Introduction

1. Fatigue of the Trunk Flexors in Position and Force-Matching Tasks in Healthy Subjects Christopher M. Wall1,Seth R. Oberst1, Derek W. Steele1, James S. Thomas1,2,3 1Division of Physical Therapy, School of Rehabilitation and Communication Sciences, Ohio University;2Ohio Musculoskeletal & Neurological Institute; 3Department of Biomedical Sciences , Heritage College of Osteopathic Medicine, Ohio University, Athens, OH Introduction Results • Muscle fatigue can be defined as a reduction in force-generating capacity of a muscle. • Enoka and Duchateau (2008) have shown that muscle fatigue is different if an individual attempts to maintain a constant joint angle with a fixed load (i.e. position-matching task) versus if an individual exerts a constant force against a rigid constraint (i.e. force-matching task). • Thomas et al. (2011) have shown that fatigue of the elbow flexors and the trunk extensors is dependent on load type (i.e. force- vs. position-matching tasks). While the elbow flexors tend to sustain a force longer than a position, the trunk extensors are able to sustain a position approximately 50% longer than force. However, the effect of load type on fatigue of the trunk flexors remains relatively unknown. • Having an understanding on the effects of load type on task failure in this muscle group may demonstrate clinical significance. We sought to explore whether the trunk flexors (as a group or as individual muscles) would behave more similar to the appendicular or axial muscles during a supine fatigue task with a varying load type. • The purpose of this study was to determine the effect of load type and gender on time-to-task failure as well as changes in muscle activation on the trunk flexors assessed using surface electromyography (EMG) on 20 healthy subjects (10 male, 10 female). • Time-to-Task Failure • The effects of load type and gender on time-to-task failure were analyzed using a 2-way mixed model ANOVA. No significance was found on time to task failure between load types. • Change in Median Power Frequency • There was a significant main effect of muscle on decrease in MPF across load type (p=.002). Specifically, the rectus abdominus was significantly different than the internal oblique (p<.001) and the external oblique (p=.006). However, the internal oblique and external oblique did not show a significant difference. Results showed that the rectus abdominus experienced the most fatigue (mean=20.36 ± 4.429), followed by external oblique (mean=5.41±5.59). Internal oblique displayed an increase in median frequency (mean= -5.54±5.59) suggesting that this muscle did not fatigue. Figure 1: A representative subject is depicted in the supine fatigue set-up. Inset is real-time feedback given to subject during fatigue task. Figure 6: Time series of integrated EMG of a representative subject’s abdominal musculature averaged across sides during a position-matching task. Figure 4: Mean decrease in Median Power Frequency (MPF)of rectus abdominus (RAB), external obliques (EXO), internal obliques (INO). * = significant difference (p<.005). Conclusion • Regardless of load type the decrease in MPF was greatest for the rectus abdominus, indicating that this is the major muscle used in a supine abdominal fatigue task. This finding suggest that the rectus may be more important in fatigue tasks and that intervention aimed at improving control and endurance of the rectus may be an important adjunct in those with LBP. McGill (2002) has found that loss of the balance between the endurance of the trunk flexors, extensors, and lateral musculature can discriminate between those with and without a history of LBP. Further exploration of the importance of the rectus abdominus in trunk flexion fatigue tasks in healthy and unhealthy subjects is necessary. • Unlike the trunk extensors, these data suggest that load type does not affect time-to-task failure of the trunk flexors. This is unlike data from the trunk extensors (axial muscles) which showed an ability to maintain a position longer than a given force – opposite that typically seen for the appendicular muscles. Woodley, et al. found that only some regions of the abdominal muscles featured multiple endplate bands consistent with typical axial muscle orientation. This is suggestive of regional differences in muscle architecture. It is possible that certain regions of the rectus abdominus (the major muscle used in supine fatiguing tasks) act similar to appendicular muscles while other regions are more involved in postural control such as axial muscles, thus negating the influence of load type on time-to-task failure. • The internal oblique, an analog to the transverse abdominus and shown to be a spinal stabilizer, did not show fatigue in this task based on changes in MPF. It is unknown whether this muscle is highly fatigue-resistant in healthy subjects or if it is simply not used in the specific fatigue task used in this experiment. • References: • 1. Enoka, R. M., and Duchateau, J. (2008). Muscle fatigue: What, why and how it influences muscle function. Journal of Physiology,586(1), 11–23. • 2. Thomas JS, Ross AJ, Russ DW, Clark BC. Time to task failure of the trunk muscles differs with load type. Journal of Motor Behavior; 2011. • 3.Woodley, S. J., Duxson, M. J. and Mercer, S. R. (2007), Preliminary observations on the microarchitecture of the human abdominal muscles. Clin. Anat., 20: 808–813. • 4. McGill, S., Low Back Disorders. 1 ed. 2002, Champaign: Human Kinetics Methods • All participants were provided written informed consent and the protocol was approved by the Institutional Review Board of Ohio University. • Time-to-task failure was assessed using a supine abdominal fatigue protocol during two sessions separated by 72 hours. • Subjects were positioned supine on a custom designed table in which the trunk segment was attached to a free-floating padded board that was counter-weighted to the subject’s trunk mass. The subjects were positioned with hips flexed 60 degrees and tibias strapped horizontally to a suspended platform. Peak trunk flexion force was then assessed in this position. Subjects were then asked to maintain a sub-maximal trunk flexion contraction at 20% of their peak force for as long as possible. • The force-matching condition consisted of pulling against a load cell while receiving visual and auditory feedback. Task failure (i.e. fatigue) occurred when they were unable to maintain their target force (± 20%) for greater than 3 seconds. • In the position-matching task, the trunk counter-weights were removed so that the subject needed to exert an effort equal to 20% of peak load to maintain the trunk in a horizontal position. The subject received visual and auditory feedback on their trunk position via software developed in LabVIEW (National Instruments, Austin, Texas, USA). Task failure occurred when they could not maintain that position (± 1º) for greater than 3 seconds. • For both load types (i.e., position- and force-matching), an MVC of the trunk flexors was performed immediately following task failure. Figure 7: Time series of integrated EMG of a representative subject’s abdominal musculature averaged across sides during a force-matching task. Figure 2: A representative subject’s submaximal force production during a force matching task and maximal flexor force production immediately following task failure. Figure 5: No significant difference in time-to-task failure between load types across gender (p>.05). Position- matching (mean= 1942.64 sec); Force-matching (mean= 1930.27) Figure 8: Time-to-task failure of 25 subjects with a history of LBP at 3 load magnitudes (20, 40, and 60 % MVC) who were asymptomatic at time of testing compared to anthropometrically-matched controls. Preliminary analyses revealed significant differences within subjects but no significant difference between LBP and control subjects. Figure 3: A representative subject’s trunk position during a position matching task. Note the fluctuations in trunk position during the isometric fatigue task. Table 1: Subject demographics. * indicates significant difference between gender.