://www.instagram.com/danielle_s_abel/?hl=en

 

 

 

 

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