Nine men and six women participated in this study. Subjects had a mean height of 172.07 (SD 11.73) cm, a mean weight of 75.2 (SD 13.1) kg, and ages ranging from 21−34 years (mean 24.53, SD 3.38 yrs). This study was approved by the human subjects committee of the University of Kansas and consent was obtained from all subjects. All the participants were healthy and reported no instance of low back pain within the last year or musculoskeletal disorder that would limit normal torso flexion.
An electromagnetic motion analysis system (Motion Star, Ascension Tech., VT) was used to collect position and orientation of three electromagnetic sensors. This system has a resolution of 0.08 cm and 0.1 degrees and an RMS accuracy of 0.76 cm and 0.5 degrees. The rotational accuracy was further validated in the laboratory by connecting one electromagnetic sensor to a 100 Kohms rotational potentiometer, rotating the sensor and potentiometer through 120 degrees of rotation at 5 degree intervals, and calculating the RMS error between the sensor and potentiometer.
The three sensors were attached to the skin with double-sided tape over the T10 spinous process, over the S1 spinous process, and over the manubrium. This kinematic data was collected for 5 seconds at a frequency of 30 Hz for each trial. Using the position of the T10 and S1 sensors, trunk flexion angle was determined as the angle between a line connecting these sensors and vertical. The difference in angular orientation between the T10 and S1 sensors in the anterior-posterior plane was defined as the lumbar angle. The manubrium marker allowed detection of trunk rotation and asymmetry of motion. This configuration is consistent with previous literature on lumbar position sense and lumbar-pelvic coordination [Gade and Wilson, 2007
; Granata and Sanford, 2000
; Wilson and Granata, 2003
Before conducting the experimental protocol, the range of lumbar angle for the three flexion angles (0, 30 and 60 degrees) was determined. A real-time visual display of torso flexion and lumbar angle was provided to the subjects (). At each of the three torso flexion angles, subjects were instructed to hold the flexion angle constant while rotating the thorax and pelvis to change the lumbar angle. Subjects were allowed to practice these movements until they became comfortable with the equipment. Subjects were then instructed to assume the maximum (kyphotic) and then the minimum (lordotic) lumbar angle while maintaining one of the three torso flexion angles. Subjects were asked to repeat these extreme lumbar angles three times at each of the three torso flexion angles. The mean of the three maximum lumbar angles and the mean of the three minimum lumbar angles at each torso flexion angle were recorded. These means were used to define the three target lumbar angles: a maximum lumbar angle, a minimum lumbar angle and a mid-range lumbar angle (midpoint between the maximum and minimum lumbar angle) for each torso flexion angle.
Figure 2 A real-time visual display of torso flexion and lumbar curvature was provided for the subjects. Both torso flexion and lumbar curvature were displayed with two parts: the target posture that the subject should attempt to reach and the actual posture measured (more ...)
A reposition sense protocol was then used to determine the subject's ability to sense and to reproduce the three target lumbar angles at the three flexion angles (0, 30, and 60 degrees). The reposition sense protocol consisted of 8 trials. In the initial training trial, subjects were asked to assume a target position by matching both prescribed torso flexion and the target lumbar angle using real-time visual feedback of torso flexion and lumbar angle provided on the computer screen (). The subjects were not told where the target lumbar angle was located within their range and were instructed only to “match the target angle displayed”. After matching both the target flexion and lumbar angle, subjects were asked to remain in that posture while kinematic data was collected for 5 seconds. Subjects were instructed to remember this lumbar angle. The training trials were repeated three times with a short flexion task in between each trial.
After the initial three training trials, an assessment trial was performed. In the assessment trial, the lumbar angle display was turned off while flexion display remained available to the subject. While maintaining the specified torso flexion, subjects were asked to reproduce the target lumbar angle from memory and data was again collected for 5 seconds. Training and assessment trials were then alternated for a total of 8 trials: 5 training and 3 assessment trials. In between each trial, subjects were asked to return to upright standing and perform a torso flexion task to prevent holding of the lumbar angle.
Each subject performed the reposition sense protocol at all combinations of three target lumbar angles (maximum, minimum and mid-range lumbar angle) and three flexion angles (0, 30, & 60 degrees) for a total of 9 separate tests. The order in which the tests were performed was randomized and subjects were asked to rest for 5 minutes before performing the next reposition sense protocol.
Reposition sense error (RSE) was defined as the difference (in degrees) between the target lumbar angles (θt
) and actual lumbar angle reproduced by the subject (θr
). The absolute value of this error (Absolute Reposition Sense Error (aRSE)) for each trial was used to determine the magnitude of the error:
Directional reposition sense error (Directional Reposition Sense Error (dRSE)) was defined as the difference between the target lumbar angles (θt
) and reproduced (mean) lumbar angle (θr
The absolute reposition sense errors and the directional reposition sense errors were averaged for the 5 training trials and for the 3 assessment trials at each of the nine experimental conditions. The repositioning errors for all nine conditions (three lumbar angle targets and three flexion angles) were compared to study the effect of torso flexion and lumbar angle on the reposition sense of the lumbar spine.
A Huynh-Feldt, repeated measures ANOVA was used to the examine the effects of the independent variables, torso flexion angle and lumbar angle, on the dependent variable, assessment absolute reposition sense error (aRSEa). Additional ANOVA were performed with the dependent variables, training absolute reposition sense error (aRSEt), training directional reposition sense error (dRSEt), and assessment directional reposition sense error (dRSEa). These tests were determined to be significant for a p<0.05.
The significant main effects from the ANOVAs on the absolute reposition sense error (aRSEa) were further investigated using one-way, Huynh-Feldt repeated measures ANOVA performed to assess the effect of flexion on aRSEa for each lumbar angle condition. This analysis was done to confirm that absolute reposition error increased with torso flexion in mid-range curvatures as reported in the literature [Wilson and Granata, 2003
]. One-way, Huynh-Feldt repeated measures ANOVA were also performed to assess the effect of lumbar angle target on aRSEa at each of the three torso flexion angles.