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°, 45°, 90°, 135°and 180°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 quasistatic 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).

The effect of sex and chronic low back pain on back muscle reflex responses

“…The trunk muscle preactivation level has been assessed only once in subjects with CLBP (Stokes et al 2006) and has been suggested as a potential confounding variable to be considered in future studies (Lee et al 2007). In partial agreement with the present results, Stokes et al (2006) showed a higher preactivation of back and abdominal muscles in subjects with CLBP than in controls.…”

“…The trunk muscle preactivation level has been assessed only once in subjects with CLBP (Stokes et al 2006) and has been suggested as a potential confounding variable to be considered in future studies (Lee et al 2007). In partial agreement with the present results, Stokes et al (2006) showed a higher preactivation of back and abdominal muscles in subjects with CLBP than in controls. Unfortunately in that study, EMG signals were normalized with the use of maximal EMG obtained during MVCs, which might be questionable since it may artiWcially increase the estimated activation level if their subjects with CLBP had painrelated fears (Thomas et al 2008).…”

Clinical Evaluation of Scapular Positioning in Patients With Nonspecific Chronic Low Back Pain: A Case-Control Study

“…Participants with CLBP adopt a stiffening strategy to increase spine stability and to reduce the exacerbation of existing pain. 10,11 Thus, in our study, increased activity of latissimus dorsi could be due to pain severity (6 of 10 based on the data in Table 1) in patients with CLBP. According to the "pain-spasm-pain" theory, pain increases muscle activity and a vicious cycle of muscle spasm develops.…”

“…8,9 It seems that increased activity of the trunk muscles can be due to abnormal motor recruitment patterns. 3 Stokes et al 10 reported that increased superficial trunk muscle activation was due to a "painadaptation" hypothesis. Participants with CLBP adopt a stiffening strategy to increase spine stability and to reduce the exacerbation of existing pain.…”

Abdominal Muscle Activity During a Standing Long Jump

“…The determination of muscle onset was based on the methods described by previous studies. 12,32 The onset of muscle activity was considered to occur if the average EMG amplitude every 20 milliseconds exceeded a threshold of 3 standard deviations above baseline for 20 milliseconds. A signal with baseline artifact was not determined as muscle onset, and we identified the onset with continually increased EMG signal from threshold values.…”