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A dynamometer for the measurement of the extension torque of the lower leg during static and dynamic contractions of the quadriceps femoris muscle
J E,omrrhan,cs Vol. 16. Na Prmted ,n Greal Brmm
I?. pp 1019-1023.
OOZI-9290.83 $3.00 + 00 Per~amon Pm, Lrd
1983
TECHNICAL
NOTE
A DYNAMOMETER FOR THE MEASUREMENT OF THE EXTENSION TORQUE OF THE LOWER. LEG DURING STATIC AND DYNAMIC CONTRACTIONS OF THE QUADRICEPS FEMORIS MUSCLE T. M. G. J. VAN Laboratory
of Anatomy
and Embryology,
EIJDEN and
W.
DE BOER
University of Amsterdam, The Netherlands
Mauritskade
61. Amsterdam,
and J. VERBURG Laboratory
of Medical
Physics,
University of Amsterdam, The Netherlands
Herengracht
196, Amsterdam,
Abstract-A dynamometer which makes an angular movement is described. The dynamometer enables the measurement of the extension torque of the lower leg at different knee angles during static and slow concentric and eccentric contractions of the quadriceps femoris muscle. The influence of gravity on the measured torque signal can be compensated for by another signal representing the angular movement. The application of the dynamometer is demonstrated by giving an example of measurement.
INTRODUCTION The use of dynamometers has become a reliable method in evaluating muscle strength in normal and pathological conditions (Kroemer and Marras, 1980; Lankhorst et al., 1982). However, with the majority of the dynamometers only static measurements of muscle strength can be made. A few devices have been described for the measurement of muscle strength both under concentric and eccentric conditions. Asmussen et al. (1965) constructed a dynamometer with which the maximal horizontal arm pull strength was measured in concentric or eccentric contractions of the upper arm retroflexors and forearm flexors. For measuring the forearm flexion or extension force, Doss and Karpovich (1965) constructed a dynamometer in which the forearm movement was forced upon by the experimentor. Singh and Karpovich (1966) built a dynamometer in which the movement was enforced by a motored appliance. Komi and Buskirk (1972) used an arm dynamometer in which the velocity of movement could be varied. In order to measure the leg extension strength, Komi (1973) constructed a dynamometer in which the leg of the subject was moved in either flexion or extension by a footsupport driven by a motor. Smidt (1973) used a force table to measure the extension torque and flexion torque of the lower leg. These dynamometers have some disadvantages. With the exception of the dynamometers described by Doss and Karpovich (1965) and Singh and Karpovich (1966), the movement of these dynamometers is linear, whereas the movement of the limbs is angular. This means that during the movement the lever arms of both muscle-joint system and the force measuring unit may change, and consequently a variation in the measured force is not necessarily due to changing muscle forces. The effectiveness of a muscle-joint system is in our opinion best represented by the torque the system can produce. This torque reflects both the muscle force and its iever arm. The force measured somewhere in this Received 23 Norember
1982; in revisedform
30 May 1983. 1019
system is under static conditions generally simply related to torque. In a dynamic situation, however, the lever arms of the measuring system may change, as mentioned above, and the relation between torque and force is no longer that simple. Another objection to be made is that, with the exception of Smidt (1973), investigators using these dynamometers have not taken into account the influence of gravity on their measurements. This means that the force they measure contains either a positive or a negative contribution by gravitational forces of one or more parts of the body. In our opinion, gravitational force should not be neglected, especially in those experiments where the whole body or the lower limb plays a part. With these objections in mind, a dynamometer was built for measuring the extension torque of the lower leg in static and slow concentric and eccentric contractions of the quadriceps femoris muscle.
DESCRIPTION
OF THE DYNAMOMETER
Consfruclion* The dynamometer enables the measurement of the extension torque of one ofthe lower legs in both staticand dynamic contractions of the quadriceps femoris muscle at different knee angles with intervals of 15’. For the dynamic measurements the lower leg is moved by a motor driven lever, which makes an angular movement through an angle of lo’, either in flexion or extension. The quadriceps femoris muscle is activated by pushing the lower leg against the lever and due to the movement of the lever, the contraction of the muscle will be concentrically during extension and eccentrically during flexion.
*A part of the dynamometer was built under supervision of Professor B. Bangma, Head of the Department for Rehabilitation of the Faculty of Medicine of The University of Rotterdam.
Technical
Note
-
8\
Fig. 1. The dynamometer: A: lever; B: round disc; C: bearing; D: bolt; E: adjustable legsupport; P: potentiometer; S: straingauges. The arrows indicate the size and direction of the angular movement.
The main parts of the dynamometer are given in Fig. 1. The lever A is connected with the round disc B by means of the bearing C. The axis of the bearing coincides with that of the disc. By means of the bolt D, the lever can be fixed in one of the holes along the rim of the disc and positioned at different angles with an interval of 15”. A reversable electric motor drives through a gear box the disc-and thus the lever-in a clockwise or counter-clockwise direction through a range of maximally lo”. Two limitswitches are mounted to stop the lever automatically to terminate each movement. The movement can furthermore be stopped at any point within the above mentioned range. A potentiometer P coupled to the disc, acts as transducer and thus makes a continuous recording of the angle of the lever possible. The angular velocity is nearly uniform and has a value of 2” s- ‘. The impedance of the construction is so big that this velocity will not be altered by the torque exerted by a subject. Just above the bearing, two pairs of strain-gauges S are mounted on both sides of the lever. These strain-gauges are connected in a full bridge. With an amplifier and a recorder this forms the torque measuring system. Calibration The lever is placed in a right horizontal or left horizontal position. At a distance of I m different known weights (from 5 kg to 30 kg) are suspended. The measuring system of the dynamometer is considered to be reliable, if the torque recordings with different weights are linear and identical-in an absolute sense-for the left and right horizontal position.
rotation
fixation
on dish
axis
Fig. 2. Diagram of forces working on the lever due to gravity; IV,: weight leg; IV,: weight lever; X1 ; assumed distance from rotation axis to center of gravity of the lower leg; X,: assumed distance from rotation axis to center of gravity of the lever.
As long as A4 < 5”, cos A4 x 1 (error < 0.47,) sin A~#Jz A+ (error < 0.2 7,). We therefore may write
and
Tdyn = T,,,, + C A~#Jcos 4. As in the static case the strain gauge bridge amplifier is adjusted to zero with the lever in the mid position (A+ = 0). The voltage due to gravity produced by this amplifier during a dynamic measurement is then given by E dyn = a C A@ cos 4 (a being a transducing
constant).
From the potentiometer which is coupled to the lever (Fig. 1) and is fed with a constant voltage, a voltage is derived proportional to A#, thus measuring the rotation. With a simple analogue multiplier (multiturn potentiometer G in Fig. 3) this voltage is made equal to Edyn and subtracted from Ed,,,, with a subtracting operational amplifier (SA in Fig. 3). thus compensating the gravity influences on the measured torque. Operation of this device is as follows. If a new mid position is chosen the strain gauge bridge amplifier is set to zero (static torque compensation). The lever arm and the leg coupled to it are then moved to a position either r#~- 5” or Q + 5” and with potentiometer G (Fig. 3) the torque output is adjusted to zero. Thereafter nothing is changed before a new angle or another leg or subject is chosen. Figure 4 demonstrates the effectiveness of this device. The upper part shows a compensation for an ordinary weight of 5 kg, the lower part for a leg which is inactively rotated by the lever through lo” without muscle action. The ellipsoid curve is due to passive mechanical resistance and stiffness acting on the knee joint.
Compensation for the infZuence of gravity The gravity component in the measured torque is depending on the angle of flexion. In Fig. 2 the situation is schematically drawn in an arbitrary position. In a static measurement the torque due to gravity is given by Ts,at=(WiX,+W,X,)sin+=Csin~. If in this static position the strain gauge bridge amplifier is adjusted to zero, the influence of gravity is compensated for. In the dynamic measurements the angle will change from I$ - 5” to 4 + 5” or in the opposite way. In this traject the torque due to gravity is given by Tdyn = C sin (4 + A4) = C (sin r$ cos Ad + sin AI#JCDS
I$).
Adjustment of the subject The subject (Fig. 5) is sitting on an adjustable chair, with the back placed against a vertical backsupport and the hands grasping the bars at both sides of the chair. In this way he can fixate his body on the chair. The lower legs are hanging vertically downwards. By adjusting the chair, the axis of flexion and extension of the knee (identified by the position of the femoral epicondyles) is made coaxial with the axis of the bearing C. For the measurement at other knee angles, the lower leg will be primarily adjusted in this position of 90 of flexion. From this position, the lever-and thus the lower leg-is then positioned in one of the other angles of flexion
Technical
torque signal strain
low pass
from
buffer
gauge budge
transducer
“Olt
I buffer
Fig. 3. Schematic
diagram
j amplifier
of the electronics
for the influence
for the compensation
4’50
of torques
5k
degrees
due to gravity
of flexion
of gravity between 40’ and 50 of Aexion; NC: not-compensated; C: compensated.
(0”~15~-30’~5’~0’--75’-105 -120”). By adjusting the lower leg in this way, the exerted extension force will be always directed perpendicular to the lever of the dynamometer and thus the measured torque will be equal to that exerted by the lower leg. Examplr
560 nF
filter
amplrfier
400
Fig. 4. Compensation
1021
Note
~J’measuremmt
As an example of its application the mean maximal extension torque of the right leg was determined with this dynamometer at 90 for static and between 85. and 95‘ for dynamic measurements in nine healthy female subjects. In both static and dynamic measurements they were instructed to perform a maximal voluntary contraction by pushing the lower leg as strongly as possible against the leg support. The duration of the contraction was 5s and this period was Indicated for the subjects by switching on a lamp. The sequence of the static, concentric and eccentric torque measurements was chosen randomly from the possible per-
mutations. Between these measurements there was a restperiod of 1 min. In Fig. 6 the torque registrations are given of one subject. It can be seen that the torque during the concentric contraction is lower than that during the static contraction and that the torque during the static contraction is lower than that during the eccentric contraction. From the torque registrations the peak torque was taken to calculate the mean maximal torques of the different contraction types and the mean differences between the contraction types (Table 1). Although especially the difference between the static and concentric value is small, the differences are significant as shown by a paired student f-test (c( = 0.05).
DISCUSSION With the dynamometer described above, contraction characteristics of the quadriceps femoris muscle can be investigated at static and slow concentrlc and eccentric
1022
Technical Note
Fig. 5. Position of the subject on theadjustablechair and the position of the leg in relation to the dynamometer; C: bearing; E: adjustable legsupport; B: round disc.
concentric _____ 20
I
static
. . . . . . . ., eccentric .... ........ .....
15
.;’
._-__/-._zT
10
z s -5 J P B
,
,
,
,
1
2
3
4
contractions. As far as we are aware no such a dynamometer was available. Although it is not the purpose of this paper to explain the differences between the torque values of the various contraction types, the results shown here seem to confirm the opinion that the magnitude of muscle force depends on the contraction type. Furthermore the torque due to passive mechanical resistance and stiffness acting on the knee may contribute in the differences which are seen. A dynamometer often used for the measurement of the extension torque of the lower leg in dynamic conditions is the so called Cybex dynamometer (Thirstle et al., 1967).With this dynamometer however, no torques can be measured during eccentric contractions of the quadriceps femoris muscle, because the leg cannot be forced to move by a motor driven lever. Furthermore the Cybex apparatus operates only with one range from flexion to extension and this implies that different flexion angles cannot be adjusted. Compared with the leg extension force dynamometer of Komi (1973), our dynamometer purely measures the torque exerted by the knee muscles. Moving the leg by a footsupport. the Komi dynamometer creates a closed chain and thus there will be also a movement in the hip joint. This means that the retroflexors of the hip could participate in exerting an extension force. Furthermore, the footsupport makes a linear movement and so there is a change in the angle between the lower leg and the lever which measures the torque. This means a change in lever arms and this implies that the measured torque is not simply related to the exerted muscle force. Our dynamometer does not introduce such a change of angle between the lower leg and the force measuring unit. A possible source of error is the definition of the position of the axis of flexion and extension by the epicondyles of the femur. Furthermore, the knee joint is not a simple hinge joint and this means that in movements of the lower leg there is a small displacement of the axis of rotation. However, this displacement is small relative to the length of the lever arm of the shank. One may remark that a limitation of the dynamometer is that a low velocity is used. One should, however, realize that a voluntary contraction should last about 5 s (Kroemer and Howard, 1970). This period is sufficient for the subject to build up his maximal torque. In our studies, we found a mean time of about 4 s which agrees fairly well. This means that one has to choose for a low velocity, because at high velocities the duration of the angular movement is not large enough to perform a maximal torque. The combination of angular velocity (2” s-l) and duration of the angular movement implies for our dynamometer a range of movement of 10”. Still nearly the whole range of flexion can be investigated, because the lever can be positioned at different flexion angles. With the exception of the work of Smidt (1973), the investigators with dynamometers have not taken into account the influence of gravity. Smidt eliminated this influence by a side-lying position of his subjects. The compensation for effects of gravity introduced in our measurements works fairly well for the small angles of rotation which are used. For larger rotation angles non-linear effects are no longer neglectable. Using devices like Burr Brown’s multifunction converter 4301 this problem can be solved along the same lines. Another approach is to read in torque and rotation
l_ 5
time&c.)
Fig. 6. Torque registrations for one subject performing a maximal voluntary contraction. The torque is exressed in kilogrammeter (kg@.
Table 1. Mean maximal torque and standard deviation (+ S.E.) of the different contraction types; mean differences and standard deviation of the differences Contraction
Mean toraue
+ S.E.
Eccentric Static Concentric
18.1 kgm 16.8 kgrn 16.0 kgm
3.5 2.9 3.6
Difference Eccen-stat Stat-concen Eccen-concen
Mean difference
f S.E.
1.3 0.8 2.1
1.4 0.9 1.2
Technical
digitally and process these data in an appropriate manner with a computer. It can be calculated from the body composition data of Dempster (1955) that in our group of subjects the weight of the lower leg would have contributed for maximally 0.2 kgm in the measured torque. This small correction is not important when large torques are measured as in our group of subjects, but is relevant when differences between eccentric, static and concentric torques are studied. Furthermore the compensation for the influence of gravity will be important in investigations where small torques are produced as e.g. in disabilities of the locomotor apparatus.
Acknowledgement-The authors are indebted to Mr. H. van Heuven and Mr. J. Meijer for technical assistance.
REFERENCES Asmussen, E., Hansen, 0. and Lammert, 0. (1965) The relation between isometricand dynamic muscle strength in man. Communications from the Testing and Observation Institute of the Danish National Association for Infantile Paralysis, No. 20. Dempster, W. (1955) Space requirements of the seated operator. WADC rech. Rep. 55, 159. Doss, W. S. and Karpovich, P. V. (1965) A comparison of
Note
1023
concentric, eccentric and isometric strength of forearm flexors and extensors. J. appt. Physiol. 20;351-353. Komi. P. V. and Buskirk. E. R. (1972) Effects of eccentric and concentric muscle conditioning on tension and electrical activity on human muscle. Ergonomics 15, 417434. Komi, P. V. (1973) A new electromechanical ergometer. Proceedings of the 3rd Internationales Seminar fiir Ergometrie (Edited by Hansen, G. and Mellerowiz, H.), pp. 173-176. Ergon, Berlin. Kroemer, K. H. E. and Howard, J. M. (1970) Towards standardization ofmuscle strength testing. Med. Sri. Sports 2, 224-230. Kroemer, K. H. E. and Marras, W. S. (1980) Towards an objective assessment of the maximal voluntary contraction component in routine muscle strength measurements. Eur. J. appl. Physiol. 45, l-9. Lankhorst, G. J., van de Stadt, R. J., van der Korst. J. K., Hinlopen-Bonrath, E., Griffioen, F. M. M. and de Boer, W. (1982) Relationship of isometric knee extension torque and functional variables in osteoarthrosis of the knee. Stand. J. Rehab. Med. 14, 7-10. Singh, M. and Karpovich, P. V. (1966) Isotonic and isometric forces of forearm flexors and extensors. J. appl. Physiol. 21, 1435-1437. Smidt, G. L. (1973) Biomechanical analysis of knee flexion and extension. J. Biomechanics 6, 79-92. Thirstle, H. G., Hislop, H. J., Moffroid, M. and Lohman, E. W. (1967) Isokinetic contraction: a new concept of resistance exercise. Archs phys. Med. Rehabil. 48, 279.282.
I?. pp 1019-1023.
OOZI-9290.83 $3.00 + 00 Per~amon Pm, Lrd
1983
TECHNICAL
NOTE
A DYNAMOMETER FOR THE MEASUREMENT OF THE EXTENSION TORQUE OF THE LOWER. LEG DURING STATIC AND DYNAMIC CONTRACTIONS OF THE QUADRICEPS FEMORIS MUSCLE T. M. G. J. VAN Laboratory
of Anatomy
and Embryology,
EIJDEN and
W.
DE BOER
University of Amsterdam, The Netherlands
Mauritskade
61. Amsterdam,
and J. VERBURG Laboratory
of Medical
Physics,
University of Amsterdam, The Netherlands
Herengracht
196, Amsterdam,
Abstract-A dynamometer which makes an angular movement is described. The dynamometer enables the measurement of the extension torque of the lower leg at different knee angles during static and slow concentric and eccentric contractions of the quadriceps femoris muscle. The influence of gravity on the measured torque signal can be compensated for by another signal representing the angular movement. The application of the dynamometer is demonstrated by giving an example of measurement.
INTRODUCTION The use of dynamometers has become a reliable method in evaluating muscle strength in normal and pathological conditions (Kroemer and Marras, 1980; Lankhorst et al., 1982). However, with the majority of the dynamometers only static measurements of muscle strength can be made. A few devices have been described for the measurement of muscle strength both under concentric and eccentric conditions. Asmussen et al. (1965) constructed a dynamometer with which the maximal horizontal arm pull strength was measured in concentric or eccentric contractions of the upper arm retroflexors and forearm flexors. For measuring the forearm flexion or extension force, Doss and Karpovich (1965) constructed a dynamometer in which the forearm movement was forced upon by the experimentor. Singh and Karpovich (1966) built a dynamometer in which the movement was enforced by a motored appliance. Komi and Buskirk (1972) used an arm dynamometer in which the velocity of movement could be varied. In order to measure the leg extension strength, Komi (1973) constructed a dynamometer in which the leg of the subject was moved in either flexion or extension by a footsupport driven by a motor. Smidt (1973) used a force table to measure the extension torque and flexion torque of the lower leg. These dynamometers have some disadvantages. With the exception of the dynamometers described by Doss and Karpovich (1965) and Singh and Karpovich (1966), the movement of these dynamometers is linear, whereas the movement of the limbs is angular. This means that during the movement the lever arms of both muscle-joint system and the force measuring unit may change, and consequently a variation in the measured force is not necessarily due to changing muscle forces. The effectiveness of a muscle-joint system is in our opinion best represented by the torque the system can produce. This torque reflects both the muscle force and its iever arm. The force measured somewhere in this Received 23 Norember
1982; in revisedform
30 May 1983. 1019
system is under static conditions generally simply related to torque. In a dynamic situation, however, the lever arms of the measuring system may change, as mentioned above, and the relation between torque and force is no longer that simple. Another objection to be made is that, with the exception of Smidt (1973), investigators using these dynamometers have not taken into account the influence of gravity on their measurements. This means that the force they measure contains either a positive or a negative contribution by gravitational forces of one or more parts of the body. In our opinion, gravitational force should not be neglected, especially in those experiments where the whole body or the lower limb plays a part. With these objections in mind, a dynamometer was built for measuring the extension torque of the lower leg in static and slow concentric and eccentric contractions of the quadriceps femoris muscle.
DESCRIPTION
OF THE DYNAMOMETER
Consfruclion* The dynamometer enables the measurement of the extension torque of one ofthe lower legs in both staticand dynamic contractions of the quadriceps femoris muscle at different knee angles with intervals of 15’. For the dynamic measurements the lower leg is moved by a motor driven lever, which makes an angular movement through an angle of lo’, either in flexion or extension. The quadriceps femoris muscle is activated by pushing the lower leg against the lever and due to the movement of the lever, the contraction of the muscle will be concentrically during extension and eccentrically during flexion.
*A part of the dynamometer was built under supervision of Professor B. Bangma, Head of the Department for Rehabilitation of the Faculty of Medicine of The University of Rotterdam.
Technical
Note
-
8\
Fig. 1. The dynamometer: A: lever; B: round disc; C: bearing; D: bolt; E: adjustable legsupport; P: potentiometer; S: straingauges. The arrows indicate the size and direction of the angular movement.
The main parts of the dynamometer are given in Fig. 1. The lever A is connected with the round disc B by means of the bearing C. The axis of the bearing coincides with that of the disc. By means of the bolt D, the lever can be fixed in one of the holes along the rim of the disc and positioned at different angles with an interval of 15”. A reversable electric motor drives through a gear box the disc-and thus the lever-in a clockwise or counter-clockwise direction through a range of maximally lo”. Two limitswitches are mounted to stop the lever automatically to terminate each movement. The movement can furthermore be stopped at any point within the above mentioned range. A potentiometer P coupled to the disc, acts as transducer and thus makes a continuous recording of the angle of the lever possible. The angular velocity is nearly uniform and has a value of 2” s- ‘. The impedance of the construction is so big that this velocity will not be altered by the torque exerted by a subject. Just above the bearing, two pairs of strain-gauges S are mounted on both sides of the lever. These strain-gauges are connected in a full bridge. With an amplifier and a recorder this forms the torque measuring system. Calibration The lever is placed in a right horizontal or left horizontal position. At a distance of I m different known weights (from 5 kg to 30 kg) are suspended. The measuring system of the dynamometer is considered to be reliable, if the torque recordings with different weights are linear and identical-in an absolute sense-for the left and right horizontal position.
rotation
fixation
on dish
axis
Fig. 2. Diagram of forces working on the lever due to gravity; IV,: weight leg; IV,: weight lever; X1 ; assumed distance from rotation axis to center of gravity of the lower leg; X,: assumed distance from rotation axis to center of gravity of the lever.
As long as A4 < 5”, cos A4 x 1 (error < 0.47,) sin A~#Jz A+ (error < 0.2 7,). We therefore may write
and
Tdyn = T,,,, + C A~#Jcos 4. As in the static case the strain gauge bridge amplifier is adjusted to zero with the lever in the mid position (A+ = 0). The voltage due to gravity produced by this amplifier during a dynamic measurement is then given by E dyn = a C A@ cos 4 (a being a transducing
constant).
From the potentiometer which is coupled to the lever (Fig. 1) and is fed with a constant voltage, a voltage is derived proportional to A#, thus measuring the rotation. With a simple analogue multiplier (multiturn potentiometer G in Fig. 3) this voltage is made equal to Edyn and subtracted from Ed,,,, with a subtracting operational amplifier (SA in Fig. 3). thus compensating the gravity influences on the measured torque. Operation of this device is as follows. If a new mid position is chosen the strain gauge bridge amplifier is set to zero (static torque compensation). The lever arm and the leg coupled to it are then moved to a position either r#~- 5” or Q + 5” and with potentiometer G (Fig. 3) the torque output is adjusted to zero. Thereafter nothing is changed before a new angle or another leg or subject is chosen. Figure 4 demonstrates the effectiveness of this device. The upper part shows a compensation for an ordinary weight of 5 kg, the lower part for a leg which is inactively rotated by the lever through lo” without muscle action. The ellipsoid curve is due to passive mechanical resistance and stiffness acting on the knee joint.
Compensation for the infZuence of gravity The gravity component in the measured torque is depending on the angle of flexion. In Fig. 2 the situation is schematically drawn in an arbitrary position. In a static measurement the torque due to gravity is given by Ts,at=(WiX,+W,X,)sin+=Csin~. If in this static position the strain gauge bridge amplifier is adjusted to zero, the influence of gravity is compensated for. In the dynamic measurements the angle will change from I$ - 5” to 4 + 5” or in the opposite way. In this traject the torque due to gravity is given by Tdyn = C sin (4 + A4) = C (sin r$ cos Ad + sin AI#JCDS
I$).
Adjustment of the subject The subject (Fig. 5) is sitting on an adjustable chair, with the back placed against a vertical backsupport and the hands grasping the bars at both sides of the chair. In this way he can fixate his body on the chair. The lower legs are hanging vertically downwards. By adjusting the chair, the axis of flexion and extension of the knee (identified by the position of the femoral epicondyles) is made coaxial with the axis of the bearing C. For the measurement at other knee angles, the lower leg will be primarily adjusted in this position of 90 of flexion. From this position, the lever-and thus the lower leg-is then positioned in one of the other angles of flexion
Technical
torque signal strain
low pass
from
buffer
gauge budge
transducer
“Olt
I buffer
Fig. 3. Schematic
diagram
j amplifier
of the electronics
for the influence
for the compensation
4’50
of torques
5k
degrees
due to gravity
of flexion
of gravity between 40’ and 50 of Aexion; NC: not-compensated; C: compensated.
(0”~15~-30’~5’~0’--75’-105 -120”). By adjusting the lower leg in this way, the exerted extension force will be always directed perpendicular to the lever of the dynamometer and thus the measured torque will be equal to that exerted by the lower leg. Examplr
560 nF
filter
amplrfier
400
Fig. 4. Compensation
1021
Note
~J’measuremmt
As an example of its application the mean maximal extension torque of the right leg was determined with this dynamometer at 90 for static and between 85. and 95‘ for dynamic measurements in nine healthy female subjects. In both static and dynamic measurements they were instructed to perform a maximal voluntary contraction by pushing the lower leg as strongly as possible against the leg support. The duration of the contraction was 5s and this period was Indicated for the subjects by switching on a lamp. The sequence of the static, concentric and eccentric torque measurements was chosen randomly from the possible per-
mutations. Between these measurements there was a restperiod of 1 min. In Fig. 6 the torque registrations are given of one subject. It can be seen that the torque during the concentric contraction is lower than that during the static contraction and that the torque during the static contraction is lower than that during the eccentric contraction. From the torque registrations the peak torque was taken to calculate the mean maximal torques of the different contraction types and the mean differences between the contraction types (Table 1). Although especially the difference between the static and concentric value is small, the differences are significant as shown by a paired student f-test (c( = 0.05).
DISCUSSION With the dynamometer described above, contraction characteristics of the quadriceps femoris muscle can be investigated at static and slow concentrlc and eccentric
1022
Technical Note
Fig. 5. Position of the subject on theadjustablechair and the position of the leg in relation to the dynamometer; C: bearing; E: adjustable legsupport; B: round disc.
concentric _____ 20
I
static
. . . . . . . ., eccentric .... ........ .....
15
.;’
._-__/-._zT
10
z s -5 J P B
,
,
,
,
1
2
3
4
contractions. As far as we are aware no such a dynamometer was available. Although it is not the purpose of this paper to explain the differences between the torque values of the various contraction types, the results shown here seem to confirm the opinion that the magnitude of muscle force depends on the contraction type. Furthermore the torque due to passive mechanical resistance and stiffness acting on the knee may contribute in the differences which are seen. A dynamometer often used for the measurement of the extension torque of the lower leg in dynamic conditions is the so called Cybex dynamometer (Thirstle et al., 1967).With this dynamometer however, no torques can be measured during eccentric contractions of the quadriceps femoris muscle, because the leg cannot be forced to move by a motor driven lever. Furthermore the Cybex apparatus operates only with one range from flexion to extension and this implies that different flexion angles cannot be adjusted. Compared with the leg extension force dynamometer of Komi (1973), our dynamometer purely measures the torque exerted by the knee muscles. Moving the leg by a footsupport. the Komi dynamometer creates a closed chain and thus there will be also a movement in the hip joint. This means that the retroflexors of the hip could participate in exerting an extension force. Furthermore, the footsupport makes a linear movement and so there is a change in the angle between the lower leg and the lever which measures the torque. This means a change in lever arms and this implies that the measured torque is not simply related to the exerted muscle force. Our dynamometer does not introduce such a change of angle between the lower leg and the force measuring unit. A possible source of error is the definition of the position of the axis of flexion and extension by the epicondyles of the femur. Furthermore, the knee joint is not a simple hinge joint and this means that in movements of the lower leg there is a small displacement of the axis of rotation. However, this displacement is small relative to the length of the lever arm of the shank. One may remark that a limitation of the dynamometer is that a low velocity is used. One should, however, realize that a voluntary contraction should last about 5 s (Kroemer and Howard, 1970). This period is sufficient for the subject to build up his maximal torque. In our studies, we found a mean time of about 4 s which agrees fairly well. This means that one has to choose for a low velocity, because at high velocities the duration of the angular movement is not large enough to perform a maximal torque. The combination of angular velocity (2” s-l) and duration of the angular movement implies for our dynamometer a range of movement of 10”. Still nearly the whole range of flexion can be investigated, because the lever can be positioned at different flexion angles. With the exception of the work of Smidt (1973), the investigators with dynamometers have not taken into account the influence of gravity. Smidt eliminated this influence by a side-lying position of his subjects. The compensation for effects of gravity introduced in our measurements works fairly well for the small angles of rotation which are used. For larger rotation angles non-linear effects are no longer neglectable. Using devices like Burr Brown’s multifunction converter 4301 this problem can be solved along the same lines. Another approach is to read in torque and rotation
l_ 5
time&c.)
Fig. 6. Torque registrations for one subject performing a maximal voluntary contraction. The torque is exressed in kilogrammeter (kg@.
Table 1. Mean maximal torque and standard deviation (+ S.E.) of the different contraction types; mean differences and standard deviation of the differences Contraction
Mean toraue
+ S.E.
Eccentric Static Concentric
18.1 kgm 16.8 kgrn 16.0 kgm
3.5 2.9 3.6
Difference Eccen-stat Stat-concen Eccen-concen
Mean difference
f S.E.
1.3 0.8 2.1
1.4 0.9 1.2
Technical
digitally and process these data in an appropriate manner with a computer. It can be calculated from the body composition data of Dempster (1955) that in our group of subjects the weight of the lower leg would have contributed for maximally 0.2 kgm in the measured torque. This small correction is not important when large torques are measured as in our group of subjects, but is relevant when differences between eccentric, static and concentric torques are studied. Furthermore the compensation for the influence of gravity will be important in investigations where small torques are produced as e.g. in disabilities of the locomotor apparatus.
Acknowledgement-The authors are indebted to Mr. H. van Heuven and Mr. J. Meijer for technical assistance.
REFERENCES Asmussen, E., Hansen, 0. and Lammert, 0. (1965) The relation between isometricand dynamic muscle strength in man. Communications from the Testing and Observation Institute of the Danish National Association for Infantile Paralysis, No. 20. Dempster, W. (1955) Space requirements of the seated operator. WADC rech. Rep. 55, 159. Doss, W. S. and Karpovich, P. V. (1965) A comparison of
Note
1023
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