Trunk Muscles Respond To Task Specific Fatigue In An Opposite Manner As Appendicular Muscles

1. Trunk Muscles Respond To Task Specific Fatigue In An Opposite Manner As Appendicular Muscles James S. Thomas1,2,3, Andrew J. Ross1, Brian C. Clark1,2, Jeffery Cowen1, Richard Pickett1, Mathew Linsenmayer1 1School of Rehabilitation and Communication Sciences, Division of Physical Therapy, Ohio University, 2Institute for Neuromusculoskeletal Research,3Department of Biomedical Sciences College of Osteopathic Medicine, Ohio University, Athens, OH Introduction Muscle fatigue is commonly defined as an exercise-induced reduction in maximal voluntary muscle force (Enoka & Stuart, 1992). It arises not only because of peripheralchanges at the level of the muscle, but also because the centralnervous system fails to drive the motorneuronsadequately. The relative contribution of the neural and muscular mechanisms to muscle fatigue varies with the specifics of the task being performed (Enoka & Duchateau, 2008; Enoka & Stuart, 1992; Hunter, Duchateau, & Enoka, 2004). For example, if an individual attempts to maintain a constant joint angle against an inertial load (i.e., a position-matching task) the time to task failure is very different when compared to exerting a constant equivalent force against a rigid restraint (i.e., force-matching task) even though the joint angles and muscle torques are identical for both tasks(Enoka & Duchateau, 2008). Numerous studies of the appendicular muscles have shown that time to task failure during sustained submaximal contractions are 50% shorter when subjects attempt a position-matching task compared to a force-matching task (Hunter, et al., 2004; Hunter, Yoon, Farinella, Griffith, & Ng, 2008; Maluf & Enoka, 2005; Maluf, Shinohara, Stephenson, & Enoka, 2005; Rudroff, Justice, Matthews, Zuo, & Enoka). While it has been suggested that limited duration of position-matching tasks in appendicular muscles is due to spinal mechanisms such as increased excitation of spinal motor neurons (Enoka & Duchateu, 2008), there are no published findings on time to task failure of the trunk extensors in position- and force matching tasks. This is of particular importance because fatigability of the trunk extensors has been used to predict a first time episode of low back pain (Alaranta, Luoto, Heliovaara, & Hurri, 1995; Biering-Sørenson, 1984), discriminate those with and without low back pain or a history of low back pain (McGill, et al., 2003; McKeon, Albert, & Neary, 2006), and predict disability in 12 months following a sub-acute episode of low back pain (Enthoven, Skargren, Kjellman, & Oberg, 2003). Understanding the effects of load type on task failure in this muscle group may have clinical significance. Thus, the purpose of this study was to first determine the effects of load type (i.e., position versus force matching) on time to task failure of the trunk extensor muscles in healthy participants during seated extension tests. Methods Eighteen healthy participants (9, males, 9 females) with a mean age of 22.8±0.92 yrs and no history of low back pain participated in this study. All subjects provided written informed consent before participating in this study. The protocol was approved by the Institutional Review Board of Ohio University. This experiment consisted of two sessions scheduled at least 72 hours apart. The order of testing (i.e., force versus position matching) was randomized and counterbalanced. Participants were seated upright in a lumbar extension apparatus (MedX, Ocala, FL) that was modified by inserting a load cell in series with the weight stack to 1) assess maximal isometric voluntary strength and 2) monitor isometric load during the force matching tasks. Specifically, the participants were positioned in the apparatus with their knees flexed 55°, their hips flexed 85°, and their trunk in a vertical position. A potentiometer was attached to the trunk resistance arm of the apparatus to monitor trunk position. Data from the load cell and the potentiometer were sampled at 1000 Hz, recorded and used to provide either force or position feedback for the participant. The real-time visual feedback of force or position was provided on a flat-panel monitor located 1.5 meters in front of the participant using software developed in LabVIEW(National Instruments, Austin, TX). The gain of the visual feedback provided was 0.5°/cm for position-matching and 5% Target Force/cm for force-matching tasks. Note the gain was the identical for the trunk extensors and the elbow flexors. Participants performed a series of maximum voluntary isometric contractions (MVIC) of the trunk extensors. In the position matching tasks participants maintained an upright sitting posture against a weight stack loaded to 15% MVIC for as long as possible while receiving visual and auditory feedback regarding trunk position. Task failure occurred when the participant was unable to match the target position (± 1 degree) for greater than 3 seconds. In the force matching tasks participants had to maintain an extension force of 15% MVIC for as long as possible while receiving visual and auditory feedback. Task failure occurred when the participant was unable to match the target force (± 10%) for greater than 3 seconds. A 2-way mixed-model ANOVA was used to determine the effect of load type and gender on time to task failure. Data Analysis A 2-way mixed-model ANOVA was used to determine the effect of load type and gender on time to task failure. As mentioned previously load type has been shown to have a marked effect on the time to task failure of the elbow flexor muscles ,(Enoka & Duchateau, 2008) with numerous studies reporting that time to task failure is significantly shorter with a position-matching task compared to a force-matching task(Maluf & Enoka, 2005; Maluf, et al., 2005; Rudroff, et al.). Accordingly, a subset of the participants (2 males and 2 females) participated in a second series of tests to assess the effect of task type on time to task failure on the right elbow flexors. Participants were positioned with the shoulder and elbow flexed 90 degrees. Using the same load parameters and system of feedback described above, time to task failure in both force and position matching tasks was analyzed using a repeated measures ANOVA. Steadiness of the contraction was quantified using the coefficient of variation of force measured at the following time points: 1st 10 sec, 20, 40, 60, 80% of task duration, and last 10 sec. Results As illustrated in Figure 2, the time to task failure for the trunk extensors was significantly longer for the position-matching task (32.6±5.6 mins) when compared to force-matching task (23.6±4.2 mins) (F=6.34, p<.05). There was no significant main effect or interaction of gender on time to task failure. However, the time to task failure for the elbow flexors was significantly shorter for the position-matching task (18.7±2.5 min) when compared to the force- matching task (28.8±4.7 mins) (F=9.36, p<.05). Conclusions This study provides the first test of the effect of load type on the time to task failure of the trunk extensor muscles. These data indicate that the time to task failure is approximately 50% longer for position-matching tasks compared to force-matching tasks for the trunk extensors, which is in contrast to that typically observed in appendicular muscles (Hunter, et al., 2008; Maluf & Enoka, 2005; Maluf, et al., 2005; Rudroff, et al.). Additionally, our findings from a subset of our subjects performing force versus position matching tasks with the elbow flexor muscles is consistent with these previous reports on appendicular muscle fatigue (Maluf & Enoka, 2005; Maluf, et al., 2005; Rudroff, et al.). Accordingly, our findings suggest that the mechanisms of task failure differ between the trunk extensor muscles and those of appendicular muscles. Figure 1.A typical subject positioned in the MedX Lumbar device and a screen shot of the visual feedback used for the position- and force-matching tasks is shown. Future Directions We seek to determine the underlying mechanisms driving the differences in time to task failure of the trunk extensor muscles. It is unknown if early task failure of the trunk extensors in force-matching tasks is due to similar spinal mechanisms reported in the appendicular muscles for position-matching tasks, or if supraspinal mechanisms such as greater alterations in intracortical inhibition and facilitation are causal factors of task failure in trunk extensor muscles. Therefore we have adapted classic neurophysiological techniques to examine the spinal and supraspinal mechanisms contributing to trunk extensor task failure under different load types (position- versus force-matching). We will examine single motor unit recruitment of the multifidus muscles (Figure 4), short latency spinal reflexes (Figure 5), and cortical excitability using paired pulse transcranial magnetic stimulation to evoke motor potentials in the erector spinae muscles (Figure 6). Figure 4. Motor unit potentials recorded from the right multifidus muscle of a healthy participant during a 5-second force-matching task at 5% MVC (left). The expanded waveforms of a single motor unit discharging repetitively during the task is shown at the right. Figure 2.The time to task failure for the different load types is shown for the trunk extensor muscles (n=18) and for the elbow flexors (n=4) which was from a subset of the larger group completing the main experiment. The time to task failure for the trunk muscles were significantly longer for the position matching tasks compared to the force matching tasks while opposite the results were found for the elbow flexors. Figure 6. A. The experimental setup for performing transcranial magnetic stimulation (TMS) to evoke motor evoked potentials (MEP) from the erector spinae muscles. B. Representative examples of motor potentials recorded from the erector spinae muscles. The left trace illustrates a MEP evoked with a single-pulse. The middle and right traces illustrate the change in MEP sizes obtained with paired pulse TMS. Specifically an example of short-interval intracortical inhibition (SICI) is shown in the middle trace, and an example of intra-cortical facilitation (ICF) is shown in the right trace. SA = Stimulus artifact. Figure 5.A. The experimental setup for revoking short latency stretch reflexes from the erector spinae muscles. B. the tip of the electromechanical tapping apparatus will be gradually pressed into the tissue until a pre-load 30 Newtons is reached and then the device delivers a rapid mechanical tap to the muscle with a net force of 90 Newtons. C. Representative examples of a short latency stretch reflex recorded from the erector spinae muscles in response to a mechanical tap. Figure 3.The coefficient of variation is plotted for both force- and position-matching tasks. time to task failure for the different load types is shown for the trunk extensor muscles (n=18) and for the elbow flexors (n=4) which was from a subset of the larger group