AN OBJECTIVE METHOD TO EVALUATE FORCE AND KNEE

JOINT MOMENTS DURING ISOMETRIC EXTENSION

F. Paez

a,b

, C.

Frigo

a,b

, E. Pavan

a,b

, E. Guanziroli

a,b

and S. Frasca

a,b

Laboratory of Movement Biomechanics and Motor Control (TBM lab)

Department of Bioengineering, Polytechnic of Milan, Milan, Italy

Address: Via Garofalo, 39 Milan Italy

Keywords: Knee extension, moment of knee joint, isometric knee extensor torque.

Abstract: A simple method to evaluate force and moments of knee joint during isometric extension has been

developed and provides to the physicians a fast and objective tool for the evaluation of patients before and

after a surgery or rehabilitative program. The experiment was made on normal young patients. Graphs of

angle-moment were obtained. The patients started from 90° of knee flexion and extended step-by-step the

knee joint until the maximum knee extension was achieved. Force, angle and moment were measured at

each step. In comparison with literature, even if significant differences of technical instrumentation, age and

activity of the patients are present, the maximum moment-angle behaviour during extension is the same but

different magnitude. Future development of this device is to make it easy to use directly in clinical

applications.

1 INTRODUCTION

When a patient with a neuromuscular disease is

subject to an intervention or physical rehabilitation,

it is always necessary to make a physical evaluation

to check the functional state of the muscles and

joints. In the specific case of spastic patients, several

methods exist to see the deficit of active extension

angle (DAE) and the maximum extension force

(MEF) (Rabaiotti, 2004). Also, most mathematical

models describe the forces in the knee under

isometric quadriceps contractions (Huss et al.,2000).

The most common methods of measure used are: A

manual force test, manual dynamometers and

isokinetic dynamometers. It is usually assumed that

the moment measured by the dynamometer is

equivalent to the resultant joint moment

(Adamantios et al.,2004). Some of those methods, as

the manual force test, are subjective and not precise

because it depends on the magnitude of the manual

force the evaluator can exert on the patient. Other

methods require a big instrumentation or are

relatively expensive because of the technology of the

machine, such as the Isokinetic Dynamometer.

The aim of the present work is to create a simple,

portable and economic device for the measure of

forces and moments of knee joint during extension,

and to provide to physicians a fast and objective tool

for the evaluation of their patients before and after a

surgery or rehabilitative programs.

2 MATERIALS

A mechanical device that is attached to the base of

the bed where the patients are lying supine (See Fig.

1).This device includes a force cell connected with a

string to the leg of the patient in order to calculate

the tension force made by the leg of the patient

during the knee extension.

An electrogoniometer made by a precision linear

potentiometer in order to measure the flexion angle

of the knee. A conventional video camera

synchronized with the electrogoniometer and the

force cell in order to acquire the different positions

of the knee during the extension. A Software (MB

Ruler) for bidimensional analysis of images

(distance and angles) in order to calculate the angle

of the force cell with respect to the ground and the

angle of the string with respect to the leg of the

patient during the knee extension. A Software

environment in MATLAB to acquire and synchronize

the data of the angles of the electrogoniometer and

the forces of the cell.

228

Paez F., Frigo C., Pavan E., Guanziroli E. and Frasca S. (2008).

AN OBJECTIVE METHOD TO EVALUATE FORCE AND KNEE JOINT MOMENTS DURING ISOMETRIC EXTENSION.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 228-231

DOI: 10.5220/0001055102280231

 SciTePress

Figure 1: Instrumentation of the patient.

PN: Patient

PT: Precision Linear Potenciometer

E : Elettrogoniometer

ST: String

PL: Pulley

CS: Cell Support and system for attach to bed

BR: Braces for fix the electrogoniometer and the

Cell Support

FC: Force cell

3 METHODS

The mechanical device is attached to the bed and

supports the force cell that is connected by a string

to the ankle of the patient. 26 normal patients

participated in the experiment. (9 boys age 10

+/-

2.24

years and 17 girls age 10.12

+/-

1.87years). The

patient is in supine position with both legs outside

the bed and flexed to 90° (See fig. 1). Considering

that there is a decline of 48

+/-

11% in the mean

dynamic flexion torque by fatigue (Beltman et al.,

2003) and that some differences are caused by the

time-of-day of the exam (Onambele-Pearson et

al.,2007) we recorded only the maximum moment

on the first trial for each angle and made the exam

to each patient at morning. The patient is

instrumented with the electrogoniometer aligned

with the axis of rotation of the knee joint, which is

defined as the midpoint of the segment connecting

the lateral and medial condyles. The knee is flexed

initially at 90° and the patient is ordered to extend

his knee and as a consequence pulling the string, the

force is then recorded by the cell. In a next step the

length of the string is manually increased by an

operator which controls the pulley and consequently

the angle of knee flexion is changed while the force

measurement continues until it arrives to the

maximum extension of the knee which is at 0°. The

conventional video system, synchronized with the

electrogoniometer and the force cell is made in order

to acquire the different positions of the knee during

the extension and be analyzed by the software for

bidimensional images in order to calculate the angle

of the force cell relative to the ground and the angle

of the string with respect to the leg of the patient

during the knee extension.

Anthropometric measurements of the patients

allowed us to calculate mass properties of the leg

and to compensate for gravitational force. A

MATLAB algorithm takes all the data

(electrogoniometer, force cell, inertia properties,

anthropometrical data, force cell-ground angles and

string-leg angles) to calculate the perpendicular

force to the leg during each measure and

consequently the resultant knee torque with planar

analysis. (See Fig. 2).

Figure 2: Free body diagram of the patient shank and force

cell.

dl : Lever arm of force Fr to the knee joint

cm: Lever arm of Wl to the knee joint

φ: Angle of knee flexion

α: Angle of resultant force F

m: Malleolus joint

k : Knee joint

h: Hip joint

: Weight of foot

: Weight

of Leg

: Weight of force cell

: Force in extension and measured by the cell

: Reaction

Force of the cell, perpendicular to

mk segment.

The reaction force F

r was to be assumed

perpendicular to the patient shank mk.

AN OBJECTIVE METHOD TO EVALUATE FORCE AND KNEE JOINT MOMENTS DURING ISOMETRIC

EXTENSION

229

4 RESULTS

4.1 Construction of the Device

A simple method to evaluate force and moments of

knee joint during isometric extension has been

developed. It provides to the physicians a fast and

objective tool for the evaluation of their patients

before and after an orthopaedic surgery or

rehabilitative program.

4.2 Extensor Torque in Normal Patients

The obtained information is useful to understand the

isometric extensor torque on normal and

pathological patients. Normalized moment [N*m/kg]

vs. angle [deg] of the normal patients of this study

are reported in figure 3. Graphs are separated in

male (thin line) and female (thick line) subjects.

0 10 20 30 40 50 60 70 80 90

0.5

1.5

2.5

Angle of Knee Flexion [Deg]

Normalized Knee Torque [Nm/kg]

Female

Mal e

Figure 3: Normalized Moment torque vs. flexion angle.

The results demonstrated significant differences

between gender according to Pincivero et al. (2004).

The highest torque was generated at 70° for men and

50° for women. Both curves have a continuous

growing behavior until his maximum value to

decrease until maximum knee flexion as reported in

literature (Beltman et al., 2003; Pincivero et al.,

2004; Welsch et al., 1998; West et al., 2005).

5 DISCUSSION

5.1 Construction of the Device

Future development of this device is to make it

usable in clinical applications. To make the process

faster and more precise, it’s specifically necessary to

eliminate the measures made by the video system

and instead install potentiometers to measure the

shank-string angle and force cell-ground angle.

5.2 Extensor Torque in Normal

Patients

If we make a comparison with literature, even if

significant differences of technical instrumentation,

age and activity of the patients are present, the

results have the same behavior but are different in

magnitude: We made the experiment with a self

constructed device on 9 occasionally active boys age

+/-

2.24 years and 17 occasionally active girls age

10.12

+/-

1.87 years, while Pincivero et al. (2004)

experimented with a Biodex Isokinetic

Dynamometer on 14 men age 25

+/-

4years and 14

women 23

+/-

4 years all physically active, as they

reported performing various types of routine

exercises. Beltman et al. (2003) doesn’t report the

data normalized, (only the torque in Nm) but the

behaviour of the curve is similar and he used an

Isokinetic dynamometer (Lido Active, Loredon

Biomedical, Davis) on 7 recreationally active male

subjects age 27

+/-

8 years. Welsh et al. (1998)

experimented with 39 active men age 29.7

+/-

12.6

years and 38 active women age 27.2

+/-

11.3 years

with an isometric knee flexion extension strength

testing device; so we can conclude that differences

in age, activity and instrumentation explains the

higher values of torque of those experiments with

respect to our study.

ACKNOWLEDGEMENTS

The Authors of this study would like to thank the

team of the Istituto Clinico Humanitas, Rozzano

Italy, Prof. Nicola Portinaro MD., Francesco Pelillo

MD. and Federica Spreafico MD. for the

collaboration in the study.

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