Training Principles Series: Strength Curves and Resistance Profiles
One of the major factors that determines the efficiency of your resistance training is exercise selection. In turn, one of the major factors that contributes to good exercise selection is understanding the intricate relationship between strength curves and resistance profiles with any given exercise. These concepts are essential for designing effective training programs to maximize training adaptations and performance. This article delves into the definitions and applications of strength curves and resistance profiles, and what else you need to consider when evaluating an exercise based on these principles. Read to the end to see examples of how to program with these considerations in mind.
What are Strength Curves?
Strength curves represent the varying force output of a muscle throughout its range of motion (ROM) during an exercise. Essentially, they’re a graphical depiction how the strength of a muscle changes at different lengths.
Like the graph above shows, a muscle will always be weakest whenever it moves toward either end of the spectrum of muscle length (fully shortened or fully lengthened), and will be the strongest somewhere in the middle.
How do you figure out a muscles strength curve?
In order to get an accurate understanding of the strength curve of a muscle it’s important to understand where exactly it’s end ranges are. However, it’s important to note that when looking at different individual muscles within a muscle group, there are nuances in their structure and physiology that can complicate the length-tension relationship conversation.
For the sake of this article, we’ll use the triceps as an example. Since the triceps cross multiple joints (the elbow and the shoulder), they have multiple functions that need to be taken into account, which are elbow extension and shoulder extension. When you’re performing a triceps pushdown and your elbow is flexed, you might feel a stretch in the triceps, but that does not mean that it’s anywhere near fully lengthened. Technically, in order to fully lengthen the triceps, you need to add in the component of shoulder flexion as well - that’s where overhead triceps extensions come in.
This is where the nuance starts to show up though, since the triceps has 3 heads that all have different origins (short heads originate on the humerus and long head originates on the shoulder blade, all converging to cross the elbow joint as one tendon). For the sake of simplifying things to illustrate the point, we’ll just focus on the long head of the triceps.
Here are the three positions in the strength curve:
Fully Shortened Triceps
To fully shorten the triceps you need to have full elbow extension as well as shoulder extension, like in the case of a triceps kickback (remember this for later).
Fully Lengthened Triceps
Conversely, in order to fully lengthen them you need to have elbow flexion and shoulder flexion (like the bottom position of an overhead triceps extension).
Midrange Triceps
Midrange would be somewhere in between those, which would be the case with cable triceps pushdowns where the elbow position is slightly ahead of the body.
This is important to consider because you want to use different loading parameters based on the exercise. I’ll get into this more toward the end.
Resistance Profiles
Resistance profiles describe the changes in resistance experienced by a muscle throughout the ROM of an exercise. It illustrates the varying levels of difficulty at different ranges throughout any given exercise, as influenced by several factors including the type of resistance used (free weight vs. machines vs. cables), the line of resistance, the position of the body relative to the line of resistance, and the biomechanics of the exercise.
Key Components of a Resistance Profile
Variability in Resistance: The profile tells us where the resistance (the load), is at it’s peak, and where it drops off. Let’s use free-weights as an example: when performing a biceps curl with a dumbbell, the resistance isn’t the actually dumbbell, it’s the force of gravity acting on the dumbbell. Since gravity is always going to be pulling in a straight line down toward the ground, the resistance is going to be the highest when the elbow is flexed at 90 degrees. At the bottom position of the exercise when the elbow is fully extended, there’s not any resistance because the biceps doesn’t have to resist the pull of gravity acting on the dumbbell and the forearm, and likewise at the top position again the resistance is very little.
Try it yourself, grab a 10lbs dumbbell and hold it in your hand down by your side, you can stay there seemingly forever. Now bring the dumbbell up and hold it up by your shoulder in the top position of the biceps curl, and again you’ll see that it doesn’t take much effort to stay there. Now let the dumbbell down and hold your elbow at 90 degrees, and pretty soon you’ll realize that you can’t stay there for long, which leads into the next point.
Mechanical Advantage: A muscles mechanical advantage will vary at different points in the range of motion of an exercise.
When using free weights and machines that don’t have a cable stack, the thing that determines the amount of load you’re using is the force of gravity acting on the weight. As a rule of thumb, whenever a weight is travelling more horizontally, it isn’t going to be affected as much by gravity (and will not be as heavy) as when it is traveling more vertically.
This can be easily demonstrated in the example of a plate-loaded T-Bar row. If you look at the image above, you’ll see that at the start of the row the weight will be moving vertically against gravity, but as you reach the top position the weight gets easier because the natural arc of the movement starts to make it travel more horizontally than vertically. This makes the bottom position the most difficult, and as you get to the top it’ll get easier and easier as the weight starts to travel in a more horizontal path, being less affected by the pull of gravity.
3. Equipment Design: Not all machines are created equal - the best machines take strength curves into account when they’re designed so that the resistance profile aligns properly with it. Machines can use cams to provide variable resistance, making certain portions of the exercise easier or more difficult to match the natural strength curve of the muscle it’s meant to target.
Putting it into practice
When examining the effectiveness of an exercise, you need to consider whether the strength curve matches the resistance profile. However, there are exceptions and I’ll get to that shortly. But first I’ll use an example of an exercise that just flat out sucks. Yes, I’m talking about triceps kickbacks. Let me explain.
If you recall, the triceps kickback biases a fully shortened position of the triceps, where the muscle will be the weakest. It just so happens that at the top position when your elbow is fully straightened and the triceps is fully shortened, the weight is going to be the heaviest (again because of the mechanical disadvantage of the muscle combined with the force of gravity acting on the weight being strongest in this position). For this exercise to be worthwhile you’d want the opposite to happen, where the difficulty drops off as the muscle moves into a weaker position.
Now for the caveat, there are plenty of exercises where the strength curve and the resistance profile don’t match up, but they are still really good exercises.
Exceptions to the rule: If the strength curve doesn’t match the resistance profile, but the exercise strengthens meaningful muscular co-contractions, then it’s still a good exercise. For example: the the bent-over barbell row has a resistance profile that doesn’t match the strength curve - however, this exercise requires meaningful co-contractions between multiple muscle groups in the shoulders, back, core, and hips, which is the characteristic that makes this exercise so good.
How to program for exercises that bias different parts of the strength curve?
For midrange exercises (where the muscle is going to be strongest) - the main variable you want to manipulate will be load.
For shortened exercises (where the muscle is going to be weaker) - the main variable you want to manipulate will be higher reps.
For lengthened exercises (where the muscle is going to be weaker) - the main variable you want to manipulate will be tempo.
Here’s an example
Exercise 1 - [midrange bias] - Bench Press - Use heavy weight and lower rep ranges (12 and less).
Exercise 2 - [lengthened bias) - Pec Fly - Use lighter weight and really slow eccentrics (3 sec minimum), pausing in that painful stretched position (it’s safe).
Exercise 3 - [shortened bias] - Clavicular Chest Press-around - Use a rep range between 15-20, you’ll really feel the burn with these.
That’s it for this topic. If you have any questions you can reach out to me on Instagram, by email, or by phone - I’ll be happy to assist any way I can.
