Document emg58ZOq1d6mJ4pmv0Xw9kv4y

96 BOSS A. MC FARLAND limits that engineering specifications should never exceed. The ranges of 24 differ ent voluntary bodily movements of the head, upper arm, forearm, hand, thigh, lower leg, and foot are presented in Table 7. The movements are defined in terms of angular degrees for the mean, the standard deviation of the. distribution, and the probable error of the mean, All measurements in this table were made by Glanville and Kreezer on a group of 10 subjects.118 b. Forces Exertable in Various Body Positions. The positions of the different parts of the body from which the maximum forces can be exerted have been deter mined by Reijs by means of a dynamometer measuring changes in muscular power during movements. His data may be summarized as follows: The foot can exert its maximum power in a plantarflexion (or downward) movement when the angle of the ankle joint is about 78; as the angle increases, the power decreases. For the hand, palmarflexion is a more powerful movement than is dorsiflexion, the average power ratio approximating 10:6 with the maxi mum for palmarflexion at a wrist angle of about 156 and for dorsiflexion at about 168. In the abduction of the leg (movement out from the body) the power drops sharply as the angle of the leg increases from 0 to 50. In adduction, or movement towards the 'body, the power is greatest at a position of 32 to 48 and drops slightly as the angle decreases. For the arm, adduction is a more powerful movement-than-is-abductiQn.Jlha..fsrmer_is_greatest_between 34 and 68, while thelatter is lowest between these same figures. Flexion of tKe arm at^tlTeTsKou'Wer'' gives greater force than does extension, having its maximum between 68 and 102. Extension power is greatest at 102. Trunk lifting power also was investigated and was found to be least when the trunk is nearly horizontal. There is a consistent increase as the trunk becomes more erect.69 c. Bate and Accuracy oj Body Movements. When linear body movements are made as rapidly and accurately as possible, both the average and the maxi mum velocities of movement increase as the length of the movement increases (at least up to 16 inches, the maximum distance studied). Variability in accuracy and speed~Slso~increaserwith-theTength-of-the-mo.vement.l0_ The percentage, of error in thecacettrS(i$ of* movements is greatest at short distances and decreases asitihe distance increases. There is, however, a slight tendency to overreach at short distances and underreach at longer distances. It was noted that the accuracy with which a movement-canjbe>.stopped..at.a..desjred^point is nqt greatly influenced -by the distance from the body at which the stop is attempted as long as the length" " J. H. O. Reijs, fiber die Veranderung der Kraft wahrend der Bewegung, Pflugers Arch, ,ges. Physiol, 191 234-357 (1921). " A. T. Slater-Hammel and J. S. Brown, Discrete movements in the horizontal plane as a function of their length and direction, Rept. No. t on Project S, Contract N5ori-57, Special Devices Center, October, 1047. HUMAN ENGINEERING AND INDUSTRIAL SAFETY 97 Mihmpvement is constant.61 For subjects who are trying for accuracy as well as -jwgfcin making movements confined to specific limits the mean maximum speeds were found to be 33,96,,and 242 cm. per second for distances of 2.5,10, and 40 cm., Respectively.82 Right-to-left movements with the right hand are slightly faster |Ka%are left-to-right movements. The time to complete movements with the '?ppiferred hand is a little less than for two-hand movements, although no'.differ- in accuracy.88 studying hand-motion speeds for an industrial task where accuracy was hfmfble and movement distance was confined, Barnes found somewhat similar w';85, 160, and 221 cm. per second for 20.3, 40.6, and 60.9 cm., respectively.64 Ii!i^lata would indicate that maximum speed will probably not be above 300 chnisfgpsecond if the movements are limited to distances of less than about 60 cm. landfifcaccuracy in stopping is required. However, if movements are unrestricted o'n/cxtent and accuracy but speed is stressed, the rate may well exceed 600 cm. ^effSecond. . B|or winding-type controls the maximum speed at which operators can turn" Tiandflii'eels and still match pointers in a continuous task appears to be about 200 fflkipiin., although large-size handwheels, increases in friction, inconvenient ||ning, and awkward hand grip will all tend to lower this maximum speed. ,,yl);;gWthere is an optimal speed band for each situation within which minimum Eijtoccurs, a speed of 180 r.p.m. may be given as a general optimal figure repre"roteiany different conditions.86___ fgjffit'ilfX-l'ccuracy oj Operation with One or Two Hands. No important difference taction time of the hands has been observed.86 However, among \y!||jp&ded people the right hand appeared to be superior to the left in perform- ^^asks'where the criterion is strength. The preferred hand is also superior to the -lii^m^eferred hand in tasks requiring continuous, accurate control.6! The use of 't lb" for a continuous control task is better thanya one-hand control (preit-k ^gjffohpreferred hand) in certain cases where acchfacy is the criterion, but ffpjt&Sfrce can probably be minimized by changes ' in'the equipment or in the ve techniques used in operating the equipment. Although the evaluation tlS-r'lSrown and E. B; Knauft, Positioning reactions of uniform length executed at ^teiianfrom the body.. Rept. No.,4 on Project g, CoPtraotr'NSori-WJJulyr 1948. ^jS^icpjtiP'and A. T. Slater-Hammel, The effect of speed-up. instructions upon the tj^fofmahjeidf.discrete movements in the horizontal plane, Rept. No. S on Project S, Contract "nlim,op'--'5S7f';'T,.Taanruiuaarrvy, 19048ft. .. Brown, A. T. Slater-Hammel, and E. A. Bilodeau, Characteristicsrof discrete move- jjjffihthe'i-h'orizontal plane when executed with one and with two hands^Rept,.Ah). 6 on f^fc"i^ntjract'N5ori-57, August, i9*48. `"s B -,MjsBarnes, Motion and Time Study. Wiley, New York, 1949. mtji W^jffioxljoro Company, Foxboro, Mass., Handwheel speed and accuracy of tracking, 1943. ~;R'-'|||^b6dworth, Experimental Psychology. Holt, New York, 1938. f igEjlspn and D. R. Craig, A comparison of one-handed and two-hanfiedtracking, S ^n MCRFXF>-891i-iM, Air Materiel Command, USAF, n.d.