You are currently exploring the Fundamentals Library, which is designed to provide a basic overview of the topics that are covered in other longer articles. This article is a part of the Mechanics section.
Dynamic Exercises
Dynamic exercises have one important characteristic that distinguishes them from static exercises. This characteristic is the range of motion, in other words, a range of angles that determines the success of performing a given exercise.
This leads to an interesting phenomenon - the dynamic exercises typically have their bottlenecks in certain ranges of motion that are more difficult to overcome than others. I call this relationship a difficulty curve.
Since the performance of the exercise is related to the torque produced in the joints, the difficulty curve depends on two separate and opposing curves. One is how the torque we produce in the joints changes with angle - which can be called the strength curve.
On the other side there is the component of how the resistance provided by gravity or some other type of resistance changes with angle - this is called resistance profile.
In this article we will take a closer look at the resistance profile. The nice thing about this particular relationship is that it is consistent and can be accurately measured and evaluated. That is why we use the word “profile”. Unlike the internal biological properties in our bodies which influence the strength curves, the physics of resistance will always work in the same repeatable way and can be universally applied.
Analyzing Force & Moment Arm Changes
Because resistance is torque, resistance profile is dependent on the changes of:
- Force Magnitude [newtons]
- Moment Arm Value [meters]
Moment arm itself can be influenced by variables like:
- Force Direction
- Force Application
Of course, all of this could be put together in a mathematical framework using trigonometry. However, this is not necessary, especially if you just want to grasp the general concept and not do any calculations.
To find out where the moment arm is greatest, we need to identify the force vector and see the perpendicular distance from that vector to the joint. Then see how it changes during an exercise as the range of motion progresses.
This is why you approach this analysis after identifying the plane in which it occurs (the axis around which rotation occurs in the joint).anatomical planes are sufficient to see the patterns. We also want to analyze the lifting (concentric) phase of an exercise for evaluation.
Take a look at this example of a squat in the sagittal plane - as I go down, the moment arm for both the knee and hip increases (despite the fact that the direction of the force remains the same).
To find out the profile of force, we need to identify the type of loading and assess how it changes. For example, a band will provide more force as it elongates.
Weights or body weights won’t change their magnitude. Therefore under conditions where we deal with free weights, or we have just one point of contact in a given plane in case of bodyweight exercise, we can assume the force magnitude is the same at each point.
That is as long as we don’t put some specific constraints, for example if we do planche push ups, we are lifting our bodyweight all the time, but when we do planche lean push ups, the magnitude of force will depend on how much we lean forward, and how much of the bodyweight we are holding on hands.
Of course, if mass is what we use as a source of resistance, we must remember the inertial effects.
Accelerating mass upward while fighting gravity (as in the initial stage of the lifting phase) requires more force & torque than maintaining constant velocity of motion in both directions and just fighting gravity. Decelerating mass while fighting gravity (as in the initial stage of the lowering phase) requires less force & torque than maintaining constant velocity of motion in both directions.
For the sake of this discussion, it's helpful to ignore the inertial effects, not only because they are different based on acceleration values. In heavy resistance training, they will not play such a large role, especially if we follow proper calisthenics execution standards.
Finally, machines can provide specific forces at specific parts of the exercise. And typically they are actually designed this way in order to manipulate the resistance profile.
When we perform this type of analysis, we can see proportionally how much resistance is being provided.
Resistance Profile Types
When we manage to analyze moment arm and force components of resistance and how they change, we can approach analyzing the resistance profile. All we have to do is to map the already mentioned components on a graph and we get the resultant.
There are a couple of main types of resistance profiles:
- Flat
- Ascending
- Descending
- Bell-Shaped
- U-Shaped
If you ever heard about the concept of “flattening the resistance profile” it basically means turning an exercise with any other resistance profile type to the flat resistance profile by manipulating some mechanical variables.
Exercise for you to try, before looking down, try to think about some examples of calisthenics exercises that have these profiles (additional equipment can be utilized).
Resistance Profile Examples
Let’s take a look at band reverse flys. In this exercise we are not fighting with gravity, but resistance provided by elastic properties of the band, which characteristics make it provide more force when the band is elongated.
The moment arm is also easy to spot, because direction of force is dictated by the band. As you can see, as we pull the band apart, the force magnitude gets larger while the moment arm gets smaller. So the resistance stays roughly the same along with the range of motion. This scenario is the flat resistance profile.
Contrary to that, let’s take a look at Reverse Hyperextension. This time, as we are looking at our lower body segments as the resistance in the context of the hip joints, the force magnitude will not change, while the moment arms will increase along with the range of motion.
This means that the resistance increases along with the range of motion. This is the example of the ascending resistance profile.
Now, let’s take a look at barbell bicep curls. Our elbow flexors are creating torque to oppose the resistance of the dumbbell and mass of the forearm, which both don’t change their force magnitudes.
The moment arm at the elbow is the largest at 90 degrees, and smaller before and after reaching this position. This scenario is the bell-shaped resistance profile.
As the last example, let's take the sissy squat. This time the exercise is purely bodyweight. Looking at the knee joint, It turns out the moment arms become smaller with the range of motion while obviously the magnitude of force dependent on our body weight will stay roughly the same. And remember, we always look at the concentric (lifting phase).
This means that the overall resistance decreases along with the range of motion. This is basically a descending resistance profile.
A U-shaped resistance profile is actually very difficult to create in classic exercises. I can't think of any exercise that has this profile. Unless we are doing a lot of mechanical maneuvering, it is primarily seen in exercises that involve antagonistic muscle groups together in a single repetition of the exercise.
The calisthenics example of such an exercise is the 360° Pull, which involves the back lever and front lever pulls together. The middle rest position, which is the inverted hang, is the least mechanically challenging, and the challenge increases as we move to the back lever or the front lever.
This is probably the only popular exercise I can think of. Frankly, I never use exercises like this in training myself or my clients, but that's a topic for another time.
