Introduction

1. The influence of chronic low back pain on muscle onset latencies in full body reaching tasks. James S. Thomas, Stephanie Gustwiller, Erica Johnson, Michelle Klewer, Ashley McCallum, Rachel Ruggeri. School of Physical Therapy, Ohio University, Athens, OH Introduction Performing multi-joint reaching tasks that necessitate some forward bending of the trunk become more complex following a low back injury due to the stresses placed on the spine. A reasonable assumption is that individuals adopt alternative movement strategies in the acute phase of an episode of low back pain. This is a normal response referred to as "protective guarding" and allows for healing of injured tissues. With chronic back pain, however, it is hypothesized that individuals become "stuck" in a dysfunctional movement pattern, which no longer protects healing tissues, but actually prevents full recovery. Consistent with this notion, preliminary data from our lab reveal that individuals with chronic pain avoid certain positions of the spine. Previously we found that low back pain also influenced trunk muscle onset latencies when performing bilateral reaches with different loads. However, it is unknown if onset latencies are different when only speed and target height are manipulated. Thus, participants reached for two targets at a comfortable and fast pace movement speed. The purpose of this study was to determine the effect of these task manipulations on the onset of trunk muscles as well as on the excursions of the spine and hip joints. Methods Forty subjects (20 chronic back pain and 20 matched healthy controls (Table 1) reached with the right hand to two targets located in a mid-sagittal plane starting from a standing posture. The target location was calculated such that the participant could reach the high target, in theory, by flexing the hips 15° with the shoulder flexed 90° and the elbow extended (See Figure 1). The low target could be reached by flexing the hips 60°.The target locations were chosen to create a task that progressively challenges the subject with larger excursions of the trunk. While standing on two force plates, subjects performed reaching movements with their right hand at a comfortable pace and were given no instructions on limb segment geometry. They were then instructed to perform the reaching movement at twice their comfortable pace. The participants completed 3 trials to each target height and leach movement speed. Kinematic data were sampled for 10 seconds at 100Hz using a seven camera Vicon MX-13 system. . Plastic plates with four-marker clusters were attached to the arms, forearms, pelvis, thighs, shanks, and feet. The arm, forearm, pelvis, thigh, shank, and foot marker clusters were attached with Velcro on elastic bands. Four markers were attached to the upper back with double-sided tape, centered around the T1-T2 spinous processes. Similarly, four markers were attached onto the lower back centered around the T12-L1 spinous processes. Four markers were attached to a hat to record head movements and one marker was attached to the tip of each index finger. A pair of markers was attached around the joint centers of the shoulders, elbows, wrists, knees, ankles, and one marker was placed over each greater trochanter. EMG data were collected bilaterally from the anterior deltoid, rectus abdominus, internal and external obliques, multifidus, and erector spinae with a DelsysBagnolisystem. Data Analysis First, we defined movement onset (t0) as the point where finger-tip velocity exceeds 2.5% of peak velocity and movement offset (t1) as the point where fingertip velocity dropped below 2.5% peak velocity. Movement time was calculated as t1-t0. Onset of EMG activity was, defined as the point where EMG signal is greater than or equal to baseline + 4*SD (baseline and SD was calculated from 100 data points beginning 200 ms prior to t0). The latencies were calculated as the difference between onset of the trunk muscles and the onset of the deltoid. For each group, muscle latencies as well as the changes in 3-D joint angles (from initial posture to target contact) of the right hip and lumbar spine were analyzed using mixed-model ANOVAs in which target height and movement speed were the within subject factors and group (low back pain or healthy) was the between subject factor. Table 1. Characteristics of participants with and without chronic LBP. B. Chronic Low Back Pain Subject A. Healthy Subject Figure 1. Targets were located such that the subject could reach each target, with the shoulder flexed to 90 and elbow extended, by flexing the hips 15, or 60 degrees. Figure 6. EMG recordings from the right deltoid and trunk musculature during a reaching trial to the low target at a fast pace for a healthy (A) and low back pain subject (B). External oblique (EXO), Rectus abdominus (RAB), Internal Oblique (INO), Erector Spinae (ERS), Multifidus (MUL) Results While there was no significant effect of group on lumbar flexion for these tasks, there was a trend for participants with chronic low back pain to use less lumbar flexion (Figure 2) and increased hip flexion (Figure 3 particularly for reaches to the low target. Interestingly, those trends are reversed for reaches to the high target. Movement time for reaches at the comfortable pace was 1090 ms (SD=57) and for reaches at the fast pace was 527 ms (SD=24). Thus participants moved twice as fast in the fast-pace reaches and there was no significant effect of group on movement time for either movement speed (Figure 4). However, for the fast paced movement trials, there was a significant effect of group on the onset latency of the left (p<.019) and right (p<.03) erector spinae. For the comfortable paced movement trials, there was only a group effect for the left erector spinae (p<0.01). On average, for fast-paced movement trials, healthy controls had a latency of the erector spinae muscles of 191 ms (SD=39) while those with chronic low back pain had a latency of 344 ms (SD=48) as shown in Figure 5. Finally, there was no effect of group on latencies of the trunk agonists. Figure 6 illustrates EMG recordings from the right deltoid and trunk musculature during a reaching trial to the low target of a typical healthy subject and a participant with chronic low back pain. These data clearly indicate that both groups turned on their antagonist trunk muscles late into the movement; but there was a much larger delay for those with chronic low back pain. Conclusions These findings provide further evidence that participants with chronic low back pain display a shift in motor behavior that is clearly evident in the timing of trunk muscle activations during volitional whole body reaching tasks. We previously have shown similar results for reaching at self-selected speeds with various loads. However, in that experiment there was significant group differences in movement time which could have accounted for some of the differences in muscles timing. Yet in this experiment in which movement time was tightly controlled we still find longer muscle onset latencies in participants with chronic low back pain. Given that participants with low back pain have increased latencies across task manipulations such as load, movement speed and target location suggest a robust finding. What is still unknown is why these latencies occur. Are they due to changes in trunk proprioception or could they due to an adapted pattern of learned disuse? Further research is needed to determine if these latencies actually contribute to continued low back pain or to the contrary serve as an adaptive protective response. This research was supported by The National Institutes of Health Grant R01-HD045512 to J.S. Thomas Figure 2. Effect of group and movement speed on lumbar flexion at each target height. Participants with chronic low back pain showed a trend of reduced lumbar spine excursions for reaches to the low target compared to healthy controls. Figure 3.Effect of group and movement speed on hip flexion at each target height. Participants with chronic low back pain showed a trend of increased hip excursions for reaches to the low target compared to healthy controls. Figure 5. Effect of group on onset latencies of the right erector spinae (ERS_R), right multifidus (MULT_R), left erector spinae (ERS_L), and L multifidus (MULT_L) Figure 4. Effect of group on movement time to reach the targets.