We analyze the moment arm. This somewhat unknown term for exercise professionals is much more linked to physics and mechanical engineering and is used for the analysis of lever systems. Often trainers, coaches, and instructors quantify the load of their clients through sets, tempo, repetitions, weight, leaving aside the fundamental principles.
Considering the human body as a lever system (joints, connective tissue…) what percentage of that applied load causes me resistance?
In this article, we try to clarify certain aspects that help to understand these issues. Although the markers mentioned above influence the volume and intensity when prescribing exercise, they are not the only nor the most determining factors.
Next, we will try to explain the relationship that the moment arm has with the exercise and why we must take it into account when working with our clients.
“tHE WEIGHT IN your hand does not indicate the forces in your body”
What is the moment arm?
In English called “moment arm,” this term refers to the perpendicular distance between the line of action of a force and an axis of rotation. It is a graphical method that combines the proportion of the lever arm that causes rotation about the axis.
In exercise, it is vitally important to handle this concept as it will allow us to quickly see how the resistance we are applying to our clients evolves.
What does the moment arm measure?
This concept arises because “trigonometric calculation is often impractical in the application in exercise with clients, as is logical to understand.” The moment arm therefore refers to the mechanical characteristics of a force applied in its action to cause rotation about an axis in a lever system(1).
How is it visualized?
In the following image, it is shown how we can easily visualize the markers that we must take into account to see this moment arm in our sessions with clients.
As can be seen at 45º of shoulder abduction, not all the force that the dumbbell causes generates rotation about the axis. In this case, it will be at 90º of glenohumeral abduction when all the weight of the dumbbell causes rotation about the axis. Conversely, the more adduction in the shoulder joint complex the dumbbell will cause less rotation.
The direction of the force (perpendicular to the ground) in this last case plays a key role in all this calculation, because with another direction of force in a similar scenario the moment arm would change completely. Below we show an example.
In this scenario, the direction of the cable causes that at 0º of shoulder abduction it will be the point where the force generates the most resistance, completely the opposite of what happened with the dumbbell.
As we can see in the previous illustrations, the moment arm helps us to know at what moment the forces applied to the different structures receive forces throughout the range of motion of an exercise (ROM).
How is the moment arm calculated?
This calculation we could say is a task that we would perform more at the table and not so much in the training with clients due to its complexity. The muscular moment arm relates two factors:
The lever arm and the angle of force. In this way, the MA describes the ability of a muscle to generate resistance around a joint axis. Below is shown the moment arm of the pectineus muscle on the anteroposterior axis of the hip. The formula for this calculation is as follows:
Moment arm = Lever arm x sin@
Having clearly understood the above, we must take into account that the theoretical moment arm can change completely if within the training process we use large accelerations. If we apply a lot of force to accelerate the load at a point where theoretically (in static) we get less “torque,” that moment becomes the peak of force, since we must be able to accelerate the weight. The rest of the path is therefore the result of the accumulated kinetic energy and not so much of the application of momentary force.
Therefore, the moment arm may be of no use if the execution speed is too high and we do not have conscious control of the movement throughout the entire range. The most optimal way for the moment arm during exercise to be equal to the theoretical one would be to apply the least possible acceleration to cause the displacement of the load.
In recent years and through the Resistance Institute this term has gained importance within the world of exercise and teaching. Biomechanics is a fundamental pillar along with physiology that must be mastered by all those who work with the health and exercise of this complex system that is the human body.
Bibliography
- Leal, L. Martinez, D. & Sieso, E.(2012).Fundamentals of exercise mechanics.Barcelona.Resistance institute.
- Purvis, T.(2001).Resistance training specialist.Focus on fitness.
- López Román,A.López Beltrán, E.(2003).Biophysics applied to the biomechanics of the human body.Biomedical University Library.
