Trunk muscular activation patterns and responses to transient force perturbation in persons with self-reported low back pain

Abstract

Trunk stability requires muscle stiffness associated with appropriate timing and magnitude of activation of muscles. Abnormality of muscle function has been implicated as possible cause or consequence of back pain. This experimental study compared trunk muscle activation and responses to transient force perturbations in persons with and without self-reported history of low back pain. The objective was to determine whether or not history of back pain was associated with (1) altered anticipatory preactivation of trunk muscles or altered likelihood of muscular response to a transient force perturbation and (2) altered muscle activation patterns during a ramped effort. Twenty-one subjects who reported having back pain (LBP group) and twenty-three reporting no recent back pain (NLBP group) were tested while each subject stood in an apparatus with the pelvis immobilized. They performed 'ramped-effort' tests (to a voluntary maximum effort), and force perturbation tests. Resistance was provided by a horizontal cable from the thorax to one of five anchorage points on a wall track to the subject's right at angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees and 180 degrees to the forward direction. In the perturbation tests, subjects first pulled against the cable to generate an effort nominally 15% or 30% of their maximum extension effort. The effort and the EMG activity of five right/left pairs of trunk muscles were recorded, and muscle responses were detected. In the ramped-effort tests the gradient of the EMG-effort relationship provided a measure of each muscle's activation. On average, the LBP group subjects activated their dorsal muscles more than the NLBP group subjects in a maximum effort task when the EMG values were normalized for the maximum EMG, but this finding may have resulted from lesser maximum effort generated by LBP subjects. Greater muscle preactivation was recorded in the LBP group than the NLBP group just prior to the perturbation. The likelihood of muscle responses to perturbations was not significantly different between the two groups. The findings were consistent with the hypothesis that LBP subjects employed muscle activation in a quasi-static task and preactivation prior to a perturbation in an attempt to stiffen and stabilize the trunk. However, interpretation of the findings was complicated by the fact that LBP subjects generated lesser efforts, and it was not known whether this resulted from anatomical differences (e.g., muscle atrophy) or reduced motivation (e.g., pain avoidance).

Fig. 1

Diagram showing a the arrangement of the apparatus relative to the subject, and b the waveform of the force perturbation generated by the single turn of the motor

Fig. 2

Sample recording of an EMG signal from a perturbation experiment. The upper panel shows the force recording, that defined the force perturbation onset time (that defined zero time). Relative to this time, pre-perturbation and post-perturbation ‘windows’ (shaded) were defined, and used to calculate the MEMGD. The EMG activity prior to the perturbation defined the Shewhart threshold used to identify the EMG response onset that occured here 113 ms after the perturbation onset

Fig. 3

Relationship between groupwise averages of the muscle activation in ramped-maximum-effort tests. Each point on the graphs represents the mean for one muscle/angle permutation. aUpper: For activation normalized by both maximum EMG and maximum effort: 15/20 of points for dorsal muscles (squares) and 23/30 points for the abdominal muscles (triangles) lie to the left of the 1:1 (equality) line, indicating that on average the subjects in the LBP group employed lesser muscle activation. bLower: Activation normalized by maximum EMG only: 18/20 points for dorsal muscles (squares) and 18/30 points for abdominal muscles (triangles) lie to the right of the 1:1 (equality) line, indicating that on average the subjects in the LBP group employed greater muscle activation

Fig. 4

Relationship between groupwise averages of the muscle preactivation in perturbation tests. Each point on the graph represents the mean for one muscle/angle permutation. aUpper: For trials with low preload, where 17 of 20 points for dorsal muscles (squares) and 26 of 30 abdominal muscles (triangles) lie to the right of the 1:1 (equality) line, indicating that on average the subjects in the LBP group employed greater muscle preactivation. bLower: For trials with high preload, where 17 of 20 points for dorsal muscles (squares) and 20 of 30 abdominal muscles (triangles) lie to the right of the 1:1 (equality) line, indicating that on average the subjects in the LBP group employed greater muscle preactivation