M. AYOUB (3) Speeds are normally
7. VARIABLES AFFECTING STRENGTH MEASUREMENTS
One procedure involved in strength testing which varies among tests is the manner in which the subject's representative score is determined. Some investigators select the best score while others use the average of several trials. Berger and Sweney (1965) and McCraw and Talbert (1952) report that reliability coefficents change if best scores are correlated rather than average scores. Henry (1967) and Jones (1972) dispute this, however. Test batteries vary in that some call for a standard testing order while others utilize a random testing order.
132 M. M. AYOUB
Another procedure involved in strength testing is the instructions given to subjects concerning how to exert maximal force. Kroemer (1970) points out that although few studies describe such instructions, they can affect the outcome of strength tests. Kroemer told subjects to exert force in one of three ways: exert and hold maximal force for five seconds; apply gradually increasing force until maximum is reached; apply maxi- mal force suddenly, twice. These three methods of applying force
resulted in distinctly different force curves. Further, Kroemer (1979) has found that subjects who are instructed to build up force rapidly achieve higher peak strength scores than subjects who are instructed to build up force slowly.
A number of other factors have been found to affect strength scores. Among these are the subject's motivation (Berger 1967;
Johnson and Neilson, 1967); the position of the body (Williams and Stutzman, 1959); environmental stimuli (Ikai and Steinhaus, 1961); and anthropometric characteristics of the subjects (Clarke, 1957). Cladwell, et al. (1974) has developed a standard procedure for testing static muscle in order to reduce variation caused by these factors.
In conclusion, there are several problems which must be addressed. These are: (1) a standardized procedure for dynamic strength, further refinements for the procedures for static strength. (2) Comparison between static and dynamic strength.
This is particularly important to establish the usefulness of each and their more appropriate applications. (3) To obtain data for specialized occupational population in addition to female data. Therefore, more effort should be given to provide these data. (4) To develop a program to evaluate the equipment designed particularly for training, but used for strength testing to determine its usefullness and hopefully establish some speci- fication for human strength measurement equipment.
REFERENCES
Asmussen, E.; Hansen, 0.; & Lammert, O. The relation between isometric and dynamic muscle strength in man.
Communications from the Testing and Observation Institute of the Danish National Association for Infantile Paralysis, 1965, 20.
Berger, R-. - A. and Sweney, A. B. Variance and correlation coefficients. Research Quarterly, 1965, 36, 368-369.
Bavard, J. F.; Cozens, F. ~.
&
Hagman, E. P. ~ests and measure- ment in physical education. W. B. Saunders Co. , Philadelphia, 1949.METHODS TO ASSESS VOLUNTARY EXERTIONS 133
Caldwell, L. S.; Chaffin, D. B. Dukes-Dobos, F. N.; Kroemer, K.
H.; Laubach, L. L.; Snook, S. H. & Wasserman, D. E. A proposed standard procedure for static muscle testing.
American Industrial Hygiene Association, 1974, 35, 201-206.
Chaffin, D. B. Ergonomics Guide for the Assessment of Human Static Strength. American Industrial Hygiene Association Journal, 505-511, July, 1975.
Clarke, H. H. Relationships of strength and anthropometric measures to physical performances involving the trunk and legs. Research Quarterly, 1957, 28, 223-232.
Doss, W. S. & Karpovich, P. V. Acomparison of concentric, eccentric, and isometric strength of elbow flexors.
Journal of Applied Physiology, 1965, 20, 351.
Flesihman, E. A. The structure and movement of physical fitness.
Prentice-Hall, Inc., Englewood Cliffs, N.J., 1964.
Hellebrandt, F. A. New devices for disability evaluation.
Archives of Physical Medicine, 1948, 29, 21-28.
Henry, F. M. "Best" vs. "average" individual scores. Research Quarterly, 1967, 38, 317-320.
lkai, M. & SteinhauS: A. H. Some factors modifying the expression of human strength. Journal of Applied Physiology, 1961, 16, 157-163.
Johnson, B. L. & Nelson, J. K. Effect of different motivational techniques during training and in testing upon strength performance. Research Quarterly, 1967, 38, 630-636.
Jones, R. E. Reliability of muscle strength testing under varying maturational condition. Journal of American Physical Therapy, 1962, 42, 240-243.
Kroemer, K. H. Human strength: terminology, measurement, and interpretation of data. Human Factors, 1970, 12, 297-313.
Kroemer, K. H. Personal communication, 1979. --
Larson, L. A. and Yocom, R. D. Measurement and evaluation in physical, health, and recreation education. C. B. Mosby Co., St. Louis, 1951.
McCraw, L. W. and tolbert, W. I. A comparison of the reliabi- lity of methods of scoring tests of physical ability.
Research Quarterly, 1952, ~, 73-81.
Neilson, N. P. and Jensen, C. R. Measurement and statistics in physical education. Wadsworth publishing Co. , Inc. , Belmont, CA,
1972.
Ramos, M. U. & Knapik, J. Instrumentation and techniques for the measurement of muscular strength and endurance in the human body. W.S. Army Research Institute of Environmental Medicine, Natick, MA.
Wasserman, D. W., Germann, T., Goulding, P.V. & Pizzo, F. An instrument for testing isometric strength and endurance.
HEW (NIOSH) Tech. Report 74-109, Nat'l Inst. for Occup.
Safety & Health, Rockville, MD (1974).
Williams, M. & Stuzman, L. Strength variations through the range of joint movements. Physical Therapy Review, 1959,
E..,
142-152.
POSTURAL CONSIDERATIONS IN MAXIMUM VOLUNTARY EXERTION
Don W. Grieve and Stephen T. Pheasant Biomechanics Laboratory
Royal Free Hospital School of Medicine Pond Street, Hampstead, London NW3, England
This paper is concerned with maximal manual exertions in the sagittal plane while standing; the whole body is therefore involved.
Body weight and a choice of posture in which weight can be used to advantage constitute one factor governing exertion. Its effect is analogous to the force that a log of wood, leaning against a handle, would exert in the Dead Axis (Fig.l), perpendicular to the line between the handle and the ground support. In the 'dead weight' analogy, the force depends only upon the weight of the log and where its mass is centred relative to the ground support and the handle.
Muscular capacity constitutes a second factor in exertion, which also depends upon posture; an analogy is the force that a Jack-in- a-Box exerts. The force is in the Live Axis (Fig. ,1) which is the line between the centre of pressure at the ground and the handle.
Although ded-weight and Jack-in-a-Box effects both operate in
exertion, they cannot be readily separated. The log of wood retains its shape by virtue of tensions and compressions in its fibres. Man retains his posture by means of tensions in muscles and ligaments and by compressions within the skeleton. Man's muscles are required both to retain the posture and to create the 'Jack-in-a-Box' forces.
If we measure the vertical or horizontal force-components in maximal static lifts and pulls respectively, with various placements of the hands and feet (together)(Fig. 2), we may calculate the Dead- Weight Fractions of the variance of strength which are accounted for by variances of weight and height in the population (Pheasant, 1977).
Nearly 70% of the variance is accounted for when the measurement is close to the Dead axis, but only 20% when it is close to the Live axis. It is clear that forces in the Dead axis counterbalance the action of body weight about the centre of foot pressure, while any force can exist in the Live axis without disturbing the equilibrium.
135
136
Fig. 1. Left:
Centre:
Right:
D. W. GRIEVE AND S. T. PHEASANT
Manual force in exertion, showing Live & Dead axes.
Dead-weight analogy of exertion in Dead axis.
Jack-in-a-Box analogy of exertion in the Live axis.
1·0
·8 DWF
• =30d'
o=43~
,""'T
·6 PULL w,'~o}
"" .,tI
·4 Ijf" a,/
~ _____ -O"
... ---9
·2 ••••• lLiFT
._. _____ ..0- ____ ...
00 30
Fig. 2. Left: Measurements of the horizontal component of a pull and its angle to the hand-toe line, or the vertical component of a lift and its angle, for various place~ents
of the hands and feet.
Right: The Dead-Weight Fractions (DWF) as a function of the angle between the measured exertion and the hand-toe line for 9 different static lifts and pulls.
POSTURAL CONSIDERATIONS 137
In the general case of exertion in any direction, the man in Fig. 3 is exerting a force which has components of LIFT and PUSH
(PRESS & PULL if negative), and also a torque TWIST. The torque may be important if the hands are separated in the sagittal plane, but were negligible in the experiments for which results are presented. The experimental rig incorporates a bar handle, fitted with force transducers to measure the manual forces. The LIFT and