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BIOMECHANIC ANALYSIS OF ARCHER ALIGNMENT

2.2 FORCES AT THE BOW SHOULDER - ANGLE B

All forces applied at the bow arm are transferred to the bow shoulder. This includes the moment, and the compressive force along the arm. Figure 2.2-1 shows the forces and the reactions at the bow shoulder.

The compressive force is applied to the shoulder by either, direct compression skeletally into the torso and shoulder girdle, or through lateral loading on the shoulder. The lateral forces try to push the shoulder to the back, and out of alignment. This requires muscular effort to resist. Here, angle B is critical, since it determines what portion of the load is directly transferred skeletally at the shoulder, and what portion requires muscular control.

Bringing angle B to 0 degrees, as shown in Figure 3.2-2, eliminates all adverse lateral loading. This takes the lateral muscular support out of the equation, and makes the form more stable, and more consistent.

2.3 FORCES AT THE DRAW HAND - ANGLE C

Figure 2.3-1 shows the Basic Trapezoidal Form, with the draw arm aligned with the line of force. For archers in this form, the forces at the draw hand are simple. There are only two: the force of the bow pulling in one direction, and the force of the draw arm pulling equally in the opposite direction.

But, as we saw before, few archers actually attain this form. The real form that most archer attain is more like that shown in Figure 2.3-2. In this form, note the angle C at the draw hand, between the draw arm and the line of force. This angle C can be very detrimental to an archer’s hold and release.

Let’s discuss why this angle C is such a problem. Figure 2.3-3 shows a close up of this location.

At the nocking point, the bowstring pulls on the draw hand. If angle C is 0 degrees, then all loading is in line tension along the draw forearm, and the system is as simple as possible. Holding an angle at this point requires that the hand be pulled in to the chin. This requires a lateral force at the hand, that is applied by a moment about the draw elbow. This moment is powered by the bicep. Most archers are not aware that they are pulling in with the arm, but in order to hold this angle it is required and it is real.

TECHNICAL NOTE – Offset Load at the Draw Hand:

For an archer holding a mere 5-degree angle and a 40-pound bow, this requires over 3 pounds of force. For the archer in an Open Form with a 10-degree angle, this force is as much as 7 pounds.

Holding an angle at the draw hand is even more detrimental than at other locations. Not only does it require an adverse lateral force, to hold the bow at full draw, but it also adds another muscular activity, that must be coordinated with other actions, at the point of release.

2.3.1 FURTHER PROBLEMS WITH ANGLE C

The lateral force caused by angle C, creates additional problems associated with angle C. These occur at the wrist. Figure 2.3.1-1 shows the draw arm and wrist.

As stated earlier, the force FC, must be applied through bicep action, and this applies a moment to the forearm about the elbow (M2). The bones of the forearm end at the wrist. If the wrist is kept relaxed then it bends back awkwardly, as in Figure 2.3.1-2

Holding the wrist straight, as shown in Figure 2.3.1-3 requires wrist tension. This tension applies a pre-stress to the hand that can greatly complicate the release process.

2.4 FORCES AT THE DRAW ELBOW - ANGLE E

Angle E, The angle at the draw shoulder, is a dependent angle. The archer should not make this angle a priority in his alignment. It is simply an angle that results from aligning the other angles.

This is one of the few angles that does not provide a benefit by reducing it. In fact, if angle E becomes too small, it makes it very difficult to provide draw hand motion to move through the clicker.

Figure 2.4-1 shows the basic forces at the draw elbow. The force of the upper arm rotating about the shoulder, pulls the elbow back. The force pulling back on the elbow must equal the draw weight of the bow. But the elbow is pulling through an arc, so the actual rotational force of the elbow is pulling at an angle to the line of force, and the required rotational force is therefore greater. When the angle E is at 90 degrees, all of the rotational force is pulling straight back, and the rotational force equals the draw force. As angle E gets smaller the component of the rotational force that is pulling back against the bow gets smaller, so the rotational force must increase.

Angle E must be coordinated with the other angles, and the physical capabilities of the archer. If it is too large, it will take other angles out of alignment. If it becomes too small, the archer will not be able to pull through the clicker.

TECHNICAL NOTE – Draw Force as a Function of Angle E:

Angle E, for most reasonably ‘in line’ archers, ranges between 35 and 55 degrees. The more in line the archer, the smaller this angle becomes. Perfectly ‘in line’ archers are often at a minimum value, between 35 and 40 degrees. Some archers, in the full Open Form, will find this angle as high as 60 to 70 degrees.

The required rotational force is inversely related to the SIN of the angle. This means that, for a good, inline archer, in the 35 – 40 degree range, he is required to pull rotationally about 1.50 to 1.75 times the draw weight of the bow.

2.5 FORCES AT THE DRAW SHOULDER – ANGLE D

Figure 2.5-1 shows the draw shoulder. This is an ‘active’ joint. It provides the motion, and the energy, that draws the bow. This angle D, at the bow shoulder, is dependent upon the angles E and B, so angle D is a secondary angle. When the archer adjusts angles B and E, the final angle D results.

TECHNICAL NOTE – ANGLE D:

Angle D, for most reasonably ‘in line’ archers, ranges between 40 and 60 degrees, with 45 degrees being a typical angle. Some archers, in the full Open Form, find this angle as high as 70 - 75 degrees.