a difference of borderline significance (F = 1.55,
0.10 >p > 0.05 with 54 X 54 df), indicating that
the deltoid slope is slightly more variable.

The means of the right and left trapezial slopes
were also compared, and the difference was signif-
icant at the 0.5 probability level (¢= 2.19, 0.5 >
Pp > 0.02, 108 df).

V. Discussion.

The present study reveals no significant differ-
ences in the means, between the angles of the
trapezial and deltoid segments of the shoulder
profile; however, the smaller standard deviation
of the trapezial segment angle indicates that it
is slightly less variable and hence would be the
preferable measurement for use in future studies.

The statistically significant differences between
the means of the right and left trapezial angles
are of some anatomical interest. Hypertrophy
of the trapezius, as an examination of physical
culturists will reveal, results in a more acute
trapezial angle. The slight unilateral hypertro-
phy associated with handedness should also have
the same result; i.e. in right-handed persons, the
right trapezial angle should be slightly smaller
than the left. In the present sample, handedness
was not recorded, but we may assume it conforms

reasonably well with the findings of previous
workers,? in which left-handedness occurs with a
frequency of 8.6% in American males. That be-
ing the case, we would expect a slightly larger
trapezial angle in a predominantly right-handed
sample, as does, in fact, occur in the present series.

From a practical standpoint, such a small dif-
ference in the right and left trapezial angles is
not critical, and the weighted mean of the two
sides (67.5°) can be considered adequate for the
design of shoulder harness.

VI. Conclusions.

A. No significant differences exist between the
deltoid and trapezial angles of shoulder slope as
herein defined and measured.

B. A statistically significant differences exists
between the right and left trapezial angle and is
probably associated with handedness. From the
practical standpoint, the difference between right
and left trapezial angles is negligible from the
standpoint of shoulder-harness design.

C. The mean shoulder-slope angle (measured
from the vertical body axis) of normal males,
based on the weighted means of the right and
left trapezial segments of 55 ATC trainees, is
67.5° with a standard deviation of 5.0°.

REFERENCES

1. Svow, Crypx ©., and Snyper, Rickarp G.: Anthropo-
metry of Air Traffic Control Trainees, Civil Aero-
medical Research Institute Report. (In press).

2. Herrzesene, H.'T. E., Daniens, G. S., and CHURCHILL, E, :
Anthropometry of Flying Personnel—1950. WADC

06-3174

Technical Report 52-321, Wright Air Development
Center, Dayton, Ohio, 1954.

3. Karprnos, B. D., and Grossman, H, A.: Prevalence of
Left-Handedness Among Selective Service Registrants.
Human Biology, 25 :36-49, 1953.



AXIAL
REFERENCE LINE

DELTOID SLOPE
M. TRAPEZIUS

J TRAPEZIAL
M. DELTOID SLOPE

Posterior shoulder profiles showing trapezial (a) and deltoid (b) angles of slope.

IV. Results. Student’s ¢ tests were used to compare the

In the Table are presented the statistical results TMeans of the deltoid and trapezial slope angles of
of the present study. Values recorded range from the right shoulder. The results were not signifi-
52° to 88° in this comparatively small sample of cant (¢ = 0.83, 0.6 > p >0.5 with 108 df). How-
55, with means in the neighborhood of 67° and ever, an F test of the differences between the
standard deviations of 5.0°. standard deviations of the two techniques revealed

Table. Shoulder Slope Angles of 55 ATC Trainees

Measurement X 8B, 8 48H, Vv +80, Range

degrees degrees % degrees
R Shoulder (deltoid segment) 65.6 £08 5.8 +06 8.8 +0.9 52.0 — 80.5 55.
R Shoulder (trapezial segment) 66.4 +0.6 46 +04 70 £0.7 55.5 — 88.0 55
L Shoulder (trapezial segment) 68.5 +0.7 5.2 £05 TT £07 59.0 — 80.5 55
R+L Shoulder (trapezial segmet -- 675 £0.7 5.0 +0.5 74 £0.7 55.5 — 88.0 55





AM 65-14

THE ANGLE OF SHOULDER SLOPE IN NORMAL MALES
AS A FACTOR IN SHOULDER-HARNESS DESIGN

Clyde C. Snow, M. S.
A. Howard Hasbrook

Approved by Released by

Stele A Venohbam I df Wade

Srantey R. Mouter, M.D. M. S. Wurre, M.D.
Dmecror, CARI Feperat Arr Surceon

March 1965

FEDERAL AVIATION AGENCY

Office of Aviation Medicine
Civil Aeromedical Research Institute
Oklahoma City, Oklahoma



Qualified requestors may obtain Aviation Medical Reports from Defense Documentation
Center. The general public may purchase from Clearinghouse for Federal Scientific and
Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151


THE ANGLE OF SHOULDER SLOPE IN NORMAL MALES
AS A FACTOR IN SHOULDER-HARNESS DESIGN

I. Introduction.

A factor in the proper fitting and design of
shoulder harnesses is the angle of the shoulder
slope. Shoulder straps of relatively inflexible or
coarse material, if not fitted correctly, will tend to
concentrate the load toward one edge, thereby in-
creasing discomfort and the probability of func-
tional interference resulting from the compres-
sion of underlying soft parts, particularly nerves
and blood vessels that supply the arm. Pain,
congestion, and mild paralysis of the affected ex-
tremity can result from such compression and can
materially impair pilot performance by increas-
ing fatigue and stress and, in emergency situa-
tions, might critically affect strength for operat-
ing controls.

A survey of the literature fails to disclose data
on the normal value of shoulder-slope angles.
The present study was undertaken to provide such
information for use in shoulder-harness design.

II. Materials and Methods.

The materials for the present study were a
random sample of 55 from a series of some 500
somatotype photographs of male Air Traffic Serv-
ice trainees collected by the Anthropology Labo-
ratory of the Protection and Survival Labora-
tories of the Civil Aeromedical Research Institute.
Details of the composition of the overall collection
are described elsewhere.’ Here is will suffice to
note that it corresponds closely in age, stature,
weight, and other anthropometric variables to the
sample composing the Air Force Anthropometric
Survey of Male Flying Personnel conducted in
1950.2 All subjects had recently passed a physical
examination essentially equivalent to that given
annually to commercial airline pilots. Therefore,
both the overall collection and the subsample used
in the present study probably provide an adequate
representation of the male pilot population. The
mean age of the sample was 27.9 years; the
weight, 161.8 pounds; and the stature, 69.56
inches.

The measurements were taken from 5- 7-inch
negatives of the posterior view of each subject
in the standard somatotype pose: standing nor-
mally erect, with head in the eye-ear horizontal
and the arms held laterally with the fingers ex-
tended and the hands about 6 to 8 inches from
the sides. Care was taken in posing the subject
to see that the shoulders were in a relaxed and
natural position.

The shoulder profile of each subject, enlarged
to approximately normal body size, was projected
upon tracing paper, and the silhouette was traced
on the paper. Care was taken to alternate the
presentation of the negatives so that any sys-
tematic errors in projection technique would be
compensatory.

III. Definition of Measurement.

The profile of the shoulder is determined by
the contour of the underlying soft parts, the
skeletal structure and, particularly, by two. mus-
cles, the deltoid and the trapezius (Figure).

In the present study, the angle of shoulder slope
was defined as the angle between a line tangent
to the shoulder and the axial reference line (a
perpendicular connecting the midpoint of the
gluteal furrow with the center of the spine of the
seventh cervical vertebra).

This angle might, however, vary somewhat ac-
cording to whether the trapezial or deltoid seg-
ments, or both, were used to determine the line of
shoulder inclination. Therefore, on the right
shoulder, the angles determined by both of the
latter segments were measured so that statistical
comparisons of the two techniques could be made.
On the left shoulder, only the angle of the trape-
zial segment was determined.

In all cases, the measurements were made by
extending the slope line to its junction with the
vertical axis and recording the angle to 0.5° with
a drafting machine.