Instrumented gait analysis records the process of walking with measurable parameters collected through the use of equipment. Such basic techniques would have enabled measurement of walking velocity (distance traversed per unit of time) and cadence (steps per unit of time). Marks,18 a New York City prosthetist, offered a more precise qualitative description of pathological gait in 1905, when he described the gait process in eight organized phases and discussed the implications of prosthetic component design on walking function. Marks praised “kinetoscopic” photography as a potential diagnostic tool for optimizing pathological gait.
Today we record gait parameters with instruments as common as a stopwatch or as complex as the simultaneous integration of three-dimensional kinematics, kinetics, and electromyographic (EMG) methods. The primary emphasis of clinical assessment has been on accessible techniques and inexpensive technologies. A simple, inexpensive footprint mat has been used for decades to record barefoot plantar pressures. Clinics use individual or multiple mats to record step and stride length as well as walking base width. Early on, video technology with slow-motion capabilities made more precise qualitative description of the gait cycle possible. The continued development of inexpensive video gait assessment software has made clinical quantitative applications more practical as well. Most quantitative and qualitative video systems, however, measure joint angles in two dimensions, which does not offer a complete analysis of the three-dimensional walking activity.
Kinematic and Kinetic Systems
Most kinematic systems provide joint and body segment motion in graphic form. This information includes sagittal, coronal, and transverse motions that occur at the ankle, knee, hip, and pelvis. The patient is instrumented with reflective spheres that are placed on well-recognized anatomical landmarks (Figure 5-8). Typically, an infrared light source is positioned around each of several cameras. This light is directed to the reflective spheres, which in turn are reflected into the cameras. Each field of video data is digitized, an operator manually identifies the markers, and the coordinates of the geometric center of each marker are calculated with computer software. Resultant data are displayed as animated stick figures that represent the actual motions produced by the patient. The operator can freeze any frame and enlarge the image at any joint to examine gait patterns in greater depth. The operator can extract raw numbers that represent joint placement and motion in space or produce a printout showing joint motion in all planes plotted against the percentage of the gait cycle (Figure 5-9). Angular velocities, accelerations, and joint and segment linear displacements can be calculated. Data from other systems (force platforms and EMG) collected during the same time sequence as the motion data are often integrated with the kinematics. Advanced systems like these can be a very expensive component of the gait lab, but the data collected provides some of the most in-depth and valid data. In the gait lab or a clinical lab, the motion system setup serves as the technological core. A variety of Vicon motion systems have been used to evaluate the joint motion in patients with spastic diplegic cerebral palsy and various other patient populations.32 Similarly the EvaRT motion analysis system has been used to collect data comparing mechanical and microprocessor knees in patients with gait and balance deficits.33
The Dartfish system is another motion analysis tool that is used in gait laboratories and clinical settings.34 The Dartfish system allows for two- and three-dimensional joint motion analysis. It is portable, less expensive, and requires less time to set up when compared with other motion analysis systems. Dartfish has been used to record and immediately evaluate the effects of various prosthetic feet on knee flexion during normal walking. These tools can allow for improved technique and transmission of information to patients and optimally a decrease in recovery time.35
When an individual takes a step, he is exerting force against the surface he is walking on. This kinetic information is obtained from one or more force platforms, which collect data on the three components of the ground reaction force: vertical, fore-aft (anterior-posterior), and medial-lateral (Figure 5-10). The contribution of kinetic data can be significant. Fore-aft shear is quite useful in establishing appropriate transtibial prosthetic alignment in the sagittal plane. For this purpose the clinician would anticipate a balanced magnitude and timing of the braking and propulsive patterns. Data collection from two consecutive steps, one gait cycle, requires dual force plates. Some kinetic software packages also offer specialized programs for specific purposes such as stability analysis, which provides information about center of gravity shift relative to time.
While the typical force platform system provides data about forces and moments occurring at the ground, or center of pressure progression, it can be combined with kinematic data to provide additional information. By combining these two data sets, the moments and power acting at the joints can be calculated. This information is useful in measuring the dynamic joint control of an individual throughout stance, particularly when used in conjunction with EMG. Similarly, information about joint moments, sometimes referred to as torque, is also often reported as an outcome measure in research studies. While this information can be potentially important in the evaluation of pathological gait, it is also necessary to have a basic understanding of how these values are derived. As mentioned, as an individual ambulates, the individual exerts force on the walking surface; differing degrees of this force are similarly exerted on each of the joints in the lower extremity. With the exertion of these forces comes an associated moment that is also acting at the joint, along with a power value. In its most basic form, a moment is the result of a force multiplied by a distance or lever arm.6 Joint power is then calculated by multiplying the moment acting at a joint by the joint’s angular velocity. Additionally, the moment acting on a particular segment is most frequently calculated with reference to the center of mass of that segment. This means that the lever arm is the distance from where the forces are acting at the joint to the center of mass of the segment. In order to calculate these values, the lower extremity must be broken down into segments, often the ankle, shank or calf, and thigh. By doing this, a link-segment model is being applied and the parameters of interest can be calculated.36
To further illustrate, the interrelated nature of these measures, the calculation path for forces, moments, and power is also presented (Figure 5-11). It is important to note within the diagram where the different data sources originate. There are very few directly measured values that are then combined with biomechanical models to calculate these variables.
The calculation process begins with the determination of the ground reaction forces, which are obtained through the direct measurement of an individual stepping on a force platform. Once that information is available, it is combined with kinematic data, derived from a two- or three-dimensional motion capture system for each lower extremity body segment, so that the joint reaction forces can be calculated. As the forces at each of the joints are determined, then the associated moments acting on each segment can also be calculated. Ultimately, the power can be calculated as well (Figure 5-12).
In many cases, instrumented kinetic and kinematic systems have included an inverse dynamics model that is applied to determine the forces acting at each of the lower extremity joints. Like virtually all biomechanics models, certain assumptions must be made in order for the calculation to be carried out in a practical manner. With assumptions come the opportunity for additional error introduction throughout the process. This is why it is important to understand the limitations associated with them. A fundamental point is that frequently these calculations all rely upon data that are calculated using general body proportions and anthropometric models for whole bodied individuals. Because of this, certain assumptions are made about the mechanical properties of the segments and joints being evaluated. For example, many of the commonly used models assume that the subject has no limb deficiencies and essentially normal musculature. While this may be acceptable for evaluations of individuals without pathology, these assumptions can become a source of error when evaluating an individual with an amputation or other limb dysfunction. There is also the issue that the knee and ankle joints are frequently modeled as simple hinge joints. By doing this it makes the calculations more practical to perform but does not completely represent the anatomical reality. Particularly in the case of the knee, the joint center does not stay in a fixed position during stance, but many of the models for calculating joint moments assume that it does (Figure 5-13). As a result, there can be variation in the distance used to calculate the moment at the knee. Considering the physical location, even a small variation in the estimated joint center could result in a significant change in the value calculated. Because many of the calculations rely upon the model assumptions, the inherent errors can be easily compounded. This is not to say that these variables should be ignored but that their value should be tempered with an understanding of the process for obtaining them.
