GAIT ANALYSIS:
Gait analysis is a sophisticated laboratory technique that illustrates and analyses the dynamics of the pattern in which the person walks. This helps the clinician identify abnormal characteristics of movement in a patient with cerebral palsy and hence assists in formulation of a treatment plan that may include physical therapy, bracing and/or surgery (Davis R.B., 1997). Gait analysis provides the surgeon with critical information that would otherwise not be available using conventional techniques such as clinical examination and visual observation (Lofterod, 2007).
Clinical gait analysis is performed to identify the disorder, assess the severity of the dysfunction, extent or nature of the disease, monitor the progress in the presence or absence of intervention, predict the outcome of intervention, identify the joint, muscle or tendon; where the surgery would be required, suggest an orthotic approach to improve the condition of the patient and suggest changes in the patient's exercise plan (Baker, R., 2006). Another significant advantage of gait analysis is that it is not painful, no drugs are required and normal activity can be resumed once the analysis is over.
Gait analysis makes use of four techniques, namely visual observation, quantitative measurements (Kinematics, time distance parameters and oxygen consumption), biomechanical analysis (Kinetics, i.e. considering the effects of forces on gait patterns) and electromyography (Kawamura et al., 2005).
Kinematics helps predict spatial joint positions for calculation of joint angles and translations throughout the gait cycle by making use of reflective markers placed on anatomical landmarks which are captured with the help of infrared cameras. Kinetics helps calculate the joint moments and joint power by measuring the foot interaction with the ground i.e. ground reaction forces using force plates. The Electromyographic (EMG) recording during gait analysis illustrates muscles activity and the difference in the action of each muscle during the gait, hence determining its contribution to the dysfunction. The measurement of oxygen consumption during gait analysis can notify energy utilized while walking, because a primary problem with patient's with CP is increased energy utilization.
However, gait analysis is not only time consuming but also a fairly expensive procedure. The set up cost of the laboratory is approximated at ₤300,000 and the expense of a one time gait analysis is approximately ₤800. The results produced by gait analysis lack standardization i.e. there is no set of directives for performing gait analysis, for example in the way the markers are attached. Variability between tests performed on a patient at different times, inaccuracy in interpretation of gait results and lack of reproducibility would be the other major drawbacks of gait analysis (Simon, S.R., 2004).
KINETICS:
Kinetics is defined as the study of forces which cross the joints and illustrates the mechanism that causes movement. Joint kinetics in gait analysis provides a better understanding of the role of joints and limbs during gait and hence the reasons for the aberrant gait in cerebral palsy patients. It facilitates calculation of joint moment of force, which can provide an insight into the resultant muscle contribution about a joint to move a particular body segment during gait and joint power, which determines the involvement of muscle groups in producing and controlling movement of the whole body (Siegel, K et al., 2000; Vaughan, C., 1996). Muscles provide force, which acts on the lever arm in order to rotate a joint. This action is known as internal joint moment. The internal joint moment is balanced by the corresponding equal and opposite external moment, produced by the ground reaction force (GRF) that the body applies on the floor through the foot. The aim of surgical management is to balance these forces across the joint (Aiona, M., 2000). Normally while walking the ground reaction force is maintained close to the centre of the hip, knee and ankle in order to reduce the demands of anti-gravity muscles. Hence, an unbalanced moment or a deficient/ misdirected lever arm would interfere with the person's ability to walk (Gage, J et al., 1995).
Force Plates: The reaction forces are measured in a gait laboratory with the help of force plates, which provide a dynamic measurement of ground reaction force, vertical and shear forces and the centre of pressure; when the patient steps on it during the gait cycle. A force plate consists of two stiff plates, approximately 600×400×35mm (Kistler, 9286A) separated by four strain gauged or piezoelectric force transducer at its corners. The top plate of the force plate is made with a relatively light weight, durable and strong material such as 6061 Aluminium alloy and the bottom plate is made with a stiff material such as 304L Stainless steel. The force transducer provides a voltage that is proportional to the deformation of the top plate with respect to the lower plate. Clinical gait analysis makes use of two force plates, mounted flush with the floor of the laboratory on an independent concrete platform in order to minimize the effect of floor vibration on the force measurement. The use of two platforms permits the recording of consecutive steps and measuring of the double stance phase (Claeys, R., 1983). The load capacity of the force plate (AMTI OR6 or Kistler 9286A) must be approximately 2.5 to 2.5 KN in the FX (medio-lateral shear force) and FY (antero- posterior force) directions and around 10 KN in the FZ (normal force) direction with natural frequency of 550 Hz (FX, FY) and 1000 Hz (FZ). The cost of a single force plate is approximately ₤7500 to ₤15000 (AMTI, Kistler).
Calibration: It is essential to calibrate the force plate before using it to measure ground reaction force. Static calibration is done by applying a known load in the centre of the force plate, i.e. step input and measuring the output. Dynamic calibration is achieved by loading and unloading the force plate within a certain time interval and assessing both the load and time factors at the output.
Joint Kinetics Calculation: A Force plate does not permit direct measurement of joint kinetics, i.e. joint moment and joint power. Hence, joint kinetics are estimated by using joint centre locations and body segment spatial orientations form kinematic data in conjunction with the ground reaction forces and the centre of pressure obtained from the force plates. The centre of pressure is defined as the origin of the ground reaction or the point where there is no movement from all the applied forces. These measurements are applied to the biomechanical model and the estimation of joint moment is obtained using inverse dynamics approach.
Joint Moment: While computing the joint moments using a biomechanical model, each segment is considered independently at a time assuming that the segment rotates about a transversal axis through the segment's centre of mass and hence it is known as Free Body Segment Method. To begin with, the ankle joint moment is calculated with the help of simple moment equations (Moment = Force× Lever Arm) obtaining: vertical and shear forces along with the centre of pressure obtained from the force plates, the joint centre from the kinematic data and the segment length, segment mass and the centre of mass are obtained from anthropometric data. The ground reaction forces are used only while calculating the ankle joint moment and these are then transferred to the shank and thigh through the joint reaction forces; similarly the knee joint moments and hip joint moments are calculated. During the swing phase of the gait cycle since the foot is not in contact with the ground, no distal forces act on the foot segment (Ariel, G., www.sportsci.com, 15 November 2007).
(http://www.sportsci.com/adi2001/adi/services/support/tutorials/gait/chapter2/2.3.asp)
Joint Power: The joint power, i.e. the power between segments across the joints can be calculated by finding the dot product of the reaction force at a joint and the absolute velocity of the joint centre. A positive value of joint power indicates inflow of energy whereas a negative value would signifies outflow of energy. Hence, outflow of energy from one segment across a joint must be equal to the inflow of energy into the adjacent segment.
Assumptions: However in order to calculate the joint kinetics there are a number of assumptions that need to be made.
Thus, kinetic measurements made with the help of force plate do not produce an immediate diagnosis of the disorder affecting the patient, but it allows a functional diagnosis of the way in which the patient is coping with the disability (Claeys, R., 1983).
Limitations: Kinetics measurement with the help of force plates provide a reliable estimate of all the components of the resultant ground reaction force and present the displacement of the instantaneous centre of pressure, however, there are a few limitations that need to be considered. Force plates might disrupt natural gait pattern by enforcing foot placement position which can be eliminated by warm-up sessions prior to the main recording, in order to determine the starting position (Femery, V. et al., 2002). Force plates do not provide any information about the location of the foot on the platform and the local distribution of loads under the foot and hence have limited relevance to the anatomy and pathology of the foot (Rosenbaum, D et al., 1997). A direct relationship between joint moment/torque and the muscles generating that moment/torque cannot be obtained due to the action of antagonist and agonist muscles; for example, the joint moment would be zero when the antagonist and agonist muscles are quiescent as well as when the two muscles provide equal force. Although force plates provide a reasonable estimate of the centre of pressure, the measurement is often inaccurate. Thus in order to provide a reliable loading characteristic under the foot, foot location and accurate centre of pressure location; plantar foot pressure measurement can be implemented (Vaughan, C., 1996; Giacomozzi, C. et al., 1997)
PLANTAR FOOT PRESSURE MEASUREMENT:
Foot deformity due to spasticity and an imbalance of muscles is very common in patients suffering from cerebral palsy. This often results in an uncharacteristic distribution of load on the plantar surface of the foot. Hence, it is important to measure plantar pressure distribution in patients with cerebral palsy to understand the effect of deformity in individual segments of the foot. Plantar pressure measurement during gait can provide a better understanding of the foot deformity by demonstrating the patho-mechanics of the abnormal foot. It can also present information of the effect of spasticity on stance phase (Rosenbaum, D. et al., 1997; Urry, S., 1999).
Measurement of plantar foot pressure: Since direct distribution of foot plantar pressure and stress using in-vivo pressure sensors are not clinically feasible, foot pressure measurements are made either using insole pressure sensor or using a pressure platform.
Insole pressure sensor: In-sole pressure measurement can be exceptionally useful while assessing the patient's foot for construction of specially designed footwear or foot orthoses. Since insole pressure sensor can record data of several successive footsteps, the patient does not need to walk over the plate and hence the problem of enforcing exact foot placement position can be eliminated (Rosenbaum, D. et al., 1997). Another important advantage of insole pressure sensor over pressure plate is that it is portable. Although an insole pressure data collection seems to be very effective for clinical plantar pressure measurement, there are a number of limitations associated with it. The sensors need to be small and the number of sensors needs to be limited. Hence, there is a trade-off in the resolution of the output. The sensors are more vulnerable to mechanical breakdown or damage due to repetitive loading and due to the hot and humid environment inside the shoe (Orlin, M. et al., 2000; Chesnin, K. et al., 2000).
Example: ‘F-Scan' (Tekscan Inc.) is an in-sole pressure device constructed with flexible printed-circuit boards' using a Mylar substrate which cost ₤2000 approximately, with 960 sensors and a scan rate of 850Hz (www.tekscan.com; Nicolopoulos, C. et al., 2000).
Pressure platform: Pressure platforms are very similar to force platforms since they can be used to measure static as well as dynamic foot plantar pressure in both normal as well as pathological patients. The main advantage of using a pressure platform is that it consists of a large number of sensors in a matrix which provides a higher resolution. Also since the pressure sensors are always in parallel to the plantar surface it provides an accurate vertical force measurement (Orlin, M. et al., 2000; Urry, S., 1999). On the other hand the pressure platform is restricted for use in the laboratory only. Also, similar to the force plates the patient needs to target the platform which might alter the gait pattern. Therefore, in-case of a pressure platform a compromise could be made between spatial resolution, the size of the platform, the speed and the cost depending upon the clinician (Rosenbaum, D. et al., 1997).
Example: Footscan (RSscan Lab Ltd.), Musgrave Footprint™ pressure plates (Musgrave Medical Ltd; Llangollen, UK) are pressure plate with dimensions approximately 685mm×1300mm×13mm having a matrix (i.e. 32 columns by 64 rows or 2048 sensors) of high quality force sensitive resistive sensors (Ledoux, W., 1994). The cost of the pressure plates depend on the resolution i.e. number of sensors and the size. Most of the gait laboratories order custom made pressure plates.
Although both insole pressure sensor and plantar pressure platform are both very popular techniques to measure plantar pressure techniques the choice of a better pressure measurement technique is still unclear. There have been a number of studies evaluating the reliability of the two plantar pressure measuring techniques and the reports suggest variation in reliability of both the techniques. Hence, it is crucial for the clinician to realize the limitations of each system which they decide to use.
Specifications: A number of factors need to be addressed when selecting a system for measuring plantar pressure:
Resolution: Resolution refers to the size of the sensors and the number of sensors that must be used in the pressure measuring system. Ideally, a high resolution system must be used i.e. the system with a large number of sensors. But this system would be very expensive. The size of the sensor is also an important criterion when measuring pressure since it is a function of both force and area. The size of the sensor must be small enough to misinterpret forces from neighboring sensors, but large enough to measure the required information from the particular area (Stebbins et al., 2004). The significance of resolution becomes very substantial especially when considering the small feet of children. Ideally a pressure sensor to measure foot plantar pressure must be below 25mm2 (Orlin, M. et al., 2000).
Sampling Frequency: Sampling frequency characterizes the number of samples measured by each sensor in one second. It is an essential factor since it can determine the temporal resolution of the pressure measuring system. The sampling frequency of the plantar measuring system in a gait laboratory would approximately require a sampling frequency of 50-100Hz (Orlin, M. et al., 2000)
Reliability: Reliability of the pressure measuring system addresses the accuracy of the pressure sensor to evaluate pressure. Normally in a gait lab an average of 3 to 5 recordings are considered (Orlin, M. et al., 2000).
Calibration: In order to provide reliable and consistent results it is important to calibrate the system. The pressure measurement system can be calibrated in a similar way as a force plate is calibrated (Orlin, M. et al., 2000).
Plantar pressure display: The plantar pressure can be graphically displayed in a number of different ways depending upon the software used. Most of the commercial plantar pressure measuring systems make use of colour coded two dimensional representation of the foot, however three dimensional displays wire-frame displays are also very common. Isobaric representation of foot pressure is represented by connecting lines of equal pressure similar to a topographical map. It is also possible to have coloured isobaric plantar pressure representation..)
Reasons for including plantar pressure measurement in regular clinical gait analysis: The reasons for including plantar pressure measurement in regular assessment techniques for cerebral palsy patient in clinical gait analysis are:
Hence, including plantar pressure measurement in regular clinical analysis could provide the clinicians with much greater information that would otherwise not be possible using force plates on its own (Orlin, M., 2000; Giacomozzi, C., 1997; Rosenbaum, D. et al., 1997; Park, E. et al., 2006).
Albeit, plantar pressure measuring system can accurately measure the centre of pressure and also the vertical component of the ground reaction; it cannot measure the fore-aft and the medio-lateral shear forces which are accurately be measured using force plates. As described in the section of Joint kinetics calculation: joint moment the fore-aft and medio-lateral forces play a particularly significant role in calculating joint moments. Hence, using a technique that integrates the benefits of force plates and plantar pressure measuring system would provide a remarkable advantage and thereby proving it to be the ideal method for analysing cerebral palsy patients in a gait laboratory. This can be achieved by using a multifunctional force plate or a piezo-dynamometric platform.
A piezo-dynamometric platform can be constructed by clamping any pressure platform (e.g. Footscan) above any force plate (e.g. AMTI) which is used in the gait laboratory, without any mechanical intervention on the force plate. Thus, the forces and moments are transferred to the force plates unaltered. Both the platforms measure the vertical components of the ground reaction force as well as the centre of pressure, which can help spatially align the two platforms for accurate measurements. For the vertical component of the ground reaction force an average of the force and pressure platform is taken, where as the centre of pressure is estimated more accurately using the pressure plate recording (Giacomozzi, C. et al, 1997).
A multifunction force plate for example FDM- system (Zebris Medical GmbH) is one of the more recent developments. The plates facilitate the analysis of the dynamic force as well as pressure distribution underneath the feet, similar to the piezo-dynamometric platform explained. It consists of approximately 8000 capacitive force sensors, measuring a maximum of 120 N/cm2 with an accuracy of ±5%. The sampling rate of the system is between 120-240Hz (www.zebris.de).
Hence, multifunction force or piezo-dynamometric platform has great potential in the gait laboratory for assessing cerebral palsy patients. It can simultaneously provide a number of interesting parameters such as loading details on the plantar surface of the foot, time history of forces, moments and centre of pressure with high temporal and spatial resolution. However, the system is both mechanically as well as electronically complex and also expensive (Giacomozzi, C. et al, 1997).
DYNAMIC EMG MEASUREMENT:
Dynamic electromyographic (EMG) measurement has become an imperative part of clinical gait analysis to assess gait abnormalities as well as evaluate the effects of treatment modalities. An indirect approach of evaluating muscle activity is achieved with the help of dynamic EMG which provides a random like signal with critical information (Ferdjallah, M., 2000). It offers the timing information of muscles (phasic, continuous or out of phase muscle timing) along with the relative intensity of muscle activation. EMG measurement can also differentiate between individual muscle activities and hence recognize its contribution to disability. Joint kinetics provide information about muscle action through joint moments and joint power, however contribution of individual muscles cannot be determined without the knowledge of dynamic EMG. Hence, EMG measurements are useful in understanding the relationship between muscle activities of the lower limb while walking (Kadaba, M., 1985).
EMG acquisition: Electromyographic activity can be recorded with the help of surface electrodes as well as intramuscular electrodes.
Intramuscular Electrodes: Intramuscular electrodes are used to detect dynamic EMG signal from small and deep muscles with the help of either needle or fine wire electrodes. Although needle electrodes are very rarely used in gait analysis, fine wire electrodes are relatively more common. The main advantage of using intramuscular electrodes is that they are attached to the muscle directly hence eliminate cross talk. However, the intramuscular electrodes have a number of drawbacks. Since the method is invasive it is not very popular amongst patients; especially since mostly children with cerebral palsy are involved. Also only a limited number of electrodes can be attached since the children cannot tolerate the pain. Even though the electrodes are carefully attached to the muscles they might get displaced while walking giving improper results. Another significant drawback of this method is that it is only sensitive to a very small region of the muscle (Kadaba, M., 1985; Ferdjallah, M., 2000; Sutherland, D., 2001).
Surface Electrodes: Surface electrodes are non-invasive electrodes used to measure broad and superficial muscle groups. These electrodes are placed on the skin directly above the muscle along the path of the muscle fiber. The most commonly used surface electrodes in gait analysis is bipolar silver/silver-chloride surface electrodes, requiring skin preparation before applying the electrodes. The main advantage of using surface electrodes is the ease and comfort for the patient. A number of researches have proved EMG measurement with the help of surface electrode to be more reliable and repeatable than intramuscular electrodes (Komi, P., et al,. 1973). The skin motion artifacts can be minimized by filtering. However, surface electrodes cannot be used to measure fine differences between muscles and are also susceptible to cross talk. In-spite of the drawbacks, surface electrodes are more commonly used in gait analysis laboratories (Kadaba, M., 1985; Ferdjallah, M., 2000; Sutherland, D., 2001).
The muscles commonly investigated in the gait laboratory when assessing cerebral palsy patients are rectus femoris, vastus lateralis, semitendinosus (medial hamstring), anterior tibialis, and gastrocnemius. The EMG signals from the electrodes are sent to an EMG backpack unit ( MA 310, Motion Lab Systems, Baton Rouge, LA) generally attached to the back of the patient. The EMG signal obtained from the electrode is sent to the pre-amplifier stage with gain of approximately 300Hz, the signal is then conditioned using multiplexers and filters and then through a differential amplifier with a band pass filter with frequency of 10-5000Hz. The sampling rate of EMG data in a gait laboratory is approximately 1000Hz. (Nuffield Orthopedic Center Gait Laboratory, Oxford).
MyoKinetic Interface (MKI): Surface electrodes are prone to get detached while the patient is walking hence making EMG acquisition a difficult and laborious technique. In order to eliminate this drawback, a new technique has been developed recently that assesses muscle activity as a 2-dimensional map of dynamic pressure produced in limbs during gait analysis known as MyoKinetic Interface (MKI). The MKI system consists of a wearable sleeve with an array of force-sensitive resistors aligned comfortably over the major muscles of the thigh or shank, measuring kinetic activity that results from a muscle contraction.
A number of results have proved a close correlation between results of MKI and EMG during gait analysis. It has also been proved that MKI system is far more repeatable than the customary EMG system for a gait analysis.
The other advantages of MKI system over surface EMG system are: MKI system detects both superficial as well as deep muscular activity where as surface EMG cannot detect deep muscle activities. MKI system eliminates the need of skin preparation before testing and it is comfortable for the patients (Wininger, M. et al., 2005). Very little is published in literature about the MKI system; however it is a very promising technique and in order to realize the maximum potential more research need to be done.
