1. Introduction Methods Results Conclusion  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).  We have recently found that time to task failure in the axial trunk extensor muscles is about 50% longer in position-matching versus force- matching tasks (i.e., opposite the findings for the appendicular muscles) when healthy participants performed sustained sub-maximal trunk exertions in a seated posture (Thomas, Ross, Russ, & Clark, 2011).  Primary aim: To determine the effects of test position (i.e., seated versus prone) and load type (i.e., position- versus force-matching) on the time to task failure of the trunk extensor muscles in healthy participants.  However, the relative contribution of the trunk extensor and hip extensor muscles to task failure with different load types is unknown. Secondary aim: To examine changes in median frequency (MF) of the electromyographic power spectrum of the primary trunk and hip extensors across test position and load type to determine the contribution of each.  Each study consisted of two sessions scheduled a minimum of 72 hours apart. The order of testing condition (i.e., force- versus position-matching task) was randomized and counterbalanced.  All participants provided written informed consent and the protocol was approved by the Institutional Review Board of Ohio University.  Force Matching task: Participants performed an isometric trunk extension exertion of 15% MVC for as long as possible against a load cell. Task failure occurred when the participant was unable to match the target force (±10%) for greater than 3 s.  Position-Matching task: Participants had to support a weight equal to 15% of the subject’s MVC for as long as possible while maintaining posture for the test position (i.e., 0° hip angle in the modified Sorensen test versus 85 ° hip angle in the seated test). Task failure occurred when the participant was unable to match the target position (± 1 degree) measured by a potentiometer attached to the apparatus for greater than 3s.  Real-time visual feedback of force or position was provided on a flat- panel monitor using software developed in LabVIEW (National Instruments, Austin, Texas, USA).  For both load types (i.e., position- and force-matching), an MVC of the trunk extensors was performed immediately following task failure. Modified Sorensen Test  Sixteen adult participants (7 male, 9 female)  Mean age of 23.6 ±0.36 years  No history of low back pain Participants were positioned in prone (i.e., 0 degree hip angle) on a segmented table. The subject’s trunk rested on a padded board connected to a weight stack through a series of low friction pulleys permitting the trunk weight to be counterbalanced. A load cell fixed between the floor and the trunk segment of the table was used to measure strength as well as the applied isometric load during the force-matching task. Seated Extension Test  Eighteen adult participants (9 male, 9 female)  Mean age of 22.8 ±0.92 years  No history of low back pain Participants were seated upright in a lumbar extension apparatus (MedX, Ocala, Florida, USA) with their knees flexed 55◦, their hips flexed 85◦, and their trunk in a vertical position. The MedX was modified by inserting a load cell in series with the weight stack to assess MVC and to monitor isometric load during the force matching task. Figure 10: Depicted are the mean times to task failure of participants in each test position and each load type (i.e., position- versus force-matching). Error bars represent the standard error of the mean. Figure 11: Normalized MVC at task failure is depicted for each combination of test position and load type. Normalized MVC at task failure is calculated using the following formula: (MVC at task failure / initial MVC) x100. Figure 9: Normalized slope of the MF values are shown for each muscle group tested in the two test positions. ERS= erector spinae, MUL= multifidis, GLU= gluteus maximi, HS= hamstring/biceps femoris, * indiciates a signifcant difference between two muscle groups or between test positions. Time to Task Failure  Time to task failure of the trunk extensors was significantly longer during the modified Sorensen test (mean= 47.2 ±5.0 min) compared to the seated extension test (mean= 28.1 ±4.7 min) (F (1,32)= 7.86, p<.01).  Time to task failure was significantly longer in position matching tasks (58.3 ±6.6 min) compared to force matching tasks (36.1 ±5.4 min) for the modified Sorensen test (F (1,15)= 5.25, p<.05) as well as for the seated extension test (32.6 ±6.2 min versus 23.6 ±5.1 min, respectively) (F (1,16)= 6.34, p<.05). Normalized Extensor Force at Task Failure  Analysis of the simple effects revealed no significant effects of load type on normalized extensor force at task failure for either the modified Sorensen (p=.078) or seated extension test positions (p=.309). Normalized Slope of the MF  There was no main effect of test position (p=.712) or load type (p=.421) on the slope of the MF; however, there was a main effect of muscle group on the normalized slope of the MF (F (1,30)= 3.42, p=.03).  A significantly larger decline in normalized slope of the MF was found (p<.01) in the multifidi (mean slope= -.603) compared to the erector spinae (mean slope= -.236) and in the multfidi compared to the gluteus maximi (mean slope= -.355) (p<.05).  A significantly greater decline in normalized slope of the MF was found in the erector spinae in the prone test position (mean slope= -.403) compared to the seated test position (mean slope= -.070) (F (1,32)= 4.24, p= <.05). Figure 1: A participant is depicted in the modified Sorensen test (prone test position) set-up. Figure 2: A participant is depicted in the seated extension test set-up.  The findings of this study are consistent with the data from Thomas et al 2011.  These data indicate that the time to task failure is approximately 50% longer in duration for position- matching tasks compared to force-matching tasks for the trunk extensors in both test positions, 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.).  Despite the main effect of test position and load type on time to task failure, neither test position nor load type significantly affected the normalized MVC at task failure or the normalized slope of the median frequency.  Our findings reveal that the effects of load type on time to task failure are independent of test position; however, the difference in time to task failure is not explained by the normalized slope of the median frequency (i.e., degree of fatigability) of the trunk and hip extensors during the fatiguing tasks. Figure 7: A representative subject’s EMG power spectrum during the duration of a fatigue task is shown. Figure 8: The slope of the MF of the ERS in a representative subject is shown. Figure 3: A representative subject’s trunk position during a position-matching task. Note the consistent trunk position during the isometric fatigue task. Figure 4: A representative subject’s trunk position during a force- matching task. Note the consistent trunk position during the isometric fatigue task. Figure 5: A representative subject’s submaximal load during a position- matching task and maximal extensor force production immediately following task failure. Figure 6: A representative subject’s submaximal force production during a force- matching task and maximal extensor force production immediately following task failure. The Effects of Test Position and Load Type on Time to Task Failure of the Trunk Extensors Andrew J. Ross 1, Saundra Motter 1, Matthew Linsenmayer 1, Richard Pickett 1, Jeffrey Cowen 1, Brian C. Clark 2,3, David W. Russ 1,2, James S. Thomas 1,2,3 1 School of Rehabilitation and Communication Sciences, Division of Physical Therapy, Ohio University, 2 Ohio Musculoskeletal & Neurological Institute, 3 Department of Biomedical Sciences College of Osteopathic Medicine, Ohio University, Athens, OH