How do my muscles work in certain movements?

Ok, I am in a Statics course right now and am also hitting the gym hard trying to put on a few pounds. While doing bench press I got to thinking. How the heck do my pecs come into play when pushing the weight straight up? My pecs are pulling force that is perpendicular to the downward force of the wieghts, same goes for my tri’s. All of my pec force should be pulling on my shoulders basically. So why does the wieght move up?

The way I imagine it the movement for a bench press is the same as trying to move something to the right while only pushing forward on it. My pecs pull on my shoulder area and my tris try to straighten out my arms. I can understand ho your tri’s come into play after you get your arms past a certain angle, but I dont see how the pecs actually move the weight upward.

Are the muscles of the human body just in extremely inefficient positions for these exercises? The forces just don’t seem to be pointing in the mest directions to make the most of our muscles.

Pushing something to the right while only moving forward isn’t so hard if

  1. you’re not pushing on the object, you’re pushing on a stick that is connected to another stick that pushes the object. for exampe, imagine you were pushing a slanted object forward, the slant would push the other object to the side.
  2. also that other minor muscles can help the levers and such keep aligned correctly.
  3. the joines between other levers can be rigid or flexible at your choice.

sound reasonable?

Pushing something to the right while only moving forward isn’t so hard if

  1. you’re not pushing on the object, you’re pushing on a stick that is connected to another stick that pushes the object. for exampe, imagine you were pushing a slanted object forward, the slant would push the other object to the side.
  2. also that other minor muscles can help the levers and such keep aligned correctly.
  3. the joines between other levers can be rigid or flexible at your choice.

sound reasonable?

The pecs move your arms in towards your chest. Depending on how you’re doing the bench press, they’re either not doing much, or providing most of the work.

If you’re just moving your arms up and down, then it’s all triceps and anterior deltoids. If you’ve got your arms out at the bottom of the press, then your pecs are involved.

Nonetheless, you’d be hard-pressed to bench with just your pecs. Ever seen a pec fly? That’s what your pecs do. The bench requires your pecs, triceps, and anterior deltoids all moving together in a certain way. The shoulder press involves those same three muscles*, but since they move in a different way, it’s a different motion.

As a general rule, there are very few movements which involve only one muscle or muscle group.

  • The bench press uses the sternal head of the pectoralis major, while the shoulder press uses the clavical head. Although they’re not technically the same muscle, they are both considered part of the pectoralis major, and that’s good enough for me.

It is one muscle, just different ends.

The clavicular portion of the pectoralis major is primarily a flexor, but it also adducts the humerous. When the arm is elevated sideward to a position slightly above shoulder level, however, the line of pull of some of the fibers of the clavicular portion shifts from below to above the sagittal horizontal axis of the shoulder joint. Contraction of these fibers in this position contributes to abduction of the humerus, rather than to adduction. The clavicular fibers of the pectoralis major are significantly active in abduction at the level of 110 degrees. The sternocostal portion is generally antagonistic in its actions in the sagittal plane: it acts in downward and forward movements of the arm and in medial rotation when accompanied by adduction.

It’s a misperception that there is some physiological reason for a muscle to pull in a single direction. A muscle merely contracts and cannot pull in a predetermined direction. This misconception allows for the lack of understanding of seeming exceptions. There are many movements in which the insertion or distal attachment of the muscle is stationary and the origin or proximal attachment is the one that moves, such as chin-ups. The movement of the elbow is flexion, but it is the upper arm that moves toward the forearm, just the reverse of what happens when you lift a book.

Huh. I was under the impression that different heads of a muscle were actually separate muscles, just grouped together for ease of classifying. Is that always false?

I like everyone else’s answers better. Ignore mine please!

As far as the inefficiency of the muscles is concerned -

They are attached so as to maximize (mostly) the ability to move your extremities fast, not necessarily strongly. The closer your pectorals are attached to your elbows, the more strongly they can bring your elbows forward, but the closer they are attached to your shoulders, the faster your elbows will move for a given force.

I think muscles are levers of the third class, which have no mechanical advantage, but are used in situations in which you want speed rather than great force.

Regards,
Shodan

Some muscles have more than one head and each head is a separate muscle, such as the two muscles in a bicep, the three muscles in a tricep, and the four muscles in a quadricep. However, the pectoralis major does not have separate head, just (as every other muscle) an insertion end and an origin end. The distal end is considered the insertion end and the proximal end is considered the insertion end.

barbitu8: What’s a good resource for muscular anatomy, aimed at a scientifically-literate person outside of the medical field?

A good book, but makes for dry reading, is Kinesiology, subtitled “Scientific Basis of Human Motion” by Kathryn Luttgens and Katharine F. Wells, exercise physiologists. Published by CBS College Publishing (The Dryden Press) and initially copyrighted in 1950.