Finally, do all exercises follow this diagram's logic, or are there ones that have an equal contraction throughout the whole movement (no peaks at beginning, middle or end but equally easy or difficult throughout the ROM)? So this would be like a flat horizontal line on the diagram I'm guessing.
Your diagram is missing a flat strength curve.
There are four different strength curves that I'm aware of:
- Flat (cable resisted curl)
- Bell Shaped (barbell curl)
- Ascending Linear (incline curl)
- Descending Linear (band resisted curl)
**Note: You do not have to do this in every single workout. It is not 'necessary,' but it is a good idea to add variety over the course of a macrocycle of programming. **
In fact, that might be a bad idea to do it all in one workout, depending on how you've subdivided your training.
If you had to use all of them in every workout, you'd have to spend quite a bit of time training. It is not necessary for all of them to be present in any given workout, but it might be a good idea to spread that out between two workouts if hypertrophy is the goal. At the very least to utilize them all within a training cycle (macrocycle: about 6-12 months).
What you're describing about strength curves certainly exists but how it affects training is a little more complicated. For instance it's likely far more relevant to muscle hypertrophy than muscle strength.
It appears to influence muscle hypertrophy in one or all of the four following ways:
- Different exercises alter the proportional contribution of the active and passive elements within the muscle/tendon (so you may get fascicle lengthening or thickening for instance)
- Different exercises potentially affect how much stretch-mediated hypertrophy can occur for a given movement (length-tension relationship, you get more hypertrophy at muscle ends working at longer muscle lengths, so the incline curl might yield more hypertrophy at the ends of the muscle, while the barbell might yield less stretch mediated hypertrophy)
- It might determine which region of a muscle is most stimulated by an exercise, and thus where hypertrophy occurs (however, not as much research exists to support this one)
- Muscles don't work in isolation, so varying the exercise may alter the proportional involvement of each muscle involved in that movement.
Strength is more specific.
I think it's important to note that strength curves shouldn't be your only consideration. Progressive loading is the most important consideration but joint position and range of motion also alters muscle hypertrophy.
That being said, movement variability does seen to support development to specific strength. i.e. doing rack pulls if they improve a weak area of your deadlift, will likely improve your deadlift.
From what I've read in the literature movement variability in general is also a good idea. Mostly for joint health and coordination/neurological function. Most people anecdotally will report feeling better about training with more variability, as a general rule. Doing the same stuff over and over again, tends to beat up on your body and increases injury potential.
Movement variability is defined as the normal variations that occur in motor performance across multiple repetitions of a task. All of your examples are an arm curls, with subtle differences in leverage, so they all provide some variability to the same movement.
I'd encourage you to vary the movements you use semi-regularly for health and variety. Approx. once every month or two. Perhaps err to longer if hypertrophy is the goal.
can all exercises be modified to alter their peak contraction to another fase of the strength curve?
Yes, for the most part you just change the tool being used to train the movement.