Muscle Force Assessment in the Presence of Indeterminate Deficiency

Joseph Mizrahi

, Etgar Marcus

and Avi Wiener

Technion, Israel Institute of Technology, Haifa, Israel

Rambam Medical Center, Haifa, Israel

Keywords: Indeterminate Deficiency, Force Assessment, Mind Distraction, Computer Adaptive Testing.

Abstract: Indeterminate deficiency is encountered when a subject puts an artificial limit on his/her muscle abilities, in

which case his/her true muscle force cannot be assessed in a straight forward manner. Self-restriction of

muscle performance during testing may be unintentional due to lack of motivation, lack of self-confidence or

due to fear of pain. It can, however, be intentional resulting from unwillingness to cooperate. The aim of this

study was to develop an objective system aimed at directing subjects into generating more intense muscle

forces than they aimed, thus leading to their true force potential. The methodology used combines mind

distraction techniques and Computer Adaptive Testing (CAT) theory. The first element is aimed at distracting

the examinees from restricting their muscle force, or shifting the interval of convenience of their performance.

The examined subject is thus directed into generating higher muscle forces than he/she has intended to, thus

reaching the highest possible force. Practically, the tested subject is asked to perform tasks of varying

intensities, visually presented in subject-unanticipated real-time converted-scale display biofeedback. For the

second element of our methodology we made use of the Item Response Theory custom-tailored itemized test,

for people with various ability levels. A developed iterative CAT algorithm provided, individually for each

subject, rapid convergence to the highest force level possible. The results of this study bear potential

significance in two major areas: Rehabilitation Medicine, by indicating training procedures for unconfident

patients and in Occupational Medicine, for the functional evaluation of subjects who exhibit lack of

motivation or cooperation.

1 INTRODUCTION

The neuro musculo skeletal system is characterized

by redundancies, whereby a motor task can be

normally performed with the simultaneous

involvement of more muscles than strictly necessary.

Furthermore, this same task may be performed in

multiple ways, with different muscle combinations.

From the mechanical viewpoint the musculoskeletal

system is indeterminate, whereby the number of

unknown muscle forces exceeds the number of

available equations. Impairment has been associated

with reduced redundancy (Mizrahi et al 2011). The

level of indeterminacy was shown to decrease with

the reduction of redundancy, e.g. through single

stance (compared to double stance) standing or

through fatiguing of part of the muscles implying, that

higher levels in the nervous system become more

unequivocally related to lower levels (Levin et al

2000; Suponitsky et al 2008;). An interesting type of

deficiency is found when neither the identity of

impairment, nor its extent are certain, thus suggesting

the term Indeterminate Impairment.

This study deals with false impairment,

encountered when tested subjects, either

unintentionally or intentionally, exert an untrue limit

on their force, in which case their real capacity cannot

be disclosed in a straight forward measurement. The

need for force measurement arises when physical

performance is to be assessed such as in Occupational

Medicine.

Assessment of muscle force is usually performed

by evaluating the maximal voluntary contraction

(MVC) of the tested muscle (Chaffin et al, 1999),

necessitating cooperation of the tested subjects.

However, since the muscles are sub-activated because

the tested subject does not apply his/her maximal

force capacity, the accuracy by which the measured

force reflects the real muscle capacity becomes

questionable (Shechtman et al 2012, Garcia et al

2014). This may happen in either of the following two

cases: (a) unintentional self-restriction, due to lack

motivation or self-confidence, fear of pain or injury

Mizrahi, J., Marcus, E. and Wiener, A.

Muscle Force Assessment in the Presence of Indeterminate Deﬁciency.

DOI: 10.5220/0007565400810086

In Proceedings of the 12th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2019), pages 81-86

ISBN: 978-989-758-353-7

(Sullivan et al. 2009), guarding; or (b) deliberate

restriction, due to low, or no willingness to cooperate.

Muscle force malingering has been reported in the

literature. Although there are no definite indications

about the extent of malingering during physical

functional assessment, reviewed studies have

indicated that malingering does occur within the

chronic pain setting, at the rate of 1.25-10.4% of

chronic pain complaints (Fishbain et al 1999).

Here, we developed a Computer Adaptive Testing

(CAT) system (Hambleton 2000; Hays et al. 2000),

specially adapted for physical testing, and aimed at

objectively evaluating muscle forces, in those cases

where the muscles are sub-activated. Basically, CAT

is based on modern testing theory to select the optimal

next task to be administered based on the examinee’s

previous responses.

In addition, we established a computerized task

algorithm, combined with real-time visual feed-back

designed to distract the examinees’ attention away

from the actual testing objectives. We hypothesized

that, by combining CAT principles and mind-

distraction techniques, it is possible to direct subjects

into generating a greater sub-maximal force than they

intended (Sullivan et al. 2009; Wiener et al 2007). In

this way, it was expected that a more objective MVC

can achieved from the tested subject (closer to the real

capacity of the muscle), which is less dependent on

the level of cooperation of the participating subjects

in the testing procedure. The results of this study bear

potential significance in two major areas:

Rehabilitation Medicine, by indicating training

procedures for unconfident patients and, in

Occupational Medicine, for the functional evaluation

of subjects who exhibit lack of motivation or

cooperation.

2 MATERIALS AND METHODS

2.1 Apparatus

Muscle forces were measured through the

corresponding joint torques. Two experimental

systems were thus designed to measure the torques

during isometric extension efforts of the knee, or

during isometric flexion of the elbow. The systems

were adjustable with restraints on which the volunteer

subjects were seated during testing (Fig. 1). Data

acquisition was made by sampling the force from the

transducer at 200 Hz into an A/D acquisition card and

a Lab-View interface (National Instruments

Corporation, Austin, TX).

2.2 Procedure

The concept of CAT combined with mind distraction

was tested on a group of 11 able-bodied female

subjects, aged 26.2 (1.6) years, who were instructed

to cooperate in fulfilling the testing instructions, as

described in Table 1.

First, the examinee was asked to exert the highest

isometric force, denoted as the initial maximal

voluntary contraction (MVCi). This was repeated

three times with an interval time of 10 min for

averaging the actual MVCi. It was assumed that,

since the subjects were cooperative, the measured

MVCi was representative of the true initial maximal

voluntary contraction. Thus, except for adjustment

due to possible existence of fatigue, the individual

MVCi could be used as a reliable measure for force

normalization.

Table 1: Summary of the Testing protocol, indicating

subject cooperativity.

Exert MVC Subject cooperates

Training to memorize Fssl Subject cooperates

CAT tests Subject does not obey

task, but instead observes

his/her force limit Fssl

Post-test MVC Subject cooperates

Repeatability of Fssl Subject cooperates

Thereafter, each tested subject was trained to set

and memorize a self-selected force limit Fssl (usually

near 20–50% his/her MVCi). The training process

was made with real-scale feedback, as displayed on a

monitor and was not limited in time. Training was

terminated when the examinee felt confident in being

able to memorize his/her Fssl, which usually

corresponded to force reproducibility of better than

5% of the selected Fssl. This memorized force (force

or joint memory) was later used by each subject to

apply force restriction irrespective of the force being

instructed to apply in the testing algorithm. It should

be noted that successfully memorizing this limit force

and observing the limit was essential for indicating

whether the testing algorithm was capable of

distracting the tested subject to higher forces.

BIODEVICES 2019 - 12th International Conference on Biomedical Electronics and Devices

Figure 1: Isometric testing apparatus: top, for knee

extensors; bottom for elbow flexors. Experimental features

include, among others, dual visual screens and EMG

electrodes.

2.3 CAT Testing Algorithm

The CAT phase consisted of two different tests: The

algorithm-test and, the repeatability-test. The

algorithm-test was designed to verify whether the

CAT system could cause the examinee to

unintentionally exert a higher force than he/she

intended to. We developed an algorithm aimed to

distract the examinee’s attention from the objective of

keeping steady the memorized limit force on each

new task. For this purpose, the algorithm consisted of

two scales to describe the examinee’s effort. A real

scale was displayed on the operator screen monitor

and a false (converted) scale was displayed on the

examinee’s (apparent) screen monitor (Fig. 1, top).

The examinee was asked to perform a force-

application task which, on the operator monitor was

displayed in MVC percentage units (real force). This

was indicated on the screen by means of two

horizontal parallel “target lines” between which the

force should be aimed. On the examinee’s monitor,

this same task (apparent force) was represented in a

converted scale. The scaling (or distraction) factor SF

is defined as Apparent Force / Real Force. During

task performance, the examinee was reminded not to

exceed his/her self-selected limit force (Fssl).

For each testing cycle, a baseline mission was set

and, every task was considered to be equal 100%,

50% or 150% of this baseline according to the

difficulty level set by the force target factor (FTF).

On the operator screen, the real Ftarget force was

displayed. On the examinee’s screen the task was

represented as the product of the target force, the

scaling factor (SF, or distraction factor) and the force

target factor (FTF). Values of SF, and FTF in a cycle

are given in Table 2.

Table 2: Values of SF and FTF for every task, in a one

complete cycle.

One Task cycle

Task 1 2 3 4 5 6

SF 1 2 1 0.5 0.5 0.5

FTF 1 0.5 0.5 0.5 1 1.5

The operator terminated the algorithm test when

the examinee exerted a significantly higher force than

his/her self-selected limit force, or after completion of

two cycles.

2.4 Adjustment for Muscle Fatigue

After completion of the CAT, the maximal voluntary

contraction was measured again to verify whether the

muscle had undergone fatigue during the test (final,

MVCf). This was repeated three times and the

average was taken to represent the actual MVCf. Thus,

the true final maximal voluntary contraction MVCf,

provided a correction for MVCi due to the possible

development of muscle fatigue during the test. The

Muscle Force Assessment in the Presence of Indeterminate Deﬁciency

values of MVCf and MVCi were used to linearly

interpolate MVCtask, corresponding to each actual

task and to which the force values were normalized

during the test.

2.5 Repeatability of Joint Memory

Repeatability of joint memory was tested one week

after the CAT with similar number of cycles and

testing durations as in the CAT. Its purpose was to

confirm the subject’s ability to remember her Fssl.

3 RESULTS

A typical force-task during a complete test cycle is

shown in Fig. 2. The memorized force limit (0.4

MVC, in this case) is displayed by the horizontal line.

is noted that the task intensities generated by the

Figure 2: Force tasks in complete cycle (36 tests) for subject

10 (with Fssl at 0.4 MVC, shown as a horizontal line). Note

the zigzag effect of the algorithm on the task intensity

between the tasks. This effect is added to the distraction

effect caused on the subject’s screen. Force results are

shown: without fatigue adjustment (o) and with fatigue

adjustment (x).

CAT algorithm demonstrate a zigzag pattern. The

tested subject obtains these tasks on his/her screen in

a false (converted) scale, thus distracting him/her

from the real task, resulting in producing higher

forces than he/she meant to.

Fig. 3 demonstrates the force output for one

subject, as obtained using the CAT in a complete

cycle (top), as compared to the force output during the

repeatability test. The force values at the initial and

final ends of the test were normalized by using the

respective MVC values. For any intermediate task,

the MVC was interpolated. In the CAT (upper panel)

the examinee’s performance was characterized by

oscillations of the exerted forces below and over Fssl.

The applied force eventually reached a magnitude of

as high as 2.34 Fssl (score of the CAT). However, in

the repeatability test (lower panel), the fluctuations

around the level of Fssl were smaller compared to the

CAT with a maximum score of 1.88 Fssl. Thus, the

testing algorithm was successful in ‘tricking’ subject

5 and a higher force was obtained despite her attempts

to maintain the memorized force Fssl.

Summary of the scores, for the CAT and for the

repeatability tests, for all the subjects, is presented in

Table 3. The mean score values of the CAT and the

repeatability tests for all the participants were 1.995

Fssl and 1.554 Fssl, respectively, with a statistically

significant difference (p < 0.05) between them. Ten

out of the eleven subjects rendered a higher force than

intended by their self-imposed limit.

Figure 3: Force applied (normalized to base-line force Fbl):

comparison between CAT and repeatability tests (subject

5). The value of Fssl was in this case 0.3 MVCi. Test score

denotes the highest force value attained during the test

(CAT, or repeatability).

4 DISCUSSION

Accurate functional testing and physical

measurement is important for the objective diagnosis

of physical impairment. This however is being

prevented whenever the tested subject sets an

artificial upper limit on his/her muscle force, resulting

in sub-activation of the muscle. The true force

capacity cannot in this case be assessed or utilized in

a straight forward manner. In this study a model of

able-bodied subjects served to simulate muscle sub-

0 5 10 15 20 25 30 35 40

0.2

0.4

0.6

0.8

1.2

1.4

Performance - Algorithm Test - subject # 10

Normalized Force

Task [#]

Performance (Fmeasured) without fatigue adjustment

Performance (Fmeasured) with fatigue adjustment

Self Selected Limit Force [level of MVC]

Task Difficulty (Ftarget)

0 5 10 15 20 25

Performance - Algorithm Test - subject # 5

Normalized Force

Task [#]

Subject Task Performance (Fmeasured)

Self Selected Limit Force [0.3 MVC]

Score= 2.3442

0 5 10 15 20 25

Performance - Repeatability Test - subject # 5

Normalized Force

Task [#]

Subject Task Performance (Fmeasured)

Self Selected Limit Force [0.3 MVC]

Score= 1.8881

BIODEVICES 2019 - 12th International Conference on Biomedical Electronics and Devices

Table 3: Results of algorithm and repeatability tests for all

subjects.

Subject # CAT Algorithm Repeatability

1 1.979 1.403

2 2.089 1.183

3 2.436 2.674

4 0.957 0.954

5 2.344 1.888

6 1.385 1.155

7 2.027 1.552

8 3.193 1.742

9 1.850 1.590

10 2.458 1.672

11 1.225 1.279

Average (SD) 1.995 (0.64) 1.554 (0.47)

activation. These subjects were trained to memorize a

certain sub-maximal force in their muscle (via its

corresponding joint). In subsequent force tasks, each

subject was instructed to watch that during task

performance his/her memorized force is not exceeded.

The force tasks were within a series of tests

combining CAT and mind distraction. The former

element was based on IRT (item response theory), a

custom-tailored itemized test based on probability of

a favourable outcome for people of various ability

levels, and testing was done in iterative steps taking

into account the subjects abilities and task scores. The

second element, mind distraction, was designed to

distracting the examinee from restricting his/her

muscle force, or shifting the interval of convenience

of his/her performance, so as to direct him/her into

generating higher muscle forces than he/she has

intended to, and thus reaching the highest forces

possible for this subject. The results demonstrated

that in 10 of the 11 tested subjects the algorithm was

successful in revealing higher forces in knee

extensors than intended by the tested subjects. The

tests should be extended to elbow flexors as well.

These results can serve as an encouragement to apply

the method described to populations of subjects

characterized by muscle sub-activation, either from

unintentional or intentional reasons.

Unintentional muscle sub-activation may be due

to lack of motivation, lack of self-confidence, or fear

of pain, such as may take place after trauma, injury or

surgery and involving time periods of recovery and

rehabilitation. Intentional sub-activation is

encountered when the tested subject declines to

revealing his/her muscle force potential within his/her

overall physical functioning and resulting in lack of

willingness to cooperate.

The significance of revealing the true muscle

force potential is thus clear: In rehabilitation

medicine, Increased rehabilitation and treatment

progress and efficiency, design and assessment of

suitable treatment plan and increased self-esteem of

the patient, decreased burden on patients; In

occupational medicine, better functional assessment

and return to employment, decreased cost to patients,

treatment facilities, and insurance companies. Future

work should extend the suggested method to these

populations.

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BIODEVICES 2019 - 12th International Conference on Biomedical Electronics and Devices