Conflict of Interest: none.
Original Basic Science Article
Version of Record online: 20 JUL 2011
Copyright © 2011 Wiley Periodicals, Inc.
Neurourology and Urodynamics
Volume 30, Issue 8, pages 1550–1556, November 2011
How to Cite
Schabrun, S. M., Stafford, R. E. and Hodges, P. W. (2011), Anal sphincter fatigue: Is the mechanism peripheral or central?. Neurourol. Urodyn., 30: 1550–1556. doi: 10.1002/nau.21162
Heinz Koelbl led the review process.
- Issue online: 19 OCT 2011
- Version of Record online: 20 JUL 2011
- Manuscript Accepted: 3 MAY 2011
- Manuscript Received: 20 FEB 2011
- neuromuscular fatigue;
- transcranial magnetic stimulation;
- voluntary activation
Striated muscles of continence appear to exhibit marked fatigue during voluntary efforts. This is counterintuitive considering the high proportion of slow twitch muscle fibers. One explanation is that fatigue is due to central, rather than peripheral mechanisms. Here we examined the contribution of reduced voluntary activation (central fatigue) to the decline in anal sphincter (AS) and elbow flexor muscle force during voluntary contractions.
Ten healthy subjects participated. Fatigue was induced using 10 maximal voluntary contractions sustained for 20 s. During each fatiguing contraction, transcranial magnetic stimulation (TMS) was delivered over the motor cortex at 5 s intervals. Central fatigue was assessed using the superimposed twitch force elicited by TMS. Peripheral fatigue was measured using brachial plexus (elbow flexors) or sacral plexus (AS) stimulation during contraction and at rest.
Ability to maximally activate AS (75.9%) was less than for the elbow flexors at baseline (91.6%). Voluntary activation declined in both muscles, but the decline was greater in AS (AS 28%; elbow flexors 12%). There was no change in the amplitude of the twitch evoked by peripheral nerve stimulation in either muscle.
AS exhibits a greater decline in voluntary activation during fatiguing contractions than elbow flexor muscles. This is not accompanied by peripheral changes, which implies central mechanisms are responsible. Thus, we conclude that AS is susceptible to central fatigue during maximal voluntary activations. We propose this may be a protective mechanism to conserve contractile potential of the anal sphincter for function. Neurourol. Urodynam. Neurourol. Urodynam. 30: 1550–1556, 2011. © 2011 Wiley Periodicals, Inc.