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Sunday, March 11, 2012

Anal sphincter fatigue: is the mechanism peripheral or central?

Anal sphincter fatigue: is the mechanism peripheral or central? Schabrun SM, Stafford RE, Hodges PW. Neurourol and Urodynam 2011;30:1550-1556.

Beth Shelly March 7, 2012 - Thank you to primary author Siobhan Schabrun for joining us on the call.

Primary aim – does the anal sphincter (AS) have greater susceptibility to central fatigue than the biceps
Central fatigue – decrease in activation of neurons in the motor cortex leading to decreased voluntary activation of the muscle. Skeletal limb muscles have up to 25% decrease force due to central fatigue.
Peripheral fatigue - loss of muscle force related to decrease in function of the neuromuscular junction or the muscle itself.

Subjects
Healthy volunteers – 8 female, 2 male, no control group, no blinding


Study design / method
Pattern of contraction and stimulation is well represented in figure 1c page 1551
·         Force of AS was measured with homemade pressure transducer which was tested – researchers report it is possible to measure increased intra-abdominal pressure (IAP) without increase in EMG or force.
·         Motor cortex stimulation with figure 8 transcranial magnetic stimulation coil over brain areas to maximally stimulate the biceps and AS
·         Peripheral nerve stimulation of biceps and AS (using coil over S2)

Assessing the outcome
Force data was analyzed to see how it responded differently to voluntary activation, peripheral and central stimulation. 
Voluntary activation percent – compared the results of the electrical stimulation contraction to the voluntary contraction

Results
Baseline ability to generate force is lower for AS
·         AS = 75.9% - this means AS always has some reserve above and beyond its full capacity
·         Biceps = 91.6% - this means the biceps can voluntarily generate almost 100% capacity
No evidence of peripheral fatigue – muscle can always respond to electrical stimulation
Voluntary activation decreased for both AS and biceps but more so for AS 
·         AS 28%
·         Biceps – 12%
No change in AS pressure in response to cough. The muscle always has more left to respond to increased intra-abdominal pressure even in state of fatigue. 
Across the group pressure from cough was not sig greater than pressure from MVC

This is the part I am unsure about
Anal pressure decreased across contractions (steeper decline in holding ability within one contraction figure 4 a) and between repetitions (fig 5). 
Ability to voluntarily sustain force was reduced from pre to post training
·         AS 75.9 pre – 47.7 post – 46.6%
·         Biceps 91.6 pre – 79.9 post – 11.1%
Voluntary activation decreased significantly by the 4th fatiguing contraction
·         As 75.9 to 29.3
·         Biceps 91.6 to 82.3
Fig 5 pressure decreased by 10th, voluntary activation percent decreased by 4th .


Discussion
Peripheral stimulation continues to generate AS muscle activation – research supports that you cannot fatigue the PFM with electrical stimulation. 

Repeated sustained MVC of the AS are susceptible to central fatigue and therefore would not result in muscle hypertrophy.
Partial recovery of central fatigue occurs with very short rests
Thus short MVC (6 to 8 seconds) are more likely to result in training

Activation of AS by cough was maintained

Conclusion
AS is more susceptible to central fatigue during maximal voluntary contractions than the biceps.

Application to practice
Despite significant amount of slow twitch fibers and need for longer holding for decreased peristalsis, best training may be at 6 to 8 seconds with rest
Re-contracting during sustained hold may avoid some central fatigue?
Action of ES in PFM spasm is not by fatiguing the muscle?

6 comments:

  1. Does this mean that it would be more efficient to use estim rather than voluntary muscle contractions? Also, what does this mean for those with CNS diseases or those using meds that affect the motor cortex?

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  2. Lisa, I do not think that electrical stimulation is substitute for voluntary strengthening. Research on the outcome of current electrical stimulation for the pelvic floor muscle is very mixed and most Cochrane reviews do not support its use clinically. The stimulation used in the study was magnetic stimulation – this is not currently available for PTs to use in PFM training. Not sure your angle on the second questions. Please elaborate.

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  3. Thank you, I musunderstood the type of stim. My second question just refers to other possible rehab implications. If the motor cortex is acting as a limiter, and then is compromised, could you expect to see a disproportionate effect on rehab of the EAS as opposed to other skeletal muscle?

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