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BIOMECHANIC ANALYSIS OF ARCHER ALIGNMENT

0.0 INTRODUCTION

Most archers muddle through their form without giving much thought to the physics and theory of archery, and without giving much regard to the finer details. They simply settle in on a form that feels comfortable, push it a little bit toward something that they believe is a little better, and base most of their ideas and thinking upon unsubstantiated information that they have picked up around the archery range, from people who know little more if any than they themselves know.

Biomechanics allows us to approach the archer’s form differently. This scientific approach looks at the body like an engineering structure, and with this approachwe can analyze the archer’s form and alignment mathematically. With this approach we can understand what aspects of the form are physically and structurally better, and why. To accomplish this we must develop a model of the archer based upon solid principles of structure and the human body.

The purpose of this manual is to lay out what is important in this biomechanical model, and show how these parameters are quantified. This will help us to understand why certain forms and forms are better than others. Using this information, the archer will know better where to prioritize his efforts, and will understand what problems stem from what errors.

0.1 SIMPLIFYING ASSUMPTIONS

This discussion will be based upon a simplified model of an archer. This is done to keep the discussions as understandable as possible, while still conveying all of the important points. The purpose of this discussion is to obtain a mechanical understanding of the overall principles, and not to provide detailed descriptions of the biological systems. For this reason, all of the assumptions are general, and are intended to provide the overall concept, in a clear manner, so we can establish a mathematical basis to develop a model to analyze archer form and execution.

The joints, particularly the shoulders, are much more complex than this simplified model suggests, but the basic forces and reactions are still the same, and will still lead to a valid understanding of the statics and mechanics involved.

In this discussion the horizontal forces and the vertical forces and angles are segregated. This is standard to static analysis of systems, and helps to simplify the discussion. In actuality, the force along some members is a combined affect of both vertical and horizontal angles, however, the simplified 2-dimensional discussions shows all of the important trends, and relative affects of the various angles, without the added complexity of a single 3D model.

0.2 LINE OF FORCE

In this discussion the term ‘Line of Force’ is used often. This is the line of force of the bow. This line of force is a direct line from the pressure point of the bow hand on the grip, to the nocking point on the string. It is a line from the direct points of contact between the archer and the bow, and is the line of force that the archer applies to the bow and the line of force that the bow applies to the archer.

When viewed from them top this line of force is directly along the arrow. When viewed from the front the line of force is a line from the bow hand to the draw hand and is at a slight angle to the arrow. This is sometimes confusing to archers, but one must remember, when drawing the bow, and holding, at anchor, the archers is not pulling back the arrow, the archer is pulling back the bow. It is the bow that is applying a force back on the archer, and not the arrow.

1.0 THE BASIC FORM

Figure 1.0-1 shows the typical archer, from above, standing in the Basic Trapezoidal Form. With some basic variations to account for body proportions, this is a standard configuration for which most archers strive.

This is a good basic posture, but it still has a few flaws. Beyond these basic flaws, even this form is rarely truly attained. Most archers would be surprised at how far they are from this basic form. Because of physical dimensions, and lack of effort, most archers stand in a form more like that shown in Figure 1.0-2. It’s close, but there are reasons that it is inferior.

Even for those archers who do achieve what they consider the Basic Trapezoidal Form, shown in Figure 1.0-1, there are problems inherent in this form; it is not ideal. Thus, these archers are working hard to get to a form where they still have errors.

The Basic Trapezoidal Form is a good place to begin discussing form because it is relatively neutral form, with better and worse forms, and because it shows clearly all the body angles we need to consider when looking at archer form. Figure 1.0-3 shows the range of forms for the upper body, arms, and shoulders.

NOTE: In discussing Open and Closed forms in this discussion, we are only referring to the upper body (Rib cage, Shoulder girdle, Arms) and not to the feet and hips in the stance. These are discussed in detail in another manual.

Lets study this basic form in the context of the critical angles and learn more about the forces involved.

2.0 BASIC FORM AND ANGLES

Figures 2.0-1 and 2.0-2 show the basic archer, from above, and from the front.

For the archer at full draw there are 7 particular areas and associated angles of concern

(Figures 2.0-1 and 2.0-2):

Top / Horizontal:

A – Angle at the bow hand between the arrow line and the bow arm line.

B – Angle at the bow shoulder between the bow arm and the shoulder line.

C – Angle at the draw hand between the draw arm line and the arrow line.

D – Angle at the draw shoulder between the draw arm and the shoulder line.

E – Angle between the upper draw arm and the line of force.

Front / Vertical:

G – Angle at the draw hand between the draw forearm line and the line of force.

H – Angle at the bow between the bow arm and the line of force.

Another angle of interest which is driven by the riser design rather than by the archer form, is:

K – Vertical Angle between the line of force and the arrow.

When an archer stands at full draw, he is trying to hold his form in as precise, stable and repeatable of a form as possible. This is most easily accomplished if the major loads are carried skeletally by the bones in direct compression. This takes the muscles and strength out of the equations, and thereby takes the muscular variability, and muscular fatigue, out of the equation.

Taking the loads skeletally, and not muscularly, requires that the following angles be brought to 0 degrees:

B – Angle between the bow arm and the shoulder line.

C – Angle between the draw arm line and the arrow line.

G – Angle between the draw forearm line and the line of force.

Most archers find that with proper effort and normal body proportions these angles can reasonably be reduced to near 0 degrees.

The following angles can theoretically be brought to 0 degrees but a certain angle is preferable, in order to assist in holding the weight of the bow..

H – Angle between the bow arm and the line of force.

The following angles cannot be brought to 0 degrees, because of physical restrictions. Instead, these angles should be reduced as much as is reasonably possible:

A – Angle (horizontal) between the line of force and the bow arm line.

K – Angle (vertical) between the line of force and the arrow.

The following angles do not need to be brought to 0 degrees and in fact are often at an advantage when they are larger.

D – Angle between the draw arm and the shoulder line.

E – Angle between the upper draw arm and the line of force.

2.1 FORCES AT THE BOW HAND AND BOW ARM - ANGLE A

Lets start at the bow hand. This is at the intersection of angle A, the angle between the line of force and the bow arm. Figure 2.1-1 shows this point in close up.

At the grip, the bow pushes against the bow hand. This force is in the direction of the line of force of the bow. In order to allow for string and arrow clearance, the bow arm is slightly out of line with the line of force. This results in the angle A. Since the bow arm and the line of force are at an angle, the force of the bow is broken down into 2 constituent parts:

1 - the major force (FA) that is taken compressively into the arm.

2 – an adverse lateral force (FH) that wants to pull the hand to the front of the archer. This adverse lateral load must be countered, by a moment about the bow shoulder, and this requires muscular effort.

The balance of these two forces, and their reactions is determined by the angle A. Clearly if the angle A is 0 degrees, then all of the force of the bow is countered by compressive forces of the bow arm, and no muscular effort is required. But at this point a small angle is required, to allow for string and arrow clearance, so this angle can only be minimized, and never eliminated.

TECHNICAL NOTE – Adverse lateral loads at the bow hand:

As angle A increases, the amount of force taken directly by the skeletal structure decreases, and the amount that must be supported by the muscular moment at the shoulder increases. Most archers find this angle A to be relatively small, about 10-15 degrees. While the sine value at this range is relatively small, it is still critical since the long moment arm of the archer’s bow arm multiplies the force required by the shoulder.

As a result, even at these small angles, there can be significant muscular loading. For instance, for the average archer this 10 - 15 degree angle, translates into a range between 17% and 26 % of the bow force being carried by a moment at the shoulder. For an archer pulling a 40-pound bow this means a force of 7 – 10 pounds.

A 7 – 10 pound load, supported at full arm length, is not and insignificant load, and can create a considerable variability, and fatigue. Clearly, it is best to keep this load as small as possible, and this requires that this angle be kept as small as possible.

But how small is it possible to make angle A? This varies with arm length, chest depth, anchor position, and other physiological factors. The smallest that I have ever seen this angle successfully reduced to, is 5 degrees. Most people should be able to get it down to 8 degrees or less, without compromising other factors.

The limiter to reducing angle A is usually string clearance on the chest and arm. Thin, long armed people, find it easier to get this angle down to 5 degrees. Shorter / bulkier archers, often find 8-10 degrees to be the minimum, before chest clearance becomes a problem.