Experimental studies on posture and load carriage in man

A study has been made of the skeletal muscular activity associated with maintaining the erect posture in Man, using twenty medical students as experimental subjects. The extent and nature of this muscular activity was assessed by the mechanical end-result it produced, using for this purpose a strain gauge accelerometer mounted on a freely-suspended and optimally-damped platform. Evidence was found for the existence of continuous 10 o.p.s. oscillatory forces exerted at the feet during normal standing. The extent of these oscillatory forces showed considerable subject-to-subject, and day-to-day variation, but they were nevertheless always present. It is considered that these forces result from skeletal muscular activity. It was also found that ischaemic pain was readily produced in the leg muscles of subjects standing normally, whereas over a similar period of time, ischaemia produced virtually no painful sensations if the subject was lying down. This finding was regarded as evidence that the erect posture is associated with active muscular contractions in the legs. It was concluded from this study that the erect posture in Man is a dynamic state, associated with continuous muscular activity, especially in the lower limbs. Gravity is a constant force, and continuous activity by the postural muscles is required to counteract its effects. It is denied that normal standing in Man is dependent primarily on the mechanical arrangements of the skeleton, or the passive elastic tension in the postural muscles, as has been claimed. It is believed that some workers, such as Clemessen (1951) & Ralston & Libet (1956), have tended to place too much emphasis on the findings of electromyography of the postural muscles, and have been led into the false position of delving the existence of active muscle tone. The findings of the present experiments would suggest that there is good evidence that the upright posture in Man i» maintained by low-grade and continuous muscular activity in the anti-gravity muscles ("postural tonus").
A study was made of the postural sway occurring during normal standing in ten subjects. The procedure adopted involved measuring the continuous oscillations of the centre of gravity of the body. It is considered that measuring the movement of the centre of gravity is the most logical way to measure the postural sway that occurs during standing in the erect posture. It was found that b far the greatest amount of sway occurred in an antero-posterior direction; with the foot position adopted in these experiments (heels set 20 cms. apart), very little lateral sway was observed. To obtain an estimate of the angular displacement of the body occurring at the waist during postural sway, an inclinometer was mounted on a waistbelt. It was found that the angular displacement measured was considerably in excess of the angular displacement estimated on the assumption that the body swayed rigidly between the ankle joint axis and the centre of gravity. It was concluded from this observation that, contrary to what has been commonly assumed, the body must sway in a pliant manner over the ankle joint axis, and not as a rigid column. If the postural sway is measured by detailed planimetry of the records, and the results expressed as a mean deviation of the movement of the centre of gravity from a fitted baseline, the relative constancy of the degree of body sway is noteworthy. In the subjects examined, the difference between the minimum and maximum antero-posterior sway recorded (expressed as a mean deviation of the movement of the centre of gravity) ma lass than 35 mms. It is concluded that the body is remarkably successful in resisting the effects of gravity, and in maintaining the normal static equilibrium associated with the erect posture.
The effect of load carriage on the normal standing posture was studied using a minature camera photographic technique. The displacement from the vertical of four marked anatomical landmarks (right shoulder, hip, knee and ankle) was measured on enlarged negative images, during normal standing and when loads of 12 kg. and 24 kg. were carried. It was found that there was a highly significant forward displacement of the shoulder with load carriage, while there was no significant change in the position of the hip. There was a highly significant backwards displacement of the knee, while the position of the ankle was not significantly affected. A weight of 24 kg. was found to cause further forward displacement of the shoulder when it was carried low on the back than when it was carried high on the back. It was concluded that a highly significant realignment of the body position takes place during load carriage above a certain minimum weight, thus contradicting the findings of previous workers who had claimed that the body adapted itself to load carriage without significant realignment of the body segments. The suggestion was made that the trunk acts as a kind of counterbalance, altering its inclination according to the position of the load on the back, so that the vertical projection of the combined centre of gravity to the foot remains relatively undisturbed.
The antero-posterior displacements of the centre of gravity of the body (postural sway) of ten subjects was studied under varying load conditions. The rate of working of the muscles controlling postural sway was arbitrarily assessed on a mechanical basis, using a strain gauge accelerometer mounted on a freely-suspended platform. It was found that carrying a load of 12 kg. in a position high on the back caused a significant increase in the amount of postural sway of the body, and also in the rate of working of the muscles controlling the sway. The addition of another 12 kg. caused further significant increases in both the sway and the rate of working of the muscles.
The movement of a 24 kg. load from a position high on the back to a position low on the back, over a distance of 30 cm., did not significantly affect either the amount of postural sway recorded, or the rate of working of the postural muscles. It was concluded that the site of a load on the back did not affect the amount of work that was performed by the muscles maintaining the erect posture. The body simply altered its normal alignment according to the position of the load, so as to keep the vertical projection of the centre of gravity at the foot in a relatively constant position.
It is concluded that the maintenance of the upright posture in Man is a dynamic process, and should not be regarded as primarily a passive phenomenon. Gravity is always threatening to upset Man's postural equilibrium, and this equilibrium depends upon, and is largely controlled by, the continuous contractions of the postural muscles. The interplay between gravity and the restraining contractions produces the normal postural stray, whioh is usually remarkably small in amount. During load carriage, wherever a load may be portioned on the back, the body adapts itself to one position of the load as rail as to another, and succeeds in keeping the vertical projection of the centre of gravity relatively constant. It is suggested that this successful adaptation is achieved by the pliant manner in which the body reacts to the stresses of gravity and load carriage.