King's College, Newcastle-upon-Tyne

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1 353 J. Physiol. (I949) io8, I2.74I:6I2.222 THE METABOLIC COST OF PASSIVE CYCLING MOVEMENTS BY J. A. SAUNDERS From the Department of Physiology and Biochemistry, The Medical School, King's College, Newcastle-upon-Tyne (Received 12 July 1948) Benedict & Cathcart (1913), in their comprehensive work on the efficiency of cycling, used various base-lines for determining net efficiency. One of these was the metabolic rate of the subject sitting on the ergometer while the legs were moved passively by means of a motor which rotated the pedals. They were not satisfied that their results represented truly passive movement and placed little emphasis on them. Lindhard (1915), commenting on this paper, contended that such leg movements, if truly passive, would involve no calorie expencditure at all, although he produced no experimental evidence to support this view. Hill (1922) considered that the metabolism, while the pedals are being driven by a motor, is the most suitable base-line for use in calculating efficiency during cycling, and used Benedict & Cathcart's figures in his estimate of the actual maximum efficiency of human muscle. Garry & Wishart (1931) are of the opinion that it is impossible to determine the real efficiency of the effector muscles in any human effort, owing to the difficulty of getting an accurate base-line. As no experimnental work has been done on this point since that of Benedict & Cathcart, the observations reported here were made in an attempt to determine the metabolic cost of truly passive cycling movements at two rates of pedal rotation. APPARATUS AND METHODS The machine (an 'exercycle') consisted of a triangular bicycle frame with an electric motor which rotated the pedals. The pedals could be left free or engaged at 'slow' or 'fast' speeds. The 'slow' speed was 56 r.p.m. and the 'fast' speed 74 r.p.m. One revolution means a complete (3600) revolution of the pedal axle, both legs undergoing flexion and extension. It was possible to adjust the positions of the saddle and handlebars in relation to each other to suit the comfort of the subjects, but unfortunately the distance from saddle to pedals was fixed and could not be altered. The subjects' feet were kept on the pedals by means of straps. The metabolic rate of the subjects was determiied by the indirect method, using a Douglas bag (60 1. capacity) and a mouthpiece

2 354 J. A. SAUNDERS with rubber flap valves, which were tested before use. The gas analysis was done with 10 ml. Haldane apparatus. Analyses were done in triplicate by two individuals. The volumes of gas in the Douglas bags were measured on a 'wet' type flowmeter. Plan of experiments. The experiments were carried out starting lj-2 hr. after a light meal, and the routine given below was carried out in all cases, except that the length of time of collecting expired air varied from 4 to 7 min., depending on the ventilation of the subject. hr. min Subject sits on 'exercycle'. Pedals still Mouthpiece in Start collecting expired air: resting sample 'before' (5 min.) MIouthpiece out. R6tation of pedals started Mouthpiece in Start collecting expired air: fi,st work sample (5 min.) Start collecting expired air: second work sample (5 min.) Mouthpiece out. Pedals stopped Mouthpiece in Start collecting expired air: resting sample 'after' (5 min.) Mouthpiece out. The mouthpiece was not retained by the subject during the whole experiment to avoid discomfort and soreness due to salivation, while 15 min. was allowed for adjustment of breathing through the mouth before a sample of expired air was collected (Campbell, Douglas & Hobson, 1921). Subjects. The subjects were all niedical students and their physical measurements are given in Table 1. All subjects appeared normal although subject 5 subsequently developed pulmonary tuberculosis. Particular efforts were made to ensure that the subjects did not have to endure any TABLE 1. Physical data of subjects Age Weight nude Height Subject (years) Sex (kg.) (cm.) 1 25 M F M M M avoidable discomfort. The saddle in which they sat was broad and shaped, and although for one subject padding was provided, the other four were comfortable without it. During the rest periods the pedals were fixed in a horizontal position. A reading desk was attached to the handlebars and the subjects were encouraged to read, in order to obviate boredom. Small movements were not prohibited, but any obvious restlessness was noted and its cause removed. All the subjects volunteered the information that they liked acting as subject as they were able to read quietly and with concentration during the experiments. The mouthpiece was supported by a clamp which took the weight of the mouthpiece and attached tubing and could be moved into any position. The Douglas bags were suspended from above. 'Computation of calorie expenditure. The computing of the metabolic rate was done in the usual way, the composition of inspired air being deduced from the percentage of nitrogen expired, using Carpenter's tables (1939). The values used for kg.cal./l. oxygen consumed at different R.Q.'S were those of Cathcart & Cuthbertson (1931), which are based on the composition of human body fat. RESULTS The total number of observations made on the five subjects was 122, and details are given in Table 2 of the experiments which gave minimum values for the metabolic cost ofpassive movement, while Table 3 gives the corr'esponding

3 COST OF PASSIVE MOVEMENTS 355 maximum and minimum values. The minimum metabolic cost may be due either to a high metabolic rate before and after or to a low value during passive movement. Both factors may play a part. Table 3 shows that the minimnum values reported in Table 2 are derived from a low 'worklanetabolism and a low or average resting metabolism. The last column in Table 3 gives a measure of the effect of discomfort on the metabolic cost. The high maximum values for subject 1 were probably due to 'keeping time' with the pedal -movement. TABLE 2. Minimum metabolic rate (kg.cal./hr.) Pedals driven Metabolic cost by motor at of passive Rest 56 r.p.m. Rest movement Subject (a) (b) (c) (d)=b-i(a+c) * * * r.p.m *4 58: *8 5 79* *3 TABLE 3. Minimum and maximum metabolic rates (kg./cal./hr.) Pedals driven Rest by motor Rest Metabolic cost Subject (a) (b) (c) (d) =b - j(a +c) *1 68*2-89' * * * * * *2-114* *3 The upper figures in columns (b) and (d) give the range of values at 56 r.p.m., the lower figures at 74 r.p.m. It will be noted that the metabolic rate at rest after the passive movement is less than that at rest before the movement. This was found quite consistently and is presumably due to the elapse of a further 11 hr. since the previous meal. As the samples during passive movement are taken half-way through this period, the metabolic cost of the passive movement is computed by subtracting the mean of the values at rest before and after the movement from the metabolic rate during the passive movement. The values given are, as far as it is possible to judge, close to the true values for passive leg rotation with the exception of that for subject 2 at 74 r.p.m. In

4 356 J. A. SAUNDERS * this case the subject found the movement very difficult indeed, aiid despite considerable effort was unable to relax to a degree subjectively comparable to that achieved at the slower rate. DISCUSSION The metabolic rate determined during passive leg rotation will vary inversely with the degree of relaxation achieved by the subject. Any resistance on the part of the subject to the pedal movement or any tendency to 'keep time' with the pedals will result in a greater muscle tone and a greater calorie expenditure. It follows, therefore, that, providing the subject co-operates effectively in maintaining relaxation of the leg muscles, the minimum value recorded will give the most satisfactory measure of the cost of truly passive movements. Comparison with other values for passive cycling movements. It is difficult to compare the results reported here with those of Benedict & Cathcart, as they report only one experiment with a change from rest on the cycle to passive rotation of the legs on the same day, and the results of this experiment are unreliable because of belt slipping. They emphasize (1913, p. 118) that not only did their subject find the passive movements difficult, but even (pp ) that he became strained and uncomfortable while sitting on the cycle at rest. In the photograph given in Benedict & Cathcart's paper, the subject has-his trunk inclined forwards, his arms bent at the elbow, and appears to be supporting the weight of his trunk mainly with his arms. My subjects were sitting upright and did not use their arms at all as a means of support. The position of the subject, which is determined by the structure of the bicycle, is probably an important factor in the comfort of the subject and the cost of passive movement. The mechanical efficiency of cycling. The 'exercycle' used in this work could not be used as an ergometer, so that it was not possible to find the efficiency of the subjects. Garry & Wishart (1931) determined efficiency, using rest on the ergometer as the base-line, and found 20-0 and 21-1 % at 52 r.p.m. and 18-2 and 18-7 % at 70 r.p.m. for their two subjects. Adding the metabolic cost of passive movement reported here for subject 1 (who most closely resembles Garry & Wishart's subjects in size) to their base-line values, the efficiencies become 20-7, 21F7, 19-2 and 19-4%. Similarly, Campbell et al. (1921) found a mean efficiency at 50 r.p.m., for two subjects at two loads, of 24-4% using rest on the ergometer as base-line. This is increased to 25-2 % by adding the value reported here (subject 1) for passive movements. The cause of the increased metabolism during passive movements. The metabolic cost of passive leg rotation is positive and increased by increasing the rate of leg movement. The cost of this passive movement may be due to (a) local factors in the legs, (b) changes in the rest of the body.

5 COST OF PASSIVE MOVEMENTS 357 The local factors are (1) friction in the joints, (2) viscosity of tissues, including muscle, and (3) variation in muscle tone. In passive movement the force needed to overcome friction and viscosity is provided by the motor and would not appear as extra metabolism. It is probable, however, that some of this energy will be degraded to heat and will stimulate metabolism due to a local rise in temperature. Although this factor seems unlikely to be important in the present experiments, it would increase in importance at high rates of leg movement. Some degree of tone exists in the muscles of any conscious subject, and stretching of these muscles during passive movements may result in increased metabolism. Changes in the rest of the body caused by the leg movement may be due to the extra effort involved in maintaining the erect sitting posture, or to increased circulatory-respiratory movements resulting from the movement or from increased metabolism produced by the movement. The extra effort required to maintain the sitting position during passive leg movement can only be assessed subjectively and was not noticed by any of the subjects except subject 2 at the higher rate. In this case the difficulty was probably due to the fact that subject 2. was much shorter than the others, and that her legs were fully extended at the lowest position of the pedals. With this exception, the maintenance of posture did not seem to be an important factor in the metabolic cost at the moderate rates of leg rotation used. Passive leg movements may increase respiration reflexly (Comroe, 1944) or by increased C02 production. The circulatory system may also be affected. In these experiments no records of circulatory changes were made, but the ventilation increased in all the experiments reported in Table 2 except with subject 4 at the slow rate, when it decreased. In this instance the extra metabolism must have been due to other causes, but in the other experiments the extra muscular effort involved in the increased ventilation certainly contributed to the increased metabolism. SUMMARY 1. The metabolic rate of five subjects was determined before, during and after passive c*ycling movements, using motor-driven pedals on a special type of cycle. The metabolic cost of the movement was found by subtracting the mean of the values before and after from the value during the leg rotation. 2. The minimum metabolic cost in five subjects of passive cyclingmovements at 56 r.p.m. is between 7 and 15 kg.cal./hr./subject and at 74 r.p.m. between 19 and 26 kg.cal./hr./subject. 3. Using these figures as a base-line increases the value for mechanical efficiency in cycling by less than 1 % compared with a base-line of rest on the cycle.

6 358 J. A. SAUNDERS 4. The increase in metabolism is probably due to increased tone in muscles and possibly to increased respiration and the extra effort required to maintain posture. The author is grateful to Prof. D. Burns for his interest in the work, to the subjects for their patient co-operation, and especially to Mr A. Bone, whose technical skill and care contributed so much to the success of the experiments. A grant from the Research Fund of King's College is acknowledged with thanks. REFERENCES Benedict, F. G. & Cathcart, E. P. (1913). Publ. Carneg. Instn, no Campbell, J. M. H., Douglas, C. G. & Hobson, F. G. (1921). Philos. Tran8. B, 210, 1. Carpenter, T. M. (1939). Publ. Carneg. Instn, no. 303 B, 3rd ed. Cathcart, E. P. & Cuthbertson, D. P. (1931). J. Phy8iol. 72, 349. Comroe, J. H., Jr. (1944). Phy8iol. Rev. 24, 324. Garry, R. C. & Wishart, G. M. (1931). J. Phy8iol. 72, 425. Hill, A. V. (1922). J. Physl. 56, 18. Lindhard, J. (1915). Pflug Arch. ges. Phy8i. 161, 233.