European Journal of

Applied Physiology

Eur J Appl Physiol (1985) 54:210--215

and Occupational Physiology 9 Springer-Verlag 1985

A dynamometer for the measurement of force, velocity, work and power during an explosive leg extension F. J. Avis 1, A. Hoving 2, and H. M. Toussaint 3 1 Department of Physical Education and Sports, University of Technologie, Delft 2 Technical Construction Centre, University of Technology, Delft 3 Interfaculty of Physical Education, Dept. of Functional Anatomy, Free University, Amsterdam, The Netherlands

Summary. A dynamometer for measurement under static and dynamic conditions is presented. At different load levels, force, velocity, work and power can be measured in explosive leg extensions. Measurements on 53 subjects at different load levels (0--125.5 kg) were carried out. Peak power ranged from 2611 to 1746 W, force from 1351 to 1899 N, velocity from 1.61 to 0.89 m- s -1 and work from 329 to 605 J. Between trial correlation coefficients ranged from 0.72 to 0.95. The dynamometer is compared with others, and it is concluded that data obtained by this dynamometer have a greater practical validity. Key words: Dynamometer -- Force -- Work -P o w e r - Legs

(Gregoire et al. 1984). In devices where only monoarticular isotonic or isokinetic measurements are made one or both aspects are neglected. With these objections in mind a dynamometer was constructed for measuring the uni- or bilateral force, velocity, work and power in accelerated leg extensions against different loads. Technical data

Apparatus The dynamometer enables measurements under static and dynamic conditions.

Static condition Introduction

The use of dynamometers has become a widely accepted method in evaluating muscle strength. However, with the majority of dynamometers only static measurements of muscle strength can be made. A disadvantage of these dynamometers is that neither work nor power can be measured, while in many sports these variables are important prerequisites for good performance. A few devices are described for the measurement of muscle strength, work and power under isotonic, isokinetic (e. g. Cybex) and other conditions (Ariel 1983; Buhrle et al. 1983). In many athletic activities like jumping, running and rowing the body is accelerated. Furthermore the transport of power by biarticular muscles plays an important role in these activities Offprint requests to: F. J. Avis, Sportstichting, Mekelweg 8, 2628 CD Delft, The Netherlands

Figure 1 shows the main parts of the dynamometer during static measurement. The foot rests (A) D w-

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1~1

U --B

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Fig. 1. Functional parts of the dynamometer during static measurements. For further explanation see text

F. J. Avis et al. : A d y n a m o m e t e r for force, w o r k a n d p o w e r m e a s u r e m e n t

are m o u n t e d on a bar to which four strain gauges are attached. Together with an amplifier, an analog-digital converter (ADC) and an Apple II microcomputer these parts form the force-registering system. The bar (B) is part of a yoke is connected to the bearings (C) and is coupled on one side to an angle transducer (D). This construction can be placed in the desired position by sliding it in the indicated directions. There it is fastened with clamps (E). Thus it is possible to measure isometric forces over a range of knee-angles and to adjust the measuring device to the specific dimensions of the different subjects. The subject is strapped to the seat (F), which is immobilized by two bars (G). Two handgrips (H) enable additional fixation of the subject.

Dynamic situation By removing the bars (G) the dynamometer is ready for dynamic measurements (Fig. 2). The seat is now movable on a rail and m o u n t e d to it with 6 ball-bearings (f--0.0025) to minimize friction and to make high stretch velocities possible (Fig. 3). Two cables are attached to the seat. One leads via a pulley-system to an electromagnet (I).

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211

The other is coupled to a counterweight (J) to compensate for the weight of the electromagnet. When the electromagnet is switched on a weightholder (K) can be coupled to it. On the weightholder (mass 25.5 kg) different weights (L) (1.125--280.0 kg) can be placed, thus different loads can be applied. To prevent the subject from damaging himself a cushion (M) is placed behind the seat at such a distance that full stretching of the legs is possible. The cushion decelerates the subject after an explosive stretch movement. At that m o m e n t the electromagnet can be switched off to prevent the weight pulling back on the subject. The weightholder then drops on the buffer (N). The displacement of the seat is registered by an incremental encoder (0) (Leine & Linde AB, Sweden, model 58, type 5806/A/15V/400) which is coupled to a non-elastic loop (P) attached to the seat (Fig. 2 and 3). The angle between the horizontal plane and the direction of the force is registered with an angle transducer (D).

Calibration The force-measuring system can be calibrated using the device depicted in Fig. 4. The construction

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Fig. 2. F u n c t i o n a l parts o f t h e d y n a m o m eter d u r i n g d y n a m i c m e a s u r e m e n t s . For f u r t h e r e x p l a n a t i o n see text

F. J. Avis et al.: A dynamometer for force, work and power measurement

212

. O N ELAST,C LOOP ENCODER

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Example of measurement Several interesting research questions can be dealt with by this versatile dynamometer. It can be

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Fig. 3. Attachment of seat to rail with ballbearings and the coupling of the displacement transducer to the seat with a non-elastic loop

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shown permits the application of a very large range of forces, by using a few calibration weights. The encoder produced 400 pulses per revolution. Since the circumference of the wheel is 0.1 m, the resolution is 0.25 mm (1/4000). The angle transducer was calibrated to produce 100 mV per degree. The force-, angle- and displacement-signals are amplified, digitized by the ADC (12-bits, 150 Hz) and processed by an Apple II microcomputer. According to the diagram representing the method of calculation (Fig. 5), velocity, power and work are computed. A further detailed description is available on request (Hoving 1981; Kramer 1982).

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\\\\\\\\\ FRAME Fig. 4. Calibration device for the calibration of the force measuring system

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DISPLACEMENT Fig. 5. Diagram of the method of calculation. S is the displacement of the seat, F is the force in the horizontal plane, V is the velocity of the seat, P is the power, and W equals work

used, for instance, to determine the influences of training, the differences between one leg and two leg movements, the effect of countermovements, and the influences of feedback or knowledge of results. As an example of the application of the dynamometer in the dynamic situation, force, velocity, work and power was registered during a simple explosive leg extension of both legs in 53 healthy males (age 23.1 years+3.1, length 184.9 cm+5.2, weight 76.3 kg+5.6) (Fig. 6). Informed consent was obtained from all subjects. The movement was started with the knees flexed (70~ An 'all out' effort was made on an accoustic starting signal 2 s after a warning signal. Seven levels of additional load (between 0 and 125.5 kg) were applied. At each level two measurements were made. Occurrence of fatigue was avoided during the course of the experiment. Figure 7 gives a typical example of the instantaneous force, velocity and power curves at loads of 45.5 kg and 105.5 kg. From the curves, force (F)

F. J. Avis et al.: A dynamometer for force, work and power measurement

213

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Fig. 6. Stroboscopic photograph of subject performing an explosive leg extension

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and velocity (V) were determined 1/75s before peak power (P) was delivered. The amount of work (W) was computed by integrating the power curve. Means and standard-deviations are given in Table 1. Because the angle transducer was not operational, the force values are overestimated ( + 10%) with respect to the horizontal plane and thus the values of work and power are overestimated as well. Pearson's correlation coefficients between trials at each load level are given for the four variables in Table 2.

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The high correlations (Table 2) show that good reproducibility of measurements can be obtained.

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Fig. 7. Typical examples of the instantaneous force (F), velocity (V), and Power (P) at a load of 45.5 (a) and 105.5 kg (b)

Table 1. Peak power (P), force (F), velocity (V) and work (W) at different load levels. Means (X) and standard-deviations (SD) of the first (1) and second (2) trial are given. (N= 53) Load (kg)

P (W)

F (N)

V ( m - s - 1)

W (J)

(SD)

X

(SD)

X

(SD)

X

(SD)

X

1 2

2216 2267

(500) (492)

1351

(255)

1.57

(0.15)

1371

(255)

1.59

(0.15)

329 340

(65) (66)

25.5

1 2

2574 2611

(474) (484)

1544 1564

(249) (237)

1.61 1.60

(0.12) (0.12)

447 453

(68) (66)

45.5

1 2

2503 2525

(495) (515)

1660 1645

(266) (282)

1.46 1.49

(0.12) (0.12)

477 492

(67) (74)

65.5

1 2

2397 2301

(531) (542)

1727 1666

(285) (297)

1.34 1.33

(0.13) (0.13)

523 522

(80) (84)

85.5

1 2

2131 2104

(497) (496)

1747 1709

(297) (298)

1.17 1.18

(0.12) (0.12)

547 553

(82) (83)

105.5

1 2

1944 1862

(484) (472)

1828 1748

(302) (326)

1.02 1.01

(0.11) (0.12)

582

(87)

580

(100)

1 2

1762 1746

(451) (442)

1899 1883

(337) (360)

0.89 0.89

(0.12) (0.11)

605 604

(113) (111)

0

125.5

214

F.J. Avis et al.: A dynamometer for force, work and power measurement

Table 2. Pearson's correlation coefficients between the two

trials (1 and 2) as a test-retest reliability of the measurements. (N=53) Load

P

F

V

W

0 25.5 45.5 65.5 85.5 105.5 125.5

0.88 0.89 0.93 0.94 0.95 0.93 0.93

0.92 0.84 0.95 0.94 0.93 0.87 0.95

0.72 0.77 0.72 0.84 0.89 0.87 0.86

0.84 0.92 0.86 0.93 0.92 0.94 0.95

The mean peak power during the simple stretch movement was 2210 Watt. This is more than twice the peak power reported by Ivy et al. 1981 for isokinetic knee extension with one leg (397.5 Watt). In our view this difference is due to the fact that the present apparatus facilitates the transport of power by polyarticular muscles and lays no artificial constraints on acceleration of the movements. This view is supported by the fair agreement between the power reported here and values for specific explosive movements like jumping (Vergroesen et al. 1982). In comparison with other means of measuring force, velocity, external mechanical work and power, such as vertical jumps with or without the use of force platforms (Asmussen and Bonde-Petersen 1974; Asmussen et al. 1976; Bosco 1981; Bosco et al. 1983; Bosco and Komi 1979; Cavagna et al. 1971; Cavagna 1975; di Prampero and Mognoni 1981; Viitasalo and Bosco 1982), stairclimbing (Margaria et al. 1966; Kalamen 1968) and ergometry (Bar-Or et al. 1980; Ikuta and Ikai 1972) this dynamometer has several attractive properties. As force and displacement are directly measured it is not necessary to make assumptions concerning, for example, take off and landing body position, the contribution of other body parts, or the relationship between horizontal and vertical velocity, which are open to criticism and to the use of inaccurate integration techniques. Another advantage is the possibility of measuring the indicated variables as functions of a continuum of loads during a simple extension of the legs unaffected by complicated (sports-) movement techniques. Very important is the fact that the excursions of the movements can be completely controlled, which is essential in the case of research on positive work as a function of eccentric loads, a fact until now overlooked in countermovement and depth jumping experiments in which subjects spontaneously chose the extent of flexion preceding the extension. Although Hubley and Wells (1983) state that

"The amplitude of knee flexion was standardized at the start of the upward movement", it remains unclear as to how this was done. In general, it can be said that data obtained by the dynamometer described have a greater validity for practical applications. The dynamometer is being used for research, and results dealing with the influences of two training schedules and the effect of countermovements will soon be reported (Avis et al. unpublished work). At present the apparatus is being used in studying the differences between one and two leg movements combined with EMG registration. Acknowledgement. The authors wish to express their gratitude to the coworkers of the Centrale Werkplaats and the Centrale Electronische Dienst, University of Technology Delft and W Rijnsburger, Interfaculty of Physical Education, Free University, Amsterdam. The work performed by HM Toussaint is part of his doctoral examination and was supervised by FJ Avis, GJ van Ingen Schenau, PA Huijing and RD Woittiez.

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Accepted April 30, 1985