trunk strength and mobility measurement for the normal and impaired backs: part ii — protocol,...

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E L S E V I E R International Journal of Industrial Ergonomics 17 (1996) 91-101

I n t e r n a t ~ o n a l ] o u r n a l o t

Industrial Ergonomics

Trunk strength and mobility measurement for the normal and impaired backs: Part II - Protocol, software logic,

and sample results

Shrawan Kumar Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Canada T6G 2G4

Received 3 November 1994; accepted 28 March 1995

Abstract

An experimental protocol for systematic assessment of the mobility and strength of the normal and the functional capacity of the impaired back has been developed. This protocol is used with the FELT-I, FELT-II, and AROT equipments described in Part I. A special purpose software has been written which permits flexibility to suit testing requirement of any device. Its modular nature allows subject information input, randomization, and provides intermediate and analyzed results in addition to instant screen display for quality control during data collection. The software also allows to pause and resume, and stop and start at any stage of testing. One hundred and seventy nine subjects have been tested using this set-up. Representative graphical and numerical outputs are presented.

Relevance to industry

The content of this paper is relevant to industry in two ways. First, the software as a product has value for the industry. Secondly, through its use, isolated planar strength capacities can be obtained among normals and injured alike. These data will be of value in designing for safety as well as for performance. Through a comparison with the norm, job accommodation for the injured can be made on a sounder basis.

Keywords: Mobility and strength-measuring software; Functional capacity evaluation software

1. Introduct ion

Due to the six degrees of f reedom of movement (rotation about and translation along the three major axes) available at every intervertebral intersegmental level in a multi-segmented flexible rod-like spine, many complex movements are performed. However, extension, flexion, lateral flexion, and axial rotation of the entire spine are the expressions of summated movements at all inter-

vertebral levels. Thus, the work capacity of the normals and the functional capability of the injured involves force application and load bearing by the spine in and through a variety of function- ally isolated movements. In order to test these activities special devices are needed. As reviewed previously in Kumar (1996) no equipment exists which can test all extension, flexion, lateral flexion and axial rotation in isometric and isokinetic modes. Moreover the devices which do exist have

0169-8141/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0169-8141(95)00041-0

92 S. Kumar / International Journal of Industrial Ergonomics 17 (1996) 91-101

some problems of stabilization, contamination from other regions or restriction in movement due to the nature of harness used. In an effort to address these limitations Kumar (1996) designed and fabricated three alternative devices. The protocol of the use of these devices, the logic of the software and sample results are presented here.

2. Experimental procedures

For flexion, extension and lateral flexion testing the subjects were asked to sit in FELT-II (Figs. 1, 2, 3). The chair was then slid on the base plate such that the spinal axis of the subject was aligned with that of the axis of the stainless steel strap of the SDST. The chair was locked in place and the subject stabilized with velcro straps at the hip, distal thigh, proximal shin and ankles. The loop of the nylon strap was then placed around the shoulder in a stable position (Figs. 1 to 3).

Fig. 2. A subject positioned in FELT-II for a extension test.

Fig. 1. A subject positioned in FELT-II for a flexion test.

The tension at the nylon shoulder loop adjusted to a force of approximately 5N. When the test condition changed from flexion to extension, the revolving plate was unlocked from the sliding plate with the subject still stabilized in the chair and swung around by 180 ° . The chair was re- volved + 90 ° for left or right lateral flexion and the sliding plate moved until the coronal axis of the spine coincided with that of the axis of the stainless steel strap of the SDST.

The A RO T was used to test axial rotation (Fig. 4). The subjects were seated in the height- adjusted chair and stabilized with velcro straps at four levels (hip, distal thigh, proximal shin and ankles). The chair was then slid on the base plate until the vertical spinal axis was concentrically aligned with the vertical axis of the spring-loaded sliding shaft of the shoulder harness. The shoulder harness was then lowered onto the shoulders of the subject and fixed. The subject's position was then secured by the velcro straps. The arms

S. Kumar / International Journal of Industrial Ergonomics 17 (1996) 91-101 93

of the tongues were positioned on the acromia bilaterally. The positions of the chair and the sliding shaft were locked. Subjects were asked to actively twist to left and right to their maximal range of motion to ascertain that there was no restriction due to the positioning of the device. Once the subjects were positioned for any of the selected tests, they were allowed to perform a couple of submaximal trials for feel, practice and judgement of range of motion. During data collection, the subjects were instructed to give their maximal effort in every trial of each condition. They were informed prior to each trial that if the effort was submaximal they would have to repeat the trial. The subjects were not given verbal en- couragement or visual feedback during the test. For all testing they were told not to jerk but to build their strength gradually. For the isometric testing, they were asked to build their strength to the maximum in the first two seconds and main-

Fig. 4. A subject positioned in AROT for axial rotation test.

Fig. 3. A subject positioned in FELT-II for a lateral flexion test.

tain it for another three seconds. For the isokinetic conditions, they were required to build their full strength in the first few centimeters and maintain it throughout the entire range of motion.

2.1. Coupling and correction

The FELT and A R O T devices were indepen- dent and separate from SDST, which was the measuring device. Therefore, they had to be coupled for the experiment. The subject seated on FELT-II was coupled through a 5 cm wide nylon shoulder loop attached to the load cell through a metal hook (Fig. 5). After positioning the subjects appropriately on the device, the position of the chair was marked and the reading of the poten- tiometer was made. Based on these data an opti- mum position for the load cell and the shoulder belt was found and maintained during the study. During extension/flexion and lateral flexion at 0 °


Fig. 5. Coupling of FELT-II with SDST.

of trunk flexion, the shoulder strap was placed at right angle to the trunk. At other positions it was at various degrees of incline. The A R O T in which the subjects were seated was coupled through pulleys and airplane cable with the SDST (Fig. 6). An airplane cable, joined through a metal buckle

to the end of the stainless steel strap of the SDST, ran over a pulley attached to the top crossbar and was turned around over three pulleys such that it came in line with the groove of the circular plate of the AROT. This circular plate had several holes, to one of which this cable was joined through a special S-shaped fixture. Near the circular plate at an appropriate distance, the load cell of the SDST was inserted. The SDST measured force in pounds. These values were converted to Newtons by using the following correction:

Force(Newtons) = 4.4482 x Force(pounds)

The next step for correction was determining the moment arm accurately and reliably. For all isometric extension, flexion and lateral flexion at 0 ° trunk flexion, the moment arm was determined by measuring the distance between the lumbosacral joint and the shoulder strap. In isometric extension, flexion and lateral flexion at angles other than 0 ° trunk flexion the moment arm was fixed, whereas it changed continually for the isokinetic extension, flexion and lateral flexion. This distance was calculated using a model for the experimental set-up. Fig. 7 represents the experimental set-up with subject at 0 ° and Fig. 8 represents the set-up at an angle other than 0 °, where:

A = axis of rotation for torque (lumbosacral joint)

B = point at which shoulder strap is positioned

C = point at which the force applied is sensed D 1 = the distance between A to C D 2 = the distance between B to C

D 2 C B

D a .....

D 3

A

Fig. 6. Coupling of AROT sith SDST. Fig. 7. Model of FELT-II and SDST coupling at 0 °.

S. Kumar / lnternational Journal of Industrial Ergonomics 17 (1996) 91-101 95

D 3 =the distance between A to B (lumbosacral level to the strap level)

a = angle at A between points B and C b = angle at B between points A and C L = moment arm length The length D 2 was measured to be 86 cm for

flexion, 97 cm for lateral flexion and 107 cm for extension. The length D 1 is constant (because both points are fixed and do not move) and can be given as follows:

D 1 = sqrt[D22 + D32- ]

and

a = cos- I (D3/D1) = (a0constant)

At angles other than 0 ° trunk flexion, this angle will be a o + the angular displacement of the subjects trunk. From the cosine law:

D2 = sqrt[D, 2 + D3 2 - 2D1D 3 cos(a)]

and from the sine law:

s i n ( b ) / D t = s i n ( a ) / D 2

Therefore, using D 2 from the above, the angle b can be given by;

b = sin- ~ [ (D t /D2) sin(a)]

and the moment arm length L is given by;

L = D 3 sin(b)

During isokinetic conditions there was a constant change in D 2 and the angle a. Both these variables were being continuously monitored. Therefore, a constant correction of moment arm length was being applied in torque calculations.

C D 2

D 1

A

Fig. 8. Model of FELT-II and SDST coupling at other than 0 °.

d I

i

d2 i !

F 1

Fig. 9, A mechanical model of AROT.

In axial rotation, the point where the force was measured with reference to the axis of rotation was offset from the point where the force was applied (Fig. 9).

According to the law of static equilibrium

O = Fld I + Fed 2

Because F 1 and F 2 are forces acting in oppo- site directions, hence the correction to the force applied by the subject can be given by the following:

F 2 = F l ( d l / d 2 )

where, F 2 = the force applied by the subject F 1 = the measured force d 2 = the distance between the point of force

application and the axis of rotation d 1 = the distance between the point of force

measurement and the axis of rotation (this was constant = 19 cm).

The torque in each of the conditions was obtained by simply multiplying the force by the moment arm. Written simply;

Torque = Force × Moment Arm Length

3. Software development and design

A customized modular software was designed for this project which handled the data acquisition (according to the experimental design), data management and appropriate predetermined data


analysis. A description of the salient features is presented (Fig. 10). The software was therefore organized to execute different stages of the project separately as desired. The first stage was concerned with subjects' demographic and mor- phometric data, and experimental data acquisition (Fig. 11)." At this stage, the software randomized the sequence of experimental conditions based on the last name of the subject and loaded this sequence for the experimental session. It performed the data acquisition of a trial of a condition and produced instant screen plots for quality control. The process was repeated twice for a total of three trials which signalled the end of one condition. At this stage, all three trials were instantly plotted on the screen to verify the consistency of the trials. The data for consistent trials were saved; otherwise the condition was repeated. At the end of the session, the program gave the option of terminating data acquisition or going to next session. At the end of one session, the experiment was terminated for the day. The process was repeated for other sessions till all data were acquired from the subject.

The raw data of each trial of each condition was reduced to extract meaningful data from the individual subjects (such as peak and average force and torques, time to peak, duration of the trial) (Fig. 12). The reduced data were saved as the individual results to be fed to the next stage

i

/ /

I lll----,,,,lt I Ic*,,nin*,, I= l ] I ±

' 1

, .

l J - ' J

" . "

IN,,

Fig. 11. The structure of stage I of functional evaluation software.

D a t a A c q u U s i t i o n

D a t a A n a l y s i s I n d i v i d u a l

S u b j e c t s

D a t a A n a l y s i s S u b j e c t s G r o u p s

D a t a A n a l y s i s N o r m a n z a t i o n

o f R e s u l t s

Fig. 10. The stages in functional evaluation software.

of analysis (Fig. 13). In the third stage of data processing, all individual results of same activities were pooled separately and descriptive statistics were calculated for each condition. In the fourth and final stage (Fig. 14), the group values were normalized against the preselected reference value. The peak isometric torque at 0 ° flexion was chosen as the reference value against which all other values were compared and expressed in percent of the reference (normalization).

4. Data collection and sample results

The sequence of the experimental conditions was randomized. The axial rotation conditions

S. Kumar / International Journal of Industrial Ergonomics 17 (1996) 91-101 97

were separated from the extension, flexion and lateral flexion as these required different devices, different stabilization and different coupling. Changing between devices was not only cumber- some and time consuming but had the potential of increasing experimental error by possible dif- ference in positioning. The velocity of the Static Dynamic Strength Tester was adjusted to be 15 cm sec -1 linear velocity (angular velocity 30 ° s ec - t ) for isokinetic extension, flexion and lateral flexion. This was reduced to 7.5 cm sec-1 linear velocity and 15 ° sec-1 angular velocity for axial rotation.

Once the subjects were positioned securely in the experimental set-up they were required to exert their maximal effort for trial of the condi-

S t a r t

e¼ec S u b j e c t t

A n a l ~ s ~ l

Input (Rmld) / Subject da ta A4te ,Ht- ,Wt . , / T h r e s h o l d s , / /

etc . /

C o n d i t i o n i ~ 1

p ~ N N I S tlh~ ~ for oondlton I

-I

/..=o,/ Y e s ~

ICondltlon I~1-1-1

Fig. 12. The structure of stage II of functional evaluation software.

~ . ~ S t a r t b _I

S e l e c t G r o u p n:mO

l_ S e l e c t S u b J e c t t o

p l a c e i n g r o u p n = n + l

4. Read i n d i v i d u a l a n a l y s i s r e s u l t s •

A c c u m u l a t e s t a t s f r o m s u b J , ~ t s i n

s a m p l e

P I ~ ) c e s s S t a t i s t i c s

ndG oup A n a l y s i s

Fig. 13. The structure of stage III of functional evaluation software.

tion being tested. Isometric conditions were set up for 5 seconds for normal subjects. The flow of data and its collection is shown in Fig. 15. All data channels, the potentiometers from the FELT, A R O T and SDST and the load cell were fed to a Tecmar A to D board, which was set to sample each channel sequentially at 50 Hz. The digital data were then saved and stored in the computer memory in files created before subse- quent processing. The sample outputs for an isometric and an isokinetic trial are shown in Figs. 16 and 17.

The outputs of the potentiometers and load cell were being continuously monitored. How- ever, the data were windowed based on initial and final force threshold. The threshold on the force monitor was set for 20N and 10N for start

98 S. Kumar / lnternational Journal of lndustrial Ergonomics 17 (1996) 91-101

S t a r t

_1 Nt

S e l e c t G r o u p S e l e c t R e f e r e n c e

C o n d i t i o n

L o a d G r o u p / S t a t i s t i c a l

~ l ' a b l e s

N o r m a l i z e

D a t a

Isometric Extension

150 1.30 f~ 110

90.

70.

50-

.30-

I o- . __=.~_ ........

- 1 0 f

- - Force(Ub) - " L J n e o r D i l ~ . ( c m )

- - A n g u l a r D i t p . ( ° )

/ - - / N o r m a l i z e d T a b l e s

C -> N o r m a l i z a t i o n

Fig. 14. The structure of stage IV of functional evaluation software.

and end respectively. The peak force was taken as the highest force exerted during the trial. For average isometric strength, the sustained level of values of force over the trial period was averaged. The average isokinetic strength was determined by dividing the area under the force curve by the time of exertion.

Tables 1 and 2 show numerical sample results

_• I I ~ t l - - I

FELT ~~o~u~ i Subject . SDST An) Co,lroh

Pot3 Fig. 15. Block diagram to show data flow and collection.

Time (.)

Fig. 16. A sample output of an isometric extension.

output from the computer for isometric and isokinetic testing. Table 3 presents peak and average values of the force and torque and their time relationship with linear and angular displace- ments. In Table 4, the values of each of the measured variables is shown at every 10% of the task cycle interval. Such information allows track- ing the variable behaviour during the whole test cycle. It is this capability of continuous quantita- tive monitoring which will enable us to identify the pattern and magnitude of impairment, quantitatively, if any.

-6

Isokinetic Extension

- - rc.~,(ub) I O0 1 - - ~ l ~ , p . ( © m )

40 ~ "~

0 Y - : . . . . . : . . . . 0 1 2 3 4 5

"rim, (s)

Fig, 17. A sample output of an isokinetic extension.

S. Kumar / International Journal of Industrial Ergonomics 17 (1996) 91-101

Table 1 Sample numerical result output for isometric conditions

99

Condition Flexion Extension

0 ° 20 ° 40 ° 60" 0 o 20 ° 40 ° 60 °

Peak force (N) 671.9 423.5 Avg. force (N) 505.3 334.6 Peak torque (n-m) 329.2 197 Avg. torque (N-m) 247.6 155.6 Time of peak (s) 3.8 4.3 Duration (s) 4.5 4.6

Condition Lateral flexion left

372.1 313.5 451.8 657.4 737 994.8 318.4 278.8 340.1 509.7 512.2 744.9 150.4 100.4 221.4 319.7 270.2 273.9 128.7 89.3 166.6 247.9 187.7 205.1

3.5 3.4 4.2 3.7 4.7 4.5 4.6 4.6 4.5 4.6 4.7 4.7

0 o 10 ° 20 °

Lateral flexion right

30 ° 0 ° 10 ° 20 ° 30 °

Peak force (N) 386.6 351.9 Avg. force (N) 310.8 289 Peak torque (n-m) 189.4 170 Avg. torque (N-m) 152.3 139.6 Time of peak (s) 3.7 2.5 Duration (s) 4.4 4.6

Condition Axial rotation left

277.3 136.1 352.6 328.7 212.9 178.1 222.1 111.6 286.9 278.4 179.2 148 128.8 59.5 172.8 158.8 98.9 77.8 103.2 48.8 140.6 134.5 83.3 64.7

3.9 3.7 4 3.5 3.4 3.7 4.5 4.5 4.6 4.5 4.6 4.5

Axial rotation right

0 o 5 ° 10 ° 15 ° 20 ° 0 ° 5 ° 10 ° 15 ° 20 °

Peak force (N) 638.4 515.4 479 407.8 411.1 474.2 537.3 541.3 364.9 Avg. force (N) 469.5 390.6 371.5 317.3 309.3 362.1 411.6 397.4 291.2 Peak torque (n-m) 108.5 87.6 81.4 69.3 69.9 80.6 91.3 92 62 Avg. torque (N-m) 79.8 66.4 63.2 53.9 52.6 61.6 70 67.6 49.5 Time of peak (s) 4.2 3.3 3.9 3.1 3.3 3.6 4.4 4.2 3.1 Duration (s) 4.5 4.7 4.7 4.6 4.7 4.7 4.9 4.8 4.7

481.5 370.7 81.9 63

3.7 4.8

Table 2 Sample numerical result output for isokinetic conditions

Condition Flexion Extension Lateral flexion Axial rotation

Left Right Left Right

Peak force (N) 485.1 612.5 267.9 190.4 383.5 335 Average force (N) 393.9 435.1 194.6 126 310.5 256.9 Peak force (N-m) 225.3 237.3 129.1 90.3 65.2 56.9 Average torque (N-m) 168 166.4 91.6 57.7 52.8 43.7 Time of peak (s) 2.6 2.2 1.2 0.7 2.6 2.6 Duration (s) 4.7 4.6 4.5 4.6 4.7 4.7 Lin disp at peak (cm) 16.2 14 8.3 5.7 8.1 8.1 Trunk angle at peak (deg) 20.5 17 11 - 15.5 - 3 0 33.1 Total lin disp (cm) 26.1 25.3 24.2 22.4 13 13 Total angle disp (deg) 41.9 47.3 26 - 27.2 - 44.1 48.4

Table 3 Peak and average values of force and torque and time relationship with linear and angular displacement

Peak Average Peak Average Time of Duration Disp. at Trunk angle force (N) force (N) torque (N-m) torque (N-m) peak (s) (s) peak (cm) at peak (deg)

Trial 1 421.4 336.0 192.5 144.4 2.9 4.7 17.4 23.4 Trial 2 521.3 423.6 240.0 182.5 2.9 4.8 17.4 23.0 Trial 3 512.6 422.1 243.5 177.2 2.1 4.7 13.8 15.2 Summary 485.1 393.9 225.3 168.0 2.6 4.7 16.2 20.5

100 S. Kumar / lnternational Journal of lndustrial Ergonomics 17 (1996) 91-101

Table 4 Sample numerical output of three trials of isokinetic flexion and changing values at 10% intervals of the task cycle

Percent of the Force (N) Torque Linear dis Trunk angle task cycle (N-m) (cm) (deg)

Trial 1 0 39.1 - 19.1 0.0 - 4 . 2 10 282.4 138.3 4.6 1.9 20 293.2 142.9 7.0 6.1 30 308.4 149.1 10.0 9.8 40 336.6 160.5 12.2 14.2 50 397.5 186.0 14.4 18.7 60 414.8 190.4 16.8 22.6 70 406.2 182.1 19.0 26.6 80 408.3 178.9 21.4 30.0 90 349.7 149.8 23.3 33.1

100 321.5 133.0 25.5 37.4 Trial 2 0 36.9 - 18.0 0.0 - 6.4

10 356.2 174.5 4.9 1.5 20 412.7 201.2 7.3 5.8 30 445.3 215.3 10.3 9.8 40 477.8 227.6 12.8 14.5 50 501.7 234.4 15.2 19.1 60 521.3 238.5 17.6 23.1 70 499.5 222.3 20.1 27.7 80 456.1 199.6 22.3 30.2 90 406.2 174.2 24.7 32.9

100 369.2 153.9 26.6 36.5 Trial 3 0 36.9 - 17.9 0.0 - 4 . 2

10 356.2 - 174.5 4.9 1.9 20 432.2 211.3 7.3 6.1 30 486.5 235.8 10.3 9.8 40 490.9 235.4 12.5 14.2 50 506.1 239.3 14.7 18.7 60 456.1 213.8 16.8 22.6 70 447.4 206.6 19.5 26.6 80 414.8 187.9 21.7 30.0 90 404.0 179.5 23.9 33.1

100 410.5 176.8 26.3 37.4 Summary 0 37.6 - 18.3 0.0 - 6.4

10 331.6 46.1 4.8 1.0 20 379.4 185.1 7.2 5.3 30 413.4 200.1 10.2 9.5 40 435.1 207.8 12.5 13.8 50 468.4 219.9 14.7 18.0 60 464.1 214.2 17.1 21.3 70 451.0 203.7 19.5 25.2 80 426.4 188.8 21.8 28.4 90 386.6 167.8 24.0 31.3

100 367.1 154.5 26.1 35.4

5. Conclusions

Using FELT-I and II, and AROT in conjunc- tion with the developed testing protocol and software, the functional assessment of an impaired or

a normal back can be done quantitatively. The relatively low cost devices and user-friendly methodology will permit the mapping out of motion coupled force exertion capability. Such an

S. K umar/International Journal of Industrial Ergonomics 17 (1996) 91-101 101

approach will allow an accurate and objective assessment of impairment, recovery and work worthiness of the injured workers. Objective data collected from an approach like this has a potential to minimize the conflicts between the injured and the insurance companies optimizing resource utilization. In addition, a database developed for normal sample may help maximizing safety and optimizing job accommodation.

the financial support of this project. The help received from Mr. Doug Garand in the development and debugging of the software is duly noted. I would also like to thank the volunteers in the project who participated in the program.

Reference

Acknowledgement

The author wishes to express his gratitude to the Workers Compensation Board of Alberta for

Kumar, S., 1996. Isolated planar trunk strength and mobility measurement for the normal and impaired backs: Part I - The devices. International Journal of Industrial Er- gonomics, 17(2): 81-90 (this volume).

trunk strength and mobility measurement for the normal and impaired backs: part ii — protocol,...

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