THE REP PROPER FORM
The key to the perfect rep follows straight from the old cliche: a picture is worth a thousand words.
It's not enough just to know "the biceps flex the elbow" or "the quadriceps extend the knee". To optimize exercise form, you need to picture exactly how each muscle acts... what line of force and planes of motion are involved, and how the muscle works against resistance. This is not difficult, but it's essential.
Lines of force
Many muscles, like the biceps or triceps consist of muscle fibers all running in the same direction and so these muscles contract along straight lines too. These muscles are called straight muscles. There is a second kind of muscle, called a fan-shaped muscle. Fan-shaped muscles, like the pectorals( the fibers of which fan out from the shoulder across the chest) act like several straight muscles arranged in a fan. Each segment of a fan-shaped muscle contract along a single line.
For example, the pectorals act as if they are divided into tree segments, upper, middle and lower. Therefore you must do three movements of three separate exercises to fully develop them. Incline bench for the upper pecs, supine bench for the middle pecs and decline for the lower pecs. The important point here is the idea of a single line of contraction per muscle segment.
To generalize: although a fan-shaped muscle can contract along several line in effect just like a straight muscle in contracts along one line at a time.
This line is called the line of force. The line of force is always parallel to the muscle fibers in a muscle segment. There is only one line of force per muscle segment.
Levers and resistance
A lever actually has four parts. In your body, bone functions as lever arm, joint, as fulcrum, muscle pull provides force, and whatever you are pushing or pulling against provides resistance.
Each muscle/joint/bone system in an individual lever.
Now, every lever must be positioned properly to do its job. A car jack, for instance, won't work if you put under the car at an angle. The weight of the car ( resistance ) is pushing straight down, and unless the jack is pushing straight up you're going to have difficult time lifting the car off the ground.
Likewise, for every exercise, there is a particular body position a particular orientation of the muscle /joint/bone lever(s) involved that lets a muscle push or pull most directly against the external resistance.
This is called an exercise's Ideal orientation.
Any deviation from ideal orientation decreases the effectives of the exercise by diminishing the stress on the target muscle segment.
For example, when doing a bicep curl, the biceps's line of force is parallel to the upper arm in this case, essentially straight up and the direction of resistance is straight down. if you put your elbow out to the side and do the curling motion, the line of force for the biceps follows the upper arm, and is no longer aligned with the resistance; they are at right angles to one another.
As far as working the bicep is concerned , one of these orientations is clearly more efficient than the other.
Exercises employing the ideal orientation minimize joint stress and wasted energy and maximize concentration on the target muscle group.
An optimal exercise for a given muscle will always involve the ideal orientation it will always align the line of force with respect to resistance. sounds simple enough. But it's surprising how many "standard" exercises break this rule.
Planes of Motion
Just as each lever system has an ideal orientation (elbow facing straight down) defines the ideal plane(Plane perpendicular to the ground).
Indeed, an efficient free weight biceps curl is one where all moving body parts move within the plane described above. If, for example, your elbow is off to the side the plane you are stressing it in a way you shouldn't be and you are decreasing the effect of the exercise on your biceps.
You may be thinking this was an awfully long way around to describing an exercise you probably already knew how to do correctly . But for many other exercises, especially those involving more than one joint action (like most Pull-down and pressing motions), the ideal lines and planes are not so obvious.
In fact in many cases the "common knowledge" way to do the exercise is just flat out wrong. And the most effective way to discover that is to visualize in terms of planes of motion and lines of force.
In the body each bone is like a door, and its corresponding muscle is like spring. And just like the door and spring, each muscle can bring more force to bear on its associated bone at certain angles than others. That's why you have different levers of strength depending on the orientation of the bone lever. For example, when you do a curling motion you are the strongest just shy of one-half the way through. When you reverse the motion and extend your arm, you are strongest closer to the beginning off the move (specifically, when upper-arm to forearm angle is about forty degrees).
Each joint action (bending the elbow, straightening the elbow, etc.) Has its own resistance curve. This curve is a picture of the associated muscle's strength at every angle in the joint's range of motion.
Likewise, each exercise has it's own resistance curve. The resistance curve is a picture of how heavy the weight feels at every point during the exercise. (Depending on the angle between the direction of resistance and the bone lever, the weight you're lifting _feels" heavier or lighter.)
Every exercise has its own resistance curve.
An efficient exercise pits a muscle against resistance that varies directly with the muscle/bone system's strength curve. More strength, more resistance . Less strength, less resistance.
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