Human Strength Terminology Measurement and

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298 June 1970 HUMAN FACTORS, namic efforts that can be measured in units of really nothing isotonic happening at the. mechanical work muscles when moving constant masses When. Dynamic and static indicate whether ever the term isotonic appears in the strength. motion accompanies muscle tension A static literature the reader should check whether. effort results in force or torque exertion but dynamic is not really the appropriate term If. not in mechanical work A dynamic effort re so the kind of dynamic effort performed must. suits in mechanical work then be explained, Concentric and eccentric refer to cer Some ambiguities stem from discrepancies. tbin dynamic efforts concentric indicates between physiological and mechanical se. that the muscle shortens actively against a resist mantics as discussed by Brunnstrom 1966. ance eccentric makes clear that the muscle Ramsey 1967 and Whitney 1 958 In me. is lengthened passively by an external force chanics work is defined either as the product. Phasic may be used in lieu of dynamic of force and displacement or energetically as. when only a single contraction takes place the amount of change in potential or kinetic. The preceding definitions may help to avoid energy If a human operator holds a weight iso. such ambiguous terms as static work iso metrically he does not work in the mechani. tonic lifting dynamic strength and similar cal sense since no motion occurs He also does. paradoxes To clarify further the definitions no work when lifting a mass and returning it. certain terms and underlying mechanical princi to its initial location since its potential energy. pies will be discussed is then the same as before Still both the static. As defined strength is exerted isometri holding and the dynamic moving of the weight. cally indicating that during the contraction strain the operator physically and increase his. period the length of the muscles involved metabolism i e energy consumption Koop. is kept constant Since muscle length does man according to Burger et al 1967 conse. not change attached body segments remain quently proposes that the word effort be. motionless If there is no motion all acting used instead of work to indicate physiologi. forces must be in balance therefore the word cal strain. static may be used interchangeably with iso In the case of dynamic efforts the amount. metric Both terms indicate that there is no of energy exerted by the operator can easily be. limb motion but do not inform about the expressed in terms of mechinical work if the. magnitude or steadiness of muscle tension absolute values of the energy changes are taken. Isotonic may be the most misused term in into account e g to the positiye work of lift. strength testing It simply means constant ing a weight could be added the negative work. muscle tension and obviously is neither an of lowering it to its original location. antonym for isometric nor does it describe There is no such easy solution for the case of. the nature of an external load or resistance Al static force exertion this just jis not static. though theoretically not limited to this combi work as it is often incorrectly called A static. nation an isotonic effort is normally found effort consists of exerting a force over a certain. combined with an isometric force exertion as period of time without producing motion The. in holding a weight motionless Muscles are force may be integrated over the time of exer. NOT otrained isotonically when moving a body tion and this integral may be called impulse. segment against a constant resistance during This procedure is mechanically correct b t not. the motion the tension of the involved muscles always very meaningful The result of the in. changes with their changing length and with the tegration is a single numerical expression con. changing mechanical advaptages If a constant cealing the well known fact that ever any given. mass is to be moved as in weightliting Clarke period of time a muscle force is not constant. 19661 this does not even mean that a con but may fluctuate considerably about a mean. stant force is required at the mass to effect its value see Figure 1. motion the resistance offered by the mass It appears that for practical pirposes force. changes with the cuming velocity There is integrals over periods of I sec each are appro. K H EBERHARD KROEMER June 1970 299, FORCE in kp lever arm are known For practical purposes. the muscle is inaccessible and therefore the,magnitude of internally developed muscle force. cannot be measured directly,However the external effect of the internal.
muscular effort is measurable as the force ex,erted at a certain distance from the joint to an. outside object In the static case this externally,measured force is proportional but not equal. to the internal muscle force If lever arms and,pull angles are known the isometric muscle. force can be calculated from the externally ap,plied force. For practical purposes only the force exert,able to an outside object is of interest Since it.
depends on the strength of the muscle and on,the prevailing mechanical advantages the loca. 0 tion of the force measuring device must be, I second specified with respect to the next joint to make. a strength measurement meaningful, Figure Record of a strength measurement The SUb in a dynamic effort the externally measured. kiet as to exert a constant maxinml force over a, period of five seconds he had no feedback of the energy output at any given instant may not. actually exerted force Note the fluctuations of the even be proportional to the internally devel. force during the supposedly constant phase ranging oped energy Muscular impulses accelerate and. from about 79 kp minimum to 94 kp peak decelerate masses this means storage and dis. charge of energy and inevitably delays and blurs, priate Strength tests require force exertion of the flow of muscular energy to the point of.
less than about 10 sec to avoid muscle fatigue application Since the mechanical advantages. as discussed later Integration periods of I sec lever arms pull angles are also changing. each are short enough to reflect the more im during the dynamic process calculation of the. portant force alterations during this test yet instantaneous internal muscle force from the. are long enough to yield useful units of measured external output is usually very dif. strength A force integral over I sec could be ficult if not impossible. called l sec force or I sec impulse or Prediction of dynamiiic performance from. specific impulse A short and handy analog static strength tests would require in terms of. to impulse may be pulse Thus pulse is mechanics the following procedure. proposed as a term in strength testing signify Step 1 The externally measured static force. ing that force is integrated or averaged over 1 F must be modified by a constant factor C. sec representing lever arms and pull angles to assess. Generally a muscle connects two body seg the internal muscle strength S. ments rotatable about a common joint If the, muscle is under tension it applies torque to FX C S 1. each segment The magnitude of torque de Step 2 The static muscle strength S must be. pends 1 on the amount of internal muscle transformed into the muscular capacity D for. force 2 on the distance between the joint and dynamic exertion of energy. the location of the muscle or tendon attach,ment to the bone ie the lever arm and 3 on. the pull angle between the vector of muscle Step 3 The capacity D has to be modified. force and the limb Brunnstrom 1966 Elft by a variable V representing pull angles and. man 1966 Usually neither pull angle nor lever arms as well as the amount of energy. 300 June 1970 HUMAN FACTORS, stored in or released from moving body parts direction must also be described This is not. The variable V changes with orientation and done in some so called strength tests see e g. speed of the limbs The product of D and V Clarke 1966 The scientific value of such tests. indicates the external mechanical work W is rather dubious. Some measuring devices are constructed so, D X V W 3 that they should be used only in a specific way. The first and third steps are theoretically simple even so a formal description of the way they. since they involve only transformation of one are used is sometimes necessary The various. unit into hnother Some complication however types of hand dynamometers for example. stems from the fact that the transformation measure the grip force exerted between the. factors are unknown and in the last step this fingers and palm of the hand If the dyna. factor is a variable The second step is actually mometer is attached to a stationary device. the most difficult how to transform static such as a table instead of being held in the. muscle strength into the capacity to perform hand the experimenter must take precautions. dynamic work Though seemingly simple in to assutre that the subjects squeeze with their. physical terms this poses a rather complicated fingers and do not pull with their arm and. physiological problem Physiological processes shoulder muscles e g Smith and Edwards. such as oxygen supply to and waste removal 1968 Then too some of the elliptical grip. from the muscle are rather different when ac strength meters cannot be calibrated in the. companying short time or sustained efforts same way the subject actually applies the force. maximal or submaximal efforts dynamic or and can therefore yield only relative not. static efforts absolute readings of grip strength, Because of these mechanical and physio All measurements of muscle strength require.
logical considerations it may be doubted that the outside resistance against which force. Kroemer 1967 whether static strength tests is applied be larger than or at least equal to the. can generally serve as accurate predictors for maximal muscular force nobody can exert a. sustained dynamic performance even though force larger than the reaction force available to. they are somewhat loosely related by the fact him Some published data are actually measure. that muscular efforts are involved in both ments of limited reaction force and are so. described Caldwell 1962 Dempster 1961,and Rohmert 1960a these data do not. MEASUREMENT OF HUMAN STRENGTH describe the intrinsic muscle strength of the. Since the externally measured strength The definition of strength specifies that. serves only as an expression of an unknown and force be exerted in a singlevoluntary con. unaccessible internal muscle force and since traction This limits the time during which. the measured strength depends on the existing strength can be measured to less than about 10. mechanical advantages the location of the sec Caldwell 1963 1964 Rohmert 1960b. point of force application i e the attachment Maximal muscular tension can be maintained. of the measuring device as well as the direction without perceptible decrease for only such a. of the exerted force must be carefully selected short period of time Muscular fatigue then sets. and clearly described in the protocol Taylor s in which reduces the exertable teksio0 de. 1954 finding that the mean grasp force of Vries 1966 Lehmann 1961 1962 Scherrer. the hand is 90 lb has been reprinted in several 1967 Even if force is required over only such. human engineering handbooks but the parts of few seconds the exerted force is not constant. the palm or finger with which this force was but fluctuates in an irregular and unpredictable. transmitted or the direction in which the force manner Figure 1 These force alterations corn. was exerted are unspecified plicate the seemingly simple task of strength. Force is a vector In addition to its magni measurements. tude the location of its line of action and its Most instruments used to measure muscle. BEST AVAILABLE COPY,K H EBERHARD KROEMER June 1970 301. strength can be categorized as either indi,cating or recording All pointer instruments. belong to the first group They display only,instantaneous measures of strength While. observing the oscillating pointer the experi AK,menter has to decide what to consider the.
score of the experiment there is no way to S, check this index later Since the needle does C D sh fIsIh. not come to a rest but oscillates continuously,the experimenter very likely will use the largest. occurring deflection of the needle the peak,force as the index for the subject s strength I. On many instruments selection of the peak is,facilitated and almost dictated by a maximum. indicator a marker moved by the primary Figure Forward leanidg push dth both hands. pointer and left at its largest deflection Force plate adjusted to a height H of 70 and to a. Recording devices are usually more expen distance D of 80 of each subject s acromialheight. sive and not as easy to use as indicator instru, ments but they yield a permanent record of period of force exertion were analyzed in.
the exerted force If the force is traced over the several ways The minimal force amplitude. total period of force exertion the experimenter occurring during the middle 37 of the 5 sec of. may check the experimental results at his ease exertion was compared to the maximal ampli. instead of being forced to rely on a hasty,reading of a peak force he can select the most. suitable index,Peak Versus Average Strength Scores. The experimenter has the choice between,two types of indices to evaluate. motion accompanies muscle tension A static literature the reader should check whether effort results in force or torque exertion but dynamic is not really the appropriate term If not in mechanical work A dynamic effort re so the kind of dynamic effort performed must suits in mechanical work then be explained

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