Motion Analysis Center Services
At the Rehabilitation Institute of Chicago, the clinical Motion Analysis Center (MAC) focuses on the analysis of human walking (gait analysis) and upper extremity function during activities of daily living.
Gait analysis, or motion analysis for the upper extremity, may include any or all of the following procedures:
Videotaping of the patient from the front, back, and both sides is performed during walking or upper-extremity functional activities. The videos are used to qualitatively assess the characteristics of a patient’s movement patterns. Videos are also useful for determining which feet are on the ground during specific portions of the walking trials, and for, potentially, selecting representative trials for computer processing.
Motion data measurement
As an individual walks, the position of each joint constantly changes. The motions of these joints are computed from the measured positions of a number of reflective markers placed on either arms or legs, and on the pelvis. These positions are recorded by our 10 special electronic cameras capturing all body positions at 120 times per second.
The cameras convey the marker positions which, once in the computer, a program is executed to determine the limb motions from the marker positions for each of the 120 frames per second of data. The computer program first functions somewhat like a game of “connect the dots,” and then calculates the exact position and angle of each joint desired. Markers are taped directly onto skin. Special medical tape is used to alleviate problems with skin reactions.
Overall performance assessment includes measurement of walking speed, length of each stride, length of each step, and number of steps taken per minute. The time that each leg spends on and off the ground (i.e., supporting the body and swinging) are also important values that are evaluated. Differences as little as several hundredths of a second are identified that can be important.
Dynamic electromyography (EMG)
During walking, muscles are monitored to record when a muscle is active (i.e., muscle contraction). Muscle activity involves detection of small electrical voltage produced during muscle contractions. The main characteristics of these recordings that are considered are the times at which each muscle turns “on” (begins contraction) and turns “off” (ends contraction). Abnormal muscle activity can contribute to abnormal motion.
We record these muscle voltage signals to determine if the monitored muscles are coordinated properly. These recordings are made by placing surface electrodes on the skin over the muscles of interest. Before placing electrodes, the skin at the electrode locations is wiped with an alcohol pad. This process removes oil and dead skin cells which would impede detection of the electrical signals we are trying to capture.
Routinely, eight to twelve muscles are monitored at the same time. A well-trained specialist or a clinician is needed to affix each of the surface electrodes to the skin at the appropriate anatomical location and to ensure adequacy of their signals.
Each muscle signal is recorded at a rate much higher than that of video recordings (typically 1200 recordings per second). Recordings typically last between three and six seconds, and are obtained for three to ten walking trials. Recorded “on” and “off’ times are compared with the timing of the major gait events (i.e., footstrike, opposite foot off, opposite footstrike, foot off, and succeeding footstrike) to decide the appropriateness of the activity.
Muscle and joint force measurements
For the walking trials, the loads that tend to cause rotation of the joints (or "joint torques") are computed. These torque values reflect the loads placed on the muscles and passive restraints at the joints. Joint torques are calculated mathematically by incorporating measurements of forces between the foot and ground with motion measurements. In combination with motion and EMG recordings, torque values can be used to estimate energy costs associated with walking.
Detailed assessment of pressure distribution of the sole of the foot is provided by this test. An impression of the foot is produced which displays different degrees of pressure (measured in Newtons/cm2) achieved in different colors. A color computer display of the weight shift over time is available in the lab. This is reduced to a graphic display of pressure per unit time at all key points for ease of inclusion in the patient’s report. Diminished or increased pressures can be easily visualized and are specifically quantified.
The computerized footprint of the patient during walking is superimposed over a regular picture of the patient’s foot to allow better visualization of the way the pressures are being distributed across different areas of the foot.
All patients who are not being referred by physicians, services associated with obtaining medical history and performing a physical evaluation, are rendered during a pre-gait evaluation visit. Physical exam includes:
- Range of Motion: This helps to determine if any tightness, obvious weakness or lack of coordination is present, all of which may contribute to problems with walking. This typically takes approximately one hour. It includes all motions about the hip, knee, ankle, and foot on both legs. A range-of-motion examination is performed on the upper extremities, if the patient’s disorder affects these areas, and the focus of the motion analysis is an upper extremity evaluation. The range-of-motion exam is necessary to allow interpretation of the data.
- Manual Muscle Exam: This includes all muscle groups about the hip, knee, ankle, and foot on both legs. Examination of the upper extremities are included if the patient’s disorder affects these areas and the focus of the motion analysis is an upper extremity evaluation. Grading of muscle strength is based on voluntary muscle effort and quantification uses the standard Manual Muscle Grade (MMG) scoring system. 5 = normal, 4 = good, 3 = fair but able to lift against gravity, 2 = poor and unable to lift against gravity, and 1 = trace there is no movement but muscle contraction can be felt with palpation.
Interpretation of biomechanical data, collected and stored on computer
All of the motion data derived from testing and stored on computer requires interpretation to reveal the cause (or causes) of the problem and to suggest appropriate treatment. To do so, the information from many different walks is reviewed. This is necessary to ensure that the information examined represents a typical walk of the subject and in order to compare several different walking conditions, i.e., barefoot, braces, assistive device, or shoes. A comparison may also be needed to understand the change in the pattern of walking of a given subject on different dates before/after treatment. In all cases, the information obtained is compared to normal values specific to the patient’s age.
Interpretations include summaries of primary, secondary, and compensatory gait deviations. Primary deviations describe those abnormalities that are associated with a patient’s primary pathology. Secondary deviations refer to those gait abnormalities associated with limb segments or joints that are not primarily pathologic, but whose motions are coupled to segments or joints with primary motion deviations.
Compensatory deviations are those gait abnormalities exhibited by a patient to compensate for one or more gait deviations at other locations. These interpretations of computerized walking data are analogous to interpretations of X-rays images that describe the severity/complexity of fractures, types of lesions, or skeletal deformations. Like X-ray interpretations, gait data interpretations are isolated from evaluations of prognoses or recommendations for interventions.
Patient review sessions: A team approach
Interpretation of the data takes place during our patient review sessions. A patient review session is a team approach that involves all members of the MAC including a trained physician, engineers, kinesiologists, and a physical therapist for the gait analysis or an occupational therapist for the upper extremity motion analyses. During each patient review, all information – including the medical history, physical exam, and the laboratory data – is used in assessing the aspects of the patient’s pathology that most hinder walking or upper extremity function, making treatment decisions (e.g., surgery, bracing, orthotics, physical therapy, etc.), evaluating the patient’s overall prognosis, and determining the effectiveness of previous treatments.
The outcome of the review of each patient is included in a written report that is forwarded to the referring physician. The report includes all motion analysis observations and recommendations for further treatment. Each written report is accompanied by a CDROM. The disk includes the graphical representation of all the information collected during the motion study and animations of the movement patterns of the patient.