Gray (1944) analysed the pattern of muscular activity in mammals, but there is practically no published experimental data which could be used to check this analysis. Elftman (1938, 1939a, b, c) working with man, and Manter (1938), working with the cat, developed various types of apparatus for recording the forces exerted on the ground, but Manter’s published results give adequate data for the calculation of patterns of muscular activity for a single stride only. Following similar work on the Amphibia (Barclay, 1946), an attempt was therefore made to record the forces exerted on the ground by various mammals and to obtain synchronous photographs of the limb positions with a view to calculating the activity in the main groups of limb muscles.

The apparatus used was a development of that employed with Amphibia (Barclay, 1946) and is shown in Fig. 1. Its main advantage over previous types of apparatus is that it practically eliminates rotation and tilt, and there is therefore no need to ensure that the feet are placed centrally on the platform.

The experiments were carried out on three dogs of different types, two goats and two sheep. All the records conformed generally to the same type, and although some variation was found in detail in individual strides, the general pattern of forces at the feet was constant. In a normal stride only one limb at a time was actually on the platform. An example is given in Fig. 2, where the axis of the limb was taken as the line drawn between the centre of the part of the foot applied to the ground and the centre of articulation of the limb with the body. In the hindlimb the centre of articulaticon is the centre of the acetabulum, and its position was easily fixed in photographs in relation to bony prominences on the body. In the forelimb the glenoid cavity moves considerably because the scapula moves against the body; here the centre of articulation was taken to be the centre of rotation of the scapula, and this was harder to fix exactly in the photographs.

The photographic records showed the position of the limbs and the forces exerted on the ground at the same moment in three axes at right angles. From the angle of inclination of the limb and the vertical force (i.e. proportion of the weight) exerted on the ground at any stage it is possible to calculate the longitudinal forces which would be exerted on the ground if the limb were a rigid strut and the extrinsic limb muscles were not active. This horizontal force may be called the horizontal strut effect or horizontal effect of the weight. In this series of experiments the photographs did not show the angle of inclination of the limbs laterally Tut only longitudinally. Calculations could therefore only be made on the horizontal forces in the longitudinal axis, and the horizontal forces referred to in the discussion are all longitudinal horizontal forces.

The difference between the calculated horizontal effect of the weight (Hw) and the horizontal force actually recorded at the foot (HR) is the horizontal effect of the activity of the extrinsic limb muscles (HM) or, as it has been called, the horizontal lever effect. Clearly HR=Hw+HM. The force records when co-ordinated with the photographs of the limb positions therefore give a measure of the activity in the extrinsic limb muscles. Since the height above the ground of the centres of articulation of the limbs with the body did not vary greatly, HM is a fairly accurate measure of the total couple exerted on the limb by its extrinsic muscles.

A series of typical calculations of this type on the fore and hind limbs of a dog, sheep and goat are shown in the Tables below. In both the fore and the hind limbs a constant pattern of muscular activity was observed, except that the exact point at which the change took place from a retractor to a protractor couple varied within narrow limits, probably in accordance with the degree of acceleration or deceleration of the animal ; it was, however, always shortly after the foot had moved to be behind the centre of articulation of the limb. The records are for normal walking.

A few records were also obtained with goats standing with one foot on the platform. Normally the goat always stands with its two fore feet in front of the scapula and the two hind feet behind the acetabulum. In this position the horizontal forces at the feet were smaller than the horizontal component of the weight (Hw) and demonstrated a small retractor couple on the forelimbs and a small protractor couple on the hindlimbs. This is the same pattern of muscular activity as that shown in walking.

The lateral forces were generally less than the longitudinal forces, but since there was no record of the lateral inclination of the limbs, no calculations could be made of the pattern of adductor or abductor muscle activity.

These results could be summarized by saying that a group of extrinsic muscles is active (i.e. exerts a couple greater than its antagonists) when the foot is on the opposite side of the proximal limb joint to the origin of the muscles concerned. The exception to this is that for a short time after the feet are behind the joint the retractors are still active in both fore- and hindlimb. This, however, only lasts while the feet are so nearly vertical that the actual horizontal forces remain low. The pattern of muscular activity is therefore always such as to maintain the horizontal forces at the feet at a very low level. Usually it reduces them to a figure very considerably below the horizontal component of the weight or the horizontal force which would be developed by any other pattern of muscular activity. These generalizations are not affected by certain erratic readings such as those shown in the tables for the dog and sheep.

Calculation on results given by Manter (1938) for the cat, and Elftman (1940) for man, show the same pattern of extrinsic muscular activity. It is also the same as that shown to be present in the toad and newt (Barclay, 1946) and demonstrated by Gray (1944) and Barclay (1946) to be in almost every respect the most efficient possible mechanically.

  1. An apparatus is described for measuring the forces exerted on the ground by mammals in three axes at right angles.

  2. Analysis of these force records and synchronous photographs of the limb positions shows a constant pattern of activity in the protractor and retractor muscles of both limbs.

  3. This pattern of activity is basically the same as that previously demonstrated in the toad and newt and shown to be mechanically in most respects the most efficient possible.

I wish to thank Prof. J. Gray for constant advice and help and Mr K. Williamson for technical assistance. The work was done while holding a Coutts-Trotter Research Studentship at Trinity College, Cambridge.

Barclay
,
O. R.
(
1946
).
The mechanics of amphibian locomotion
.
J. Exp. Biol.
23
,
117
203
.
Elftman
,
H.
(
1938
).
The measurement of the external force in walking
.
Science,
88
,
152
3
.
Elftman
,
H.
(
1939a
).
Forces and energy changes in the leg during walking
.
Amer. J. Physiol.
125
,
339
56
.
Elftman
,
H.
(
1939b
).
The rotation of the body in walking
.
Arbeitsphysiologie,
10
,
477
84
.
Elftman
,
H.
(
1939c
).
The force exerted by the ground in walking
.
Arbeitsphysiologie,
10
,
485
91
.
Elftman
,
H.
(
1940
).
The work done by muscles in running
.
Amer. J. Physiol.
129
,
672
84
.
Gray
,
J.
(
1944
).
Studies in the mechanics of the tetrapod skeleton
.
J. Exp. Biol.
20
,
88
116
.
Manter
,
J. T.
(
1938
).
The dynamics of quadrupedal walking
.
J. Exp. Biol.
15
,
522
40
.