THE POST‐CONTRACTION PROPRIOCEPTIVE REFLEX, ITS AUGMENTATION AND INHIBITION
Authors: Allen Frank , O'Donoghue Chas. H. 
Published: December 2014

1. Isometric voluntary contraction in a skeletal muscle is followed by an involuntary post‐contraction of appreciable magnitude and duration in the same muscle. The muscle or rather muscle‐group chosen is the deltoid levator complex, in which the phenomenon has been previously studied. A simple apparatus and means of procedure is described whereby the intensity and duration of the initial stimulus of the voluntary contraction and the magnitude and duration of the post‐contraction can be accurately measured. The simple nature of the experiments renders them suitable for class purposes and, it is hoped, provides a new method of attack upon the problem of muscle contraction.

2. A definite relationship between the intensity and duration of the initial stimulus and the magnitude and duration of the post‐contraction has been established. This relationship can be expressed by the equation A = k log W + c, where A is the angle traversed by the arm in the post‐contraction rise, W is the weight employed for the initial stimulus, and k and c are constants. The normal curve obtained is remarkably uniform for each individual and exhibits four divisions termed α, β, 13, γ, and δ, and three deflexion points termed I., II., and III.

3. If a normal curve be taken, and then after a rest of 20 minutes a similar series of readings be repeated with the same arm, the curve they yield is raised noticeably above the normal throughout, although of the same type. This indicates a direct, ipselateral augmentation of the end‐results.

4. If a normal curve be taken and then immediately a similar set of readings repeated with the same arm, the resultant curve is significantly below the normal throughout, although of the same type. This indicates that the phenomenon described in 3 is preceded by a direct ipselateral inhibition.

5. By plotting the means of from three to six sets of readings, employing the same procedure as for the normal curve, only varying the length of stimulation in each group of sets, it is shown that an initial stimulation of 10 seconds produces the highest and most satisfactory curve in the type of experiment chosen. Shorter or longer stimulation gives a lower curve, and that for 30 seconds is anomalous in certain respects.

6. By stimulating the left arm immediately before taking a reading with the right the resulting curve falls below the normal, showing that the inhibitory effect is also contralateral. This is crossed inhibition.

7. By stimulating the left arm and waiting 17 to 20 minutes before taking each reading with the right, the curve obtained is considerably above the normal, showing that the augmentor effect is also contralateral. This is crossed augmentation.

8. It is shown that if instead of considering the angle traversed by the arm as a measure of the end‐result we take the actual work done against gravity in raising the arm, the work‐curve as produced is of the same type as the, normal curve.

9. A review of the principal investigations into post‐contraction is given.

10. Evidence is adduced to show that voluntary isotonic contraction has probably a similar series of after‐effects, and furthermore, that all these results, both direct and crossed, are to be regarded as proprioceptive reflexes.

11. For the first time is given a direct mathematical statement of the relationship between initial stimulus and the magnitude of post‐contraction. This is A = k log w + c and is practically a statement of what is generally known as Fechner's law, which has previously been demonstrated to hold good in sight, hearing, taste and touch. Such a result may be regarded as evidence that we are dealing with a sense, the muscle‐sense, of the same nature as the other senses. Certain other comparisons are drawn.

12. Inhibition and fatigue are to be distinguished from one another. The following types of inhibition are recognised and defined: direct, transferred, direct crossed, and transferred crossed. The occurrence of four similar types of post‐augmentation inhibition is considered possible but has not been demonstrated. The following types of augmentation are recognised and defined: direct, transferred, direct crossed, and transferred crossed. Post‐inhibition augmentation is defined and suggested as a possibility and also its subdivision into the same four types, but it has not yet been demonstrated.

13. The sequence of the above manifestations decidedly suggests the swing of a pendulum or beam of a balance in coming to rest, and for this widespread phenomenon the name oscillatory effect is proposed. The time‐interval between initial stimulation and the taking of the subsequent readings is of paramount importance in the interpretation of the results.

14. The normal curves for sight, hearing, taste, touch, and the muscle‐sense all consist of several branches which are here compared. Stimuli of the intensity represented by the lowest part of the curve, ie. a, produce opposite results in sight and taste (inhibition) as compared with hearing, touch, and muscle‐sense (augmentation). Correlated with this is the fact that in the first two the left‐hand side of the equation representing Fechner's law is 1/d, whereas in the latter group it is D. Stimulation of β intensity reverses the results in the two groups. It is pointed out that failure to recognise the various sequelæ enumerated in 12, the time‐interval between initial stimulation and taking the readings, and the opposite effects of stimuli of different intensities, are responsible for many of the contradictory results recorded by previous observers.

15. The present investigations provide an explanation of a number of well‐known results, and three of these are taken as examples, namely, the muscle‐fatigue curve, the curve of work, and the Bidwell effect. The meaning of adaptation or attunement is pointed out. It is also noted that the oscillatory effect has been observed in the senses previously studied, and, indeed, in many unrelated phenomena.

This investigation is one of a series of researches relating to colour vision and other sensory actions now being conducted under the direction of Professor
Frank Allen, with the financial assistance of the National Research Council of Canada, to which body we desire to express our thanks.