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 4^th fatiguing contraction

·         As 75.9 to 29.3

·         Biceps 91.6 to 82.3

Fig 5 pressure decreased by 10^th, voluntary activation percent decreased by 4^th .

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?