BradG, to continue to impart my knowledge for the greater good, I've stumbled on more studies:

ELECTROMYOGRAPHIC EVIDENCE
An EMG or electromyographic recording of muscle activity is often done to determine the extent of muscle involvement for a particular movement. An EMG signal is a recording of a muscles electrical activity from electrodes placed on the skin or within the muscle belly itself.

Using this technology, a group of Australian scientists examined the effects of different bench angles and grip widths on muscle activity around the shoulder. For example, in comparing two functionally distinct regions of the pectoralis major muscle (i.e., clavicular head and sternocostal head), they found that the clavicular head of the pec major (the part attached to the collarbone or clavicle) was more active during a narrow grip vs. wide grip, esp. during the flat and incline bench press. Relative to the sternocostal head, the clavicular head was more active during a narrow grip incline bench. On the other hand, the sternocostal head of the pec major (part attached to the breastbone or sternum) showed its greatest activity during a flat bench; however, if you wanted greater relative involvement of the sternocostal head vs. the clavicular head, a decline press with a narrow grip seemed to work best. And for best maximal involvement of the entire pectoralis major muscle, the wide grip flat bench resulted in the greatest measured electrical activity!

Another group of scientists examined how different heads of the biceps brachii muscle are activated during supination movements. If you've forgotten, supination is a movement that occurs around the radioulnar joint and occurs when you rotate your forearm such that your palms face up. And the biceps brachii is the primary muscle involved in supination! These scientists showed that when the elbow was flexed to 120 degrees, the short head (medial side) was activated more so than the long head of the biceps during supination. And the further the elbow was extended, the more the long head of the biceps came into play. Thus, if you want to stress the medial side of the biceps muscle, do supination movements at 120 degrees of flexion. Most gyms now have machines which allow isolated supination movements. So give it a shot; it'll be a good addition to your arm workout.

Further evidence for the functional differentiation within the biceps muscle is magnetic resonance imaging which demonstrate that a standing bilateral dumbell curl with your palms up (supinated) hits the medial or short head of the biceps brachii muscle more so than the lateral or long head. On the other hand, doing the same exercise with a neutral grip (slightly pronated) results in better activation of the lateral or long head.

And what about the notion that you can train your upper vs. lower abs? Here's an area where there is a big dispute concerning the role of the ab muscles. Many believe that the rectus abdominus is one muscle (which is correct) that is activated equally when you contract it (which is incorrect!). A study done at the University of Valencia in Spain compared the average EMG activity of the upper and lower abs during a curl-up and posterior pelvic tilt exercise. They found that it is true that the stomach crunch or curl does elicit greater rectus abdominus activity in the upper abs while doing posterior pelvic tilt exercises hits the lower abs better as long as they're performed correctly!

MUSCLE GROWTH!
OK, EMG signals are fine and all, but does this mean that muscles can grow in specific regions. Or does a muscle hypertrophy by generally enlarging all parts? We know in animals that stretch overload will cause greater growth in the proximal and distal region while fiber number is greater in the middle region vs. other regions of the muscle belly. Also, that rats undergoing hypertrophy of their plantaris muscle (a plantarflexor) showed the greatest growth in the middle and distal region with the least in the proximal region. Furthermore, we now have evidence in humans that regional differences exist with regards to muscle growth.

In a study done at the University of Texas Southwestern Medical Center in Dallas, twelve weeks of training the elbow flexors in elderly men resulted in regional response with regards to hypertrophy. The greatest increase in cross-sectional area occurred in the distal belly of the elbow flexor muscles with little if any change proximally (near the origin of the muscle). This demonstrates that muscle does not respond in a uniform manner! In fact, you could say that training seemed to produce a more "lop-sided" muscle!! Is this a normal response? Did a specific exercise cause this? It's hard to say since these subjects performed various arm exercises (barbell curls, dumbell curls, hammer curls, etc.).

Moreover, a Japanese group examined five men after 16 weeks of unilateral triceps brachii exercises (consisting of the French press exercise) found that maximal muscle growth occurred at the distal end of the arm vs. the middle and proximal regions.

A study done in the United Kingdom examined ten young adults (5 men and women) and had them perform leg extension exercises concentrically with one leg and eccentrically on the other leg. They trained three times per week for twenty weeks doing four sets of ten reps with a minute rest between sets. The measured the cross-sectional area of the quadriceps muscles at two levels: at 25% and 75% of the femur's (thigh bone) length measured from the knee joint. Interestingly, both the concentric and eccentrically trained leg produced increases in muscle cross-sectional area, but only in the upper region of the quad with no change occurring closer to the knee. Although, it is generally accepted that it is the eccentric part of a muscle contraction that is "more important," it is apparent that at least in previously untrained persons, concentric contractions alone may provide a sufficient hypertrophic stimulus.

In a similar study done at the University of Nebraska Medical Center, they trained previously untrained male college students to perform six sets of 10 reps of unilateral knee flexion/extension and elbow flexion/extension of the non-dominant limbs concentrically on a isokinetic device three times a week for eight weeks. The contralateral limb served as the control. They found that there was regional hypertrophy within the same muscle! For instance, the elbow extensors (triceps brachii) experienced growth at the proximal and middle levels but not distally (near the elbow) with the greatest changes occurring in the middle. For the leg extensors (i.e., quads), only the rectus femoris (at all three levels), the vastus lateralis (middle level), and vastus intermedius (middle level) increased in cross-sectional area. For the leg flexors (hamstrings), the biceps femoris (middle level) and the semitendinosus (distal level) increased in size with no change in the semimembranosus.

It isn't clear why certain muscles grow while others did not. Furthermore, it is difficult to make comparisons between studies since they often use different exercise protocols and different technology to analyze changes in muscle growth. It isn't clear why within the same muscle, only certain parts grow. But what is clear is that you do not get a generalized of muscle in response to exercise. For instance, the University of Nebraska Medical Center study demonstrated how difficult it was to induce growth in one of the hamstring muscles, the semimembranosus. Why is this muscle such a hardgainer? Is there a better exercise for that part of the hamstring muscle group?

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