What the f*%k is a moment arm and what does it have to do with my training!?

In simple physics terms, “The magnitude of the moment of a force acting about a point or axis is directly proportional to the distance of the force from the point or axis. It is defined as the product of the force (F) and the moment arm (d). The moment arm or lever arm is the perpendicular distance between the line of action of the force and the center of moments. “

In equation form this comes out as:

Moment = Force x Distance or M = (F)(d)

In Biomechanics, The, “muscle moment arm is a measure of the effectiveness of a muscle at contributing to a particular motion over a range of configurations”

We can use the above formula to think about tissue stress as being = to the moment = torque at the axis of rotation.

Side note; tissue stress could be referring a lot of things, but for the context of this article it is NOT about volume and intensity, total lbs lifted, or muscle protein break down; it is about the torque produced around a joint. Force produced by our muscular system creates torque around the levers of the body which allows us to move. Torque is by no means a bad thing in and of itself, but it is a variable we need to think about!

For example, in the context of lifting the most weight, reducing the amount of torque needed for a constant force (bar weight) is key to lifting the most weight possible with what the strength we’ve got!

In the deadlift, for example, if the force (our barbell weight) remains constant but the distance (from hip to bar path) increases we have a greater “moment” or greater tissue stress requirement. This is why we often hear the cue “drag the bar up your body” or “keep the bar close;” by keeping our lats engaged and the bar close to us we reduce the moment arm and thus the torque. This is why lifters tend to miss lifts when the bar drifts away from them. This is also why many elite level powerlifters have lifted with rounded upper backs (it shortens the moment arm). The moment arm joint relationship is illustrated in the image of the 1st pull in a clean below:

In the squat, the moment arms are slightly different because the bar is no longer in front of both the knees and the hips, rather it sits between them, this splits the single arm into two related arms.

How often do you hear the old “don’t let your knees go past your toes” advice for the squat? Folks that say this may be well meaning, but they are not necessarily correct. The torques produced at the hips and the knees have a relationship (the ankles are involved too, but to keep it simpler we will focus on just hips and knees for now). Individual builds and sport specific pursuits will change what kind of squat position one needs to/should use thus rendering the “no knees past toes que” obsolete.

In the photo to the right we see the difference between two squatters builds, we also see the ways in which their torso and leg length’s affect their squat position. Notice how the person with long legs and a short torso has a longer moment arm between the bar and the knee and how the knees go past the toes? Imagine, if we were to give this lifter a more upright torso – the knees would be much farther past the toes in order for them to get their hips down further while driving their chest up; however, this position may weaken their ability to use their hips thus demanding more force development from the quadriceps – this is, in part, why olympic lifters (although overall very muscular) seem to be biased towards quad development visually in their thighs and powerlifters (also overall very muscular) are often a little more biased to look hamstring and glute heavier.

At the end of the day what constitutes good form for one person or another may change, but keeping our moment arms in mind can help us assess sticking points, dial things in, and make sure our execution fits our structure as well as possible. This concept can be applied to many things in training and the moment arm concept can be observed in real life all the time – as you move through your training this week keep your eyes open – how many moment arms do you see? How do they impact the world around them? How does this relate to YOUR training!?

References:

1. Bengtsson, V., Berglund, L., & Aasa, U. (2018). Narrative review of injuries in powerlifting with special reference to their association to the squat, bench press and deadlift. BMJ open sport & exercise medicine, 4(1), e000382. doi:10.1136/bmjsem-2018-000382

2. Edington, C., Greening, C., Kmet, N., Philipenko, N., Purves, L., Stevens, J., . . . Butcher, S. (2018). The Effect of Set Up Position on EMG Amplitude, Lumbar Spine Kinetics, and Total Force Output During Maximal Isometric Conventional-Stance Deadlifts. Sports,6(3), 90. doi:10.3390/sports6030090

3. Eltoukhy, M., Travascio, F., Asfour, S., Elmasry, S., Heredia-Vargas, H., & Signorile, J. (2015). Examination of a lumbar spine biomechanical model for assessing axial compression, shear, and bending moment using selected Olympic lifts. Journal of orthopaedics, 13(3), 210-9. doi:10.1016/j.jor.2015.04.002

4. Hall, S. J. (2019). Basic biomechanics. New York, NY: McGraw-Hill Education.

5. Rippetoe, M., & Bradford, S. E. (2017). Starting strength: Basic barbell training. Wichita Falls, TX: Aasgaard Company.

6. Schellenberg, F., Lindorfer, J., List, R., Taylor, W. R., & Lorenzetti, S. (2013). Kinetic and kinematic differences between deadlifts and goodmornings. BMC sports science, medicine & rehabilitation, 5(1), 27. doi:10.1186/2052-1847-5-27

7. Sherman, M. A., Seth, A., & Delp, S. L. (2013). WHAT IS A MOMENT ARM? CALCULATING MUSCLE EFFECTIVENESS IN BIOMECHANICAL MODELS USING GENERALIZED COORDINATES. doi:10.1115/DETC2013-13633