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Annelie Shield
07-06-1999, 07:29 PM
Dear all;

We have been assessing the completeness of quadriceps activation via the interpolated twitch technique. We typically superimpose a supramaximal doublet (twin 50-100 microsecond stimuli 10 ms apart) over the top of the MVC and then evoke a twitch-like control response five seconds afterwards in relaxed muscle. In our 'long-rest' protocol, two minutes rest is allowed between MVCs. We have noticed that for most subjects successive control twitches decline in size, typically by 5-10% over the course of 3-5 MVCs.

We find this perplexing given that;

1. MVC force typically does not decline at all in successive MVCs using this protocol. (The duration of each MVC is held constant by a 3 second buzzer).
2. We have not observed any progressive decline when doublet evoked responses are delivered once every 10 seconds to relaxed muscles over a two minute period.
3. The same decline does not occur during a much more demanding 'short-rest' protocol which requires subjects to perform 3- 5 * five second MVCs with ten seconds between them. (In this case the control responses, delivered five seconds after each
contraction, remain remarkably constant although a significant decline (15-25%) in MVC force is evident).

We employ self-adhesive and reusable rubber-gel electrodes and this effect seems to occur whether we have new or used (1-6 uses) ones. Our stimulator is a constant-current unit (Digitimer, DS7AH).

We initially thought of dodgy electrodes, anodal block and high frequency fatigue but can't understand why any of these would influence the long-rest protocol more than the short-rest protocol. The responses evoked in relaxed muscle are typically 3-4%larger after 5 second MVCs than after 3 second ones, so perhaps the extra potentiation compensates for whatever effect occurs during the long-rest protocol?

If anyone has any clues as to the mechanism of this phenomenon we would appreciate hearing from you. We will post all replies.

Thanks in advance


Tony Shield
Southern Cross University
NSW, Australia.












Dear all;
 
We have been assessing the
completeness of quadriceps activation via the interpolated twitch
technique. We typically superimpose a supramaximal doublet (twin
50-100 microsecond stimuli 10 ms apart) over the top of the MVC and then evoke a
twitch-like control response five seconds afterwards in relaxed muscle. In our
'long-rest' protocol, two minutes rest is allowed between MVCs. We have
noticed that for most subjects successive control twitches decline in size,
typically by 5-10% over the course of 3-5 MVCs. 
 
We find this perplexing given
that; 
 
1. MVC force typically does not
decline at all in successive MVCs using this protocol. (The duration of
each MVC is held            
constant by a 3 second buzzer).
2. We have not observed
any progressive decline when doublet evoked responses are
delivered once every 10 seconds to        
    relaxed muscles over a two minute period.

3. The same decline does not occur during a
much more demanding 'short-rest' protocol which requires subjects to perform
3-        5 * five second MVCs
with ten seconds between them. (In this case the control responses,
delivered five seconds after each  
    contraction, remain
remarkably constant although a significant decline (15-25%) in MVC
force is evident).
 
We employ self-adhesive and reusable rubber-gel
electrodes and this effect seems to occur whether we have new or used (1-6 uses)
ones. Our stimulator is a constant-current unit (Digitimer,
DS7AH).
 
We initially thought of dodgy electrodes,
anodal block and high frequency fatigue but can't understand why any of these
would influence the long-rest protocol more than the short-rest protocol. The
responses evoked in relaxed muscle are typically 3-4%larger after 5
second MVCs than after 3 second ones, so perhaps the extra
potentiation compensates for whatever effect occurs during the long-rest
protocol?
 
If anyone has any clues as to the mechanism
of this phenomenon we would appreciate hearing from you. We will post all
replies.
 
Thanks in advance
 
 
Tony Shield
Southern Cross University
NSW, Australia.