Re: Contraction Theory
Well, I agree with your friend in that regardless of what angle you train the muscle at (e.g. flat vs. incline bench vs. flyes), you are contracting all the fibers in that muscle, not just the "upper ones" for inclines, etc. Some might interpret this to mean that you could just do, say, 16 sets of bench press instead of doing different pec exercises. However, I strongly disagree with this approach for several reasons.
1.) First of all, in your case, you used the example with your calves and how the way you walked on them changed their shape. Well, don't forget you dont just have a single calf muscle, you have the larger gastroc and soleus, as well as several smaller muscles in the calf. You can contract each of these different muscles to varying degrees, and only by hypertrophy of all of them will your calf reach it's fullest potential size-wise. Now I'm not saying you should do a separate exercise for each of these muscles, as they mostly function as a group, not to mention that the quantity of type II fibers in some muscles is limited. The same concept applies to the quads, which are made up of the vastus lateralis and medialis, rectus femoris, and sartorius.
2.) Many people don't realize the importance of training on your neurons. The strength with which your muscle contracts is related to the strength of the impulse it receives from the brain. Training a muscle group with varying exercises will keep the neurons excitable, and over time you will improve the efficiency of muscle fiber recruitment for a given nerve impulse.
3.) For most [free weight] lifts, there are multiple supporting muscle groups recruited to help the main muscle group with balance, leverage, etc. An example is the role your rotator cuffs play when doing bench press. Hitting a muscle with a variety of exercises will develop those muscles, too, both their strength/size, as well as the efficiency of their recruitment (based on neural pathways... see #2). This will allow you to lift more weight, which will (hopefully) allow you to build more muscle.
There are other reasons, too, which I don't have time elaborate upon right now. But as you can see, regardless of the fact that I think your friend's theory is probably acurate, I personally train with multiple exercises due to the above logic.
--Blindsight
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