Fatigability And Steadiness Of The Trunk Flexor Muscles In Young, Healthy Adults

Abstract

The abdominal muscles are the prime movers for flexion of the trunk and are essential to several functional tasks, such as lifting. Improper function of the abdominal muscles has been linked with multiple impairments, including low back pain, incontinence and breathing dysfunction; however, measurements of fatigability and steadiness for this muscle group have not been reported. PURPOSE: To determine fatigability and torque fluctuations (steadiness) across a range of torques during isometric trunk flexion contractions in young, healthy adults. METHODS: Men (n=3) and women (n=8) (18-45 yrs) performed isometric trunk flexion tasks in a back flexion/extension attachment. Fatigability was assessed in upright sitting with an intermittent isometric protocol using 6 s contractions at 50% of maximal voluntary contraction (MVC), each followed by a 4 s rest period until task failure. MVCs were performed every 60 s. Task failure was defined as an inability to maintain the 50% submaximal contraction for 3 of 6 s or MVC ≤ 50% initial MVC. In a separate testing session, participants performed two trials of a 6 s isometric trunk flexion contraction at 5%, 10%, 20%, and 70% MVC. Steadiness was quantified as the amplitude of torque fluctuations and calculated as the standard deviation (SD) of torque and coefficient of variation (CV) for each contraction. RESULTS: Time to task failure for the trunk flexion intermittent submaximal fatigue task was 17.0 min. The SD of torque increased as contraction intensity increased. CV of torque declined with contraction intensity and was highest for the low intensity contractions (14.5% and 11.1% for 5% and 10% MVC, respectively) and lowest for the high intensity contractions (6.3% for 20% MVC and 3.3% for 70% MVC). CONCLUSIONS: The abdominal muscles demonstrated greater CV of torques than typically observed for the upper and lower limb (Jesunathadas et al, J Appl Physiol 112: 1897-1905, 2012) but the pattern of change with increased intensity was similar. Fatigability for an intermittent submaximal fatiguing trunk flexion contraction was similar to that for the elbow flexor muscles (Hunter et al, J Appl Physiol 96: 2125 - 2132, 2004) but less than for handgrip muscles (Hunter et al, Muscle Nerve 39: 42-53, 2009) in young, healthy adults.