- Email: [email protected]
Clinical use of kinetics for gait pathology in cerebral palsy
36
Gait & Posture 1994; 2: No 1
Clinical use of kinetics for gait pathology in cerebral
Ankle
Knee
Hip
palsy J R Cup
In the recent past there have been efforts to approach gait disorders in cerebral palsy on a more scientific level by assessing the child pre- and postoperatively through the study of kinesiology and with the use of computerized gait analysis. Kinesiology can be subdivided into two areas: kinematics and kinetics. Kinematics is the study of motion without regard to the forces which produce it. Kinetics deal with the forces which produce the motion. These measurements include joint moments and joint powers. By employing kinetics for the assessment of normal and pathological gait we have discovered that these measurements can provide us with a great deal of insight into the specific gait abnormalities associated with cerebral palsy. Kinetics are derived through a method known as ‘inverse dynamics’, which in turn depends upon Newton’s second law of motion. This method has been described by Winter and othersr4. Newton’s second law states that F = ma (force = mass x acceleration). The two principal kinetic outputs studied are joint moments and joint powers. A moment is defined as the product of a force times the distance of the force from the centre of its axis of rotation. In our laboratory joint moments are defined or named from the internal moment. Its units are Newton-meters per kilogram. In gait the internal moments produced by the muscles are balanced by external moments produced by the ground reaction force (GRF). If these two moments are identical there will be no motion around the ankle joint. However if the internal muscle moment is slightly greater than the ground reaction moment, the ankle will move into plantarflexion. Our muscles provide all the power needed for erect stance and propulsion, but they can function in only three ways: I.
I. 7 3.
Eccentric contraction, which implies lengthening under tension. Eccentric contraction always implies shock absorption. Concentric contraction, which is shortening under tension. All accelerators work concentrically. Isometric contraction, which is muscle tension without change in length. Postural stabilizers work in this mode.
In gait analysis it is very useful to have some estimate of muscle power. The kinetic concept that can be used to estimate this is joint power’,4. Joint power is the product obtained by multiplying the joint moment of force by the joint’s angular velocity: Joint power = joint moment x Angular velocity. or P = Ma. Its units are watts per kilogram. If power is positive, it indicates that the muscle is shortening and producing an acceleration. If power is negative, it indicates that the
-0
25
so
75
96 Gait Cycle
100
0
25
50
7s
96 Gait Cycle
100
0
25
50
75
100
% Gait Cycle
Figure 1. Kinematics and kinetics of the sagittal plane. (Reprinted with permission from: Gage JR. Gait Analysis in Cerebra/ Palsy. MacKeith Press, 1991.)
muscle is lengthening and producing a deceleration. If there is a moment present but no power, it attests to the fact that the joint is being held motionless either by ligamentous forces or by isometric contraction of muscle. It is the single variable best describing the concentric and eccentric phases of mechanical energy that muscles generate in order to accomplish movements. Since the power estimation is based on the concept of inverse dynamics, it must be understood that the estimates of power obtained by these methods are net power and hence do not provide any information about power of specific muscles or muscles groups. Kinetics may well prove to be the most useful gait analysis measurements in that they (I) provide information about the cause of the movement disorder; (2) point to the source of power for movement; and (3) provide some information about net energy consumption at individual joints. Inspection of power graphs allows an individual to determine the mode of action of a particular muscle group, i.e. eccentric, concentric, or isometric. Individual joints can also be looked at as ‘torque generators’ and integration of power curves enables one to estimate the magnitude of eccentric or concentric work done in joules per kg. If a moment is present without any generation of power, the joint must be obtaining its stability either from ligaments, e.g. the plantarflexion/knee extension couple or by isometric contraction of muscle. Commercial software which will generate graphs of joint moments and joint powers in all three planes can now be purchased with most modern gait analysis systems, (Figure I). The graphs in Figure 1 illustrate the kinematics and kinetics of the sagittal plane. By inspection of the power graphs we can see that there are three principal bursts of concentric power, two from the hip and one from the ankle. These power bursts essentially provide the energy required for walking. By looking at the moment graphs one can determine which muscle groups are supplying this power. For example the first burst of concentric power at the hip coincides with an extensor moment and the second burst with a flexor moment. Therefore the first burst of energy is being provided by the hip extensors and the second by the hip flexors. Similarly at the
Abstracts
ankle. the burst of concentric power coincides with a piantarflexion moment. Thus the power needed for propulsion during walking comes primarily from the hip and the ankle. If one integrates the area under each of these curves. it can be determined that about 45% ofthe power required from walking comes from the ankle plantar flexors. 30% from the hip extensors. and 20% from the hip tlcxor~,‘. It must be remembered that this power is being contributed bilaterally and the timing is such that the ankle plantartlexion power of the trailing limb comes just slightly before the hip extension power of the fore limb. Thus the pushing power of the trailing limb is being augmented by the pulling power of the front limb. Furthermore the acceleration of the hip flexors. which along with the plantarflexion burst launches the trailing limb into swing phase. occurs concomitantly with the acceleration of the hip extensors on the fore limb. Overlaying moment and power curves can provide additional information about normal walking such as: I 3. 3.
How the body is controlling the limbs; Support strategy for the lower limb; Power flow during the gait cycle.
When this is done inspection of the graphs will reveal that there is a flow of power that starts with the hip extensors. then moves down to the knee extensors (quadriceps). and finally reaches the ankle plantar flexors. The support strategy for the lower limb can be seen in this wave of proximal-to-distal power. since these are all antigravity muscles and this bipedal flow allows the limbs to support the trunk and avoid collapse while still delivering the smooth How of energy needed for propulsion during gait. Overlaid moments and powers of the hip and knee during running will reveal a transfer of energy from the leg to the hip. By inspecting the moment graphs to determine which muscle group is dominant and the power graphs to determine whether the muscle is working eccentrically or concentrically, it can be determined that during initial swing there is an eccentric, extensor moment at the knee and a concentric, Hexion moment at the hip. whereas in late swing there is an eccentric Aexion moment at the knee and a concentric extensor moment at the hip. What these graphs are telling us is that there is a transfer of energy from the knee to the hip during both initial and terminal swing. The body uses the two-joint muscles to accomplish this. Hence during initial swing the rcctus femoris is contracting eccentrically at the knee and concentrically at the hip. whereas during terminal swing the hamstrings are contracting eccentrically at the knee and concentrically at the hip. Since the contraction is eccentric at one end of the muscle and concentric at the other. the muscles probably remain relatively isometric with respect to net length. In both cases. however. these hiarticular muscles are essentially working as energy transfer straps and act to harness the energy of the shank and carry it proximally to the hip where it can be used to augment hip flexion. Yack and Winter have estimated that the energy transfer ofthe biarticular muscles reduces the cost of walking by about 22”,,‘.
37
Moments and powers are also useful for looking at pathological gait. In a child with cerebral palsy, a late cross-over from an extension to a flexion moment during stance phase at the hip will usually indicate hip-flexor dominance. Furthermore. children with cerebral palsy tend to have poorer distal and better proximal control. Consequently, much of the power needed for gait comes from the hip extensors and flexors rather than from triceps surae push-off. The relative contributions of each of these muscle groups can easily be evaluated with sagittal-plane kinetics, and with this knowledge one can avoid excessive surgical lengthening of muscles needed for propulsion. At the knee a continuous extensor moment through stance reveals that the knee is in so much Hexion that the individual is unable to stabilize his’her knee with the GRF as is the case in normal gait. At the ankle. the most frequently encountered abnormality is a biphasic moment in conjunction with two distinct beats of power. The usual cause for this is gastrocnemius clonus”. By using joint kinetics to assess pathological gait one can get a better estimate both of the preoperative status and of the postoperative outcome. Prior to the development of kinetics we could only gather descriptive information about gait. With this tool we can begin to acquire information about cause. Based on data acquired from these new methods of investigation. we have already begun to make some significant inroads into better treatment of cerebral palsy.
References
Davis RB 111.Tyburski DJ, &npuu S. Gage JR. The determination ofjoint moments: methodology vcrifcation. Procmdirl~s of’ t/w Fifilr Biomid Co~~/iwm~~ of’ t/w Cmarlicu~ SocYct~. for Bio/l7rc’/7rn7ic,.~. 198X: 52 3 dunpuu S, Gage JR, Davis RB. Three-dimensional 1owe1 cxtrcmity joint kinetics in normal pediatric gait. J Pctlitrtr Orthop 1991: 11: 341 -9 Winter DA. Tlrc Bir)tl7rc.l7Llr7ic..scud Motor’ Control of’ If77777tr77 Grrit. University of Waterloo Press (Waterloo. Canada) 19X7;29-43 Winter DA. Tiw Bioll7c,c,lro/7ic,.sc/rul Motor C’o77t,.r~/ ot tfwmu &lit. N01~77rrl. El~lc~rl~~.c777tl Prrtholo~~ird(2nd edn.) University of Waterloo Press (Waterloo. Canada) 1991: 35 49 Yack HJ, Winter DA. Economy of two-,joinl muscles. Procwt/i/~<~s of the Fifih B;c,7/7itr/C’otrfkww o/r 1/71’ (‘~/17&~//7SOC~;C~/,I~ fo/. Bio/77c,c~hr/77ic.s 198X: 180 I Gage JR. Gtrit .-ZII~I/~~.F~S i77C~~~hrtr/ Ptr/.y~~.MucKcith Press. 1991: 14i%Y
Kinetic gait patterns in hemiplegia in spastic cerebral palsy A4 G Hdlirl, J E Rohh. I R Lo~rrlotr
Southern Margaret
General Hospital, Rose Orthopaedic
Glasgow: Hospital.
Princess Edinburgh
The use of kinetic gait analysis is of great importance in assessing those forces applied to the limb in stance. Only if the force applied to the leg is known can an intrrprctation of the muscle response to these forces be made. in terms either of EMG or joint angle. Kinetic gait anal>,sis
Gait & Posture 1994; 2: No 1
Clinical use of kinetics for gait pathology in cerebral
Ankle
Knee
Hip
palsy J R Cup
In the recent past there have been efforts to approach gait disorders in cerebral palsy on a more scientific level by assessing the child pre- and postoperatively through the study of kinesiology and with the use of computerized gait analysis. Kinesiology can be subdivided into two areas: kinematics and kinetics. Kinematics is the study of motion without regard to the forces which produce it. Kinetics deal with the forces which produce the motion. These measurements include joint moments and joint powers. By employing kinetics for the assessment of normal and pathological gait we have discovered that these measurements can provide us with a great deal of insight into the specific gait abnormalities associated with cerebral palsy. Kinetics are derived through a method known as ‘inverse dynamics’, which in turn depends upon Newton’s second law of motion. This method has been described by Winter and othersr4. Newton’s second law states that F = ma (force = mass x acceleration). The two principal kinetic outputs studied are joint moments and joint powers. A moment is defined as the product of a force times the distance of the force from the centre of its axis of rotation. In our laboratory joint moments are defined or named from the internal moment. Its units are Newton-meters per kilogram. In gait the internal moments produced by the muscles are balanced by external moments produced by the ground reaction force (GRF). If these two moments are identical there will be no motion around the ankle joint. However if the internal muscle moment is slightly greater than the ground reaction moment, the ankle will move into plantarflexion. Our muscles provide all the power needed for erect stance and propulsion, but they can function in only three ways: I.
I. 7 3.
Eccentric contraction, which implies lengthening under tension. Eccentric contraction always implies shock absorption. Concentric contraction, which is shortening under tension. All accelerators work concentrically. Isometric contraction, which is muscle tension without change in length. Postural stabilizers work in this mode.
In gait analysis it is very useful to have some estimate of muscle power. The kinetic concept that can be used to estimate this is joint power’,4. Joint power is the product obtained by multiplying the joint moment of force by the joint’s angular velocity: Joint power = joint moment x Angular velocity. or P = Ma. Its units are watts per kilogram. If power is positive, it indicates that the muscle is shortening and producing an acceleration. If power is negative, it indicates that the
-0
25
so
75
96 Gait Cycle
100
0
25
50
7s
96 Gait Cycle
100
0
25
50
75
100
% Gait Cycle
Figure 1. Kinematics and kinetics of the sagittal plane. (Reprinted with permission from: Gage JR. Gait Analysis in Cerebra/ Palsy. MacKeith Press, 1991.)
muscle is lengthening and producing a deceleration. If there is a moment present but no power, it attests to the fact that the joint is being held motionless either by ligamentous forces or by isometric contraction of muscle. It is the single variable best describing the concentric and eccentric phases of mechanical energy that muscles generate in order to accomplish movements. Since the power estimation is based on the concept of inverse dynamics, it must be understood that the estimates of power obtained by these methods are net power and hence do not provide any information about power of specific muscles or muscles groups. Kinetics may well prove to be the most useful gait analysis measurements in that they (I) provide information about the cause of the movement disorder; (2) point to the source of power for movement; and (3) provide some information about net energy consumption at individual joints. Inspection of power graphs allows an individual to determine the mode of action of a particular muscle group, i.e. eccentric, concentric, or isometric. Individual joints can also be looked at as ‘torque generators’ and integration of power curves enables one to estimate the magnitude of eccentric or concentric work done in joules per kg. If a moment is present without any generation of power, the joint must be obtaining its stability either from ligaments, e.g. the plantarflexion/knee extension couple or by isometric contraction of muscle. Commercial software which will generate graphs of joint moments and joint powers in all three planes can now be purchased with most modern gait analysis systems, (Figure I). The graphs in Figure 1 illustrate the kinematics and kinetics of the sagittal plane. By inspection of the power graphs we can see that there are three principal bursts of concentric power, two from the hip and one from the ankle. These power bursts essentially provide the energy required for walking. By looking at the moment graphs one can determine which muscle groups are supplying this power. For example the first burst of concentric power at the hip coincides with an extensor moment and the second burst with a flexor moment. Therefore the first burst of energy is being provided by the hip extensors and the second by the hip flexors. Similarly at the
Abstracts
ankle. the burst of concentric power coincides with a piantarflexion moment. Thus the power needed for propulsion during walking comes primarily from the hip and the ankle. If one integrates the area under each of these curves. it can be determined that about 45% ofthe power required from walking comes from the ankle plantar flexors. 30% from the hip extensors. and 20% from the hip tlcxor~,‘. It must be remembered that this power is being contributed bilaterally and the timing is such that the ankle plantartlexion power of the trailing limb comes just slightly before the hip extension power of the fore limb. Thus the pushing power of the trailing limb is being augmented by the pulling power of the front limb. Furthermore the acceleration of the hip flexors. which along with the plantarflexion burst launches the trailing limb into swing phase. occurs concomitantly with the acceleration of the hip extensors on the fore limb. Overlaying moment and power curves can provide additional information about normal walking such as: I 3. 3.
How the body is controlling the limbs; Support strategy for the lower limb; Power flow during the gait cycle.
When this is done inspection of the graphs will reveal that there is a flow of power that starts with the hip extensors. then moves down to the knee extensors (quadriceps). and finally reaches the ankle plantar flexors. The support strategy for the lower limb can be seen in this wave of proximal-to-distal power. since these are all antigravity muscles and this bipedal flow allows the limbs to support the trunk and avoid collapse while still delivering the smooth How of energy needed for propulsion during gait. Overlaid moments and powers of the hip and knee during running will reveal a transfer of energy from the leg to the hip. By inspecting the moment graphs to determine which muscle group is dominant and the power graphs to determine whether the muscle is working eccentrically or concentrically, it can be determined that during initial swing there is an eccentric, extensor moment at the knee and a concentric, Hexion moment at the hip. whereas in late swing there is an eccentric Aexion moment at the knee and a concentric extensor moment at the hip. What these graphs are telling us is that there is a transfer of energy from the knee to the hip during both initial and terminal swing. The body uses the two-joint muscles to accomplish this. Hence during initial swing the rcctus femoris is contracting eccentrically at the knee and concentrically at the hip. whereas during terminal swing the hamstrings are contracting eccentrically at the knee and concentrically at the hip. Since the contraction is eccentric at one end of the muscle and concentric at the other. the muscles probably remain relatively isometric with respect to net length. In both cases. however. these hiarticular muscles are essentially working as energy transfer straps and act to harness the energy of the shank and carry it proximally to the hip where it can be used to augment hip flexion. Yack and Winter have estimated that the energy transfer ofthe biarticular muscles reduces the cost of walking by about 22”,,‘.
37
Moments and powers are also useful for looking at pathological gait. In a child with cerebral palsy, a late cross-over from an extension to a flexion moment during stance phase at the hip will usually indicate hip-flexor dominance. Furthermore. children with cerebral palsy tend to have poorer distal and better proximal control. Consequently, much of the power needed for gait comes from the hip extensors and flexors rather than from triceps surae push-off. The relative contributions of each of these muscle groups can easily be evaluated with sagittal-plane kinetics, and with this knowledge one can avoid excessive surgical lengthening of muscles needed for propulsion. At the knee a continuous extensor moment through stance reveals that the knee is in so much Hexion that the individual is unable to stabilize his’her knee with the GRF as is the case in normal gait. At the ankle. the most frequently encountered abnormality is a biphasic moment in conjunction with two distinct beats of power. The usual cause for this is gastrocnemius clonus”. By using joint kinetics to assess pathological gait one can get a better estimate both of the preoperative status and of the postoperative outcome. Prior to the development of kinetics we could only gather descriptive information about gait. With this tool we can begin to acquire information about cause. Based on data acquired from these new methods of investigation. we have already begun to make some significant inroads into better treatment of cerebral palsy.
References
Davis RB 111.Tyburski DJ, &npuu S. Gage JR. The determination ofjoint moments: methodology vcrifcation. Procmdirl~s of’ t/w Fifilr Biomid Co~~/iwm~~ of’ t/w Cmarlicu~ SocYct~. for Bio/l7rc’/7rn7ic,.~. 198X: 52 3 dunpuu S, Gage JR, Davis RB. Three-dimensional 1owe1 cxtrcmity joint kinetics in normal pediatric gait. J Pctlitrtr Orthop 1991: 11: 341 -9 Winter DA. Tlrc Bir)tl7rc.l7Llr7ic..scud Motor’ Control of’ If77777tr77 Grrit. University of Waterloo Press (Waterloo. Canada) 19X7;29-43 Winter DA. Tiw Bioll7c,c,lro/7ic,.sc/rul Motor C’o77t,.r~/ ot tfwmu &lit. N01~77rrl. El~lc~rl~~.c777tl Prrtholo~~ird(2nd edn.) University of Waterloo Press (Waterloo. Canada) 1991: 35 49 Yack HJ, Winter DA. Economy of two-,joinl muscles. Procwt/i/~<~s of the Fifih B;c,7/7itr/C’otrfkww o/r 1/71’ (‘~/17&~//7SOC~;C~/,I~ fo/. Bio/77c,c~hr/77ic.s 198X: 180 I Gage JR. Gtrit .-ZII~I/~~.F~S i77C~~~hrtr/ Ptr/.y~~.MucKcith Press. 1991: 14i%Y
Kinetic gait patterns in hemiplegia in spastic cerebral palsy A4 G Hdlirl, J E Rohh. I R Lo~rrlotr
Southern Margaret
General Hospital, Rose Orthopaedic
Glasgow: Hospital.
Princess Edinburgh
The use of kinetic gait analysis is of great importance in assessing those forces applied to the limb in stance. Only if the force applied to the leg is known can an intrrprctation of the muscle response to these forces be made. in terms either of EMG or joint angle. Kinetic gait anal>,sis