RELATIONSHIP OF GRAVITY WEIGHT LINE AND FUNCTIONAL SHORT LEG
Edward F. Owens, MS
Ronald S. Hosek, PhD
Life Chiropractic College, Marietta, Georgia
Presented at: The 3rd Annual Advances in Conservative Health Conference, Pasadena, TX, Nov. 1984
In considering the determinants of posture, it is not uncommon to assume that the frontal projection of the sagittal plane represents an axis of bilateral symmetry. This axis would also represent what has come to known as the gravity weight line (GWL), i.e., that line along which gravity acts through the center of mass of the individual. We might expect, therefore, that an erect individual would be bisected by this axis provided that limb lengths and mass are balanced side to side. Theorectically, based strictly on mechanical or geometrical considerations, we might also expect that there would be a tendecy to lean away from this axis if one leg were structurally shorter than the other; the expected direction of lean would be toward the side of short leg. Recently, Lawerence, ET AL, using a Janse-Illi four quadrant scale to measure the deviation of the GWL from midline, verified that there is a tendency to lean toward the side of structurally short leg. However, they noted that as the magnitude of shortness approached 6 mm , the liklihood of leaning to the opposite side increased, perhaps as a form of compensation to promote pelvic stability. Earlier, Illi again using the four quadrant scale demostrated that the GWL, provided a good measure of correction when heel lifts were used to correct functional and/or structural short legs. The GWL, a reloable measure of posture, has thus been established as being sensitive to leg length deficiency. Because of the apparent relationship between relative leg length and posture, we wondered whether leg length difference as determined by leg checks of the type in common clinical chiropractic usage might be also correlated with postural deviation. We reasoned that should we find such a relationship, it might be possible to develop a new objective measure that could be used to monitor patient progress during a course of chiropractic care. Information of the relationship between body lean and short leg may also aid in an understanding of the mechanism of functional short leg. The present work is our approach to examining these issues.
MATERIALS AND METHODS
Subjects for the study consisted of student volunteers. Of 75 subjects who began the study, 53 were able to complete the entire protocol. Each subject participated in three trials of the experiment, each a week apart. Each trial consisted of a supine leg check followed by two GWL determinations, one with eyes open and one with eyes closed. Subjects were not informed of measurenent results during the course of the experiment, but were free to inquire about the results at the end if they desired.
The supine leg check used is comparable to the one used by the Life Cervical and other upper cervical techniques. The usual precautions, such as sliding up on the table, gently removing inversion or eversion until resistance is met and basing the measurement on the heel/last line were observed. All student assistants performing the leg check were trained to be reliable to within 1/8 with an "expert". To further promote objectivity, three students performed each check and had to agree on the measurement. The side of shortness was recorded, as was the magnitude to nearest 1/8th inch. Gravity weight line determinations were performed using a GWL analyzer developed in our laboratories. The system consists of a platform supported by three accurate. The output of these scales is fed to an Apple computer which controls the sampling, storage, analysis and display of these data. In the computer, the individual weights read from each scale are mathematically converted to the X,Y coordinates of the intersection of the GWL with the platform (fig.1). We will refer to this point as the GWL in what follows, even though strictly speaking, the GWL is a line and not a point. The computer is capable of making many determinations of the GWL per second and can thus monitor postural sway. Also, the calculation and plotting of the GWL are rapid enough to allow feedback to the subject in real time. In the present experiment, subjects were asked to stand on the platform with their feet constrained by templates so that the medial maleoli were 7 inches apart and straddling the midline. They were required to look straight ahead at a spot on the wall and maintain a still posture until the computer screen showed the GWL to be stable for ten seconds. Stability was monitored by maintaining a running average of the GWL and the scatter of points around this average. The average was declared acceptable when all points fell within a radius of 3 mm. Following the recording of this measurement, subjects were asked to close their eyes and the measurement was repeated. This was done to provide data to assess any possible effect of visual postural freflexes on outcome. Finally the position of the subject's medial maleoli were located with respect to the Y axis of the platform. When we look at the leg legth difference and GWL lateralization data on the same plot (fig.6), we note no apparent correlation. Were such a correlation to exist, i.e., short leg and GWL on same side, we would expect to see a clustering of data points in the upper right and lower left quadrants. Nor is any relationship evident between magnitude of length difference and side of GWL, as was observed by Lawarence, ET AL, for true structural short legs. This would appear as a negative correlation on the plots in fig 6.
For all subjects, there is a relatively uneven distribution of leg length differences with a slight preference for right versus left side (Fig. 2). This appears to hold across all three trials. The average difference across all trials as seen in the total plot (fig. 2) is approximately Right 2/8", where zero would be expected. The eyes-open GWL scattergram show a relatively even distribution for X and Y coordinates (AP and laterality fo GWL) across the population for all trials (Fig. 3). The average lateral position is seen to be approximately at the midline, as would be expected. The same holds for the eyes-closed case (Fig. 4). No pattern emerges upon visual inspection, nor is one evident mathematically. The amount of change in the GWL going from eyes open to eyes closed (Fig. 5) is for the most part, very small. More change is seen to occur in the AP position than in the lateral position.
The results of this suggest that leg legth difference, observed by chiropractic leg check, is not a factor in postural deviation as guaged by the GWL. It is likely that the short leg as observed in the "check" is a reflex change not seen or well compensated for in the standing posture. The deviations seen in GWL may result from actual asymmetries of structures above the pelvis or asymmetric positioning of the trunk or pelvis with respect to the midline of the body. Asymmetric positioning of these structures could occur as a consequence of incorrect neurological processing or proprioception resulting in muscle imbalance, i.e. unilateral hypertonus or weakness. More complete information on the position of individual body parts would be needed to resolve this issue.
Even though these results suggest no positive relationship between leg checks and the GWL, it is still likely that the GWL position or stability may be a useful parameter for indicating the effectiveness of adjustments. This issue is currently under study in our laboratory and we will be reporting on it soon.