Thanks to those who replied to my questions regarding shrinking twitches. Here is the original question and the replies;
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 Tony,
I am not on Biomech-L, but a colleage has forwarded your interesting
query to me. I work with Dr. Stuart Binder-Macleod and some of your
findings may be consistent with findings from our laboratory. Please see
responses interspersed below.
First, recent work suggests that the twitch interpolation technique is not
sufficient to assess maximum quadriceps activation. Please see Kent-Braun
and Le Blanc, Muscle and Nerve, 19:861-869,1996 among others.
Additionally, work in our lab that is in preparation for submission
suggests a 13-pulse, 100pps train was the easiest tolerated by our
subjects and best assesses completeness of activation (typically we use
600 microsecond pulses). We find that the additional pulses are needed
because of the time dependent rate of rise of force in response to
stimulation.
Second, it does not sound that the intent of your MVC protocols are to
avoid fatiuge. Anecdotally, we find that if we want to avoid fatigue, we
wait 5 minutes between MVC attempts and limit are attempts to 4 or 5 at
the maximum. It sounds however, that you are studing fatigue and central
activation?
Third, the time occurance of your attenuated twitch responses are
interesting. We find that after fatigue is produced with a modifed
Burke's fatigue protocol, recovery responses to low-frequency trains
vs. high frequencies are
the most attenuated at the 2 minute recovery period. That is we see
inital recovery of responses, then maximum depression at 2 minutes, and
then further recovery. Our observations support others in that there are
two components to LFF...i.e, a rapidly recovering metabolite-dependent
portion and a slow-recovering component. Please see Binder-Macleod and
Russ. J. Appl. Physiol. 86(4):1337-1346, 1999. and look for Russ and
Binder-Macleod Muscle and Nerve in Press, August 1999.
Part of the phenomena you may be observing may also be related to the
catchlike property of skeletal muscle that would be elicitited by your
doublet pulses. In fresh, unpotentiatied muscle, and in fatigued muscle,
doublet initiated trains are highly effective in augmenting peak force and
force-time integrals when compared to comparable consant-frequency trains.
In fresh, but highly potentiated muscle, no difference in peak force or
force-time integrals are generally observed. Please see Bevan et al., J.
Physiol., 449:85-108, 1992. You might find that the discussion sections
of some of our work may be interesting to you, for example, Binder-Macleod
and Lee, J. Appl. Physiol 80(6):2051-2059, 1996; Binder-Macleod, Lee, et
al. J. Neurophysiol. 79:1858-1868, 1998; Binder-Macleod, Lee, Baadte,
Arch. Phys. Med. and Rehabil 78:1129-1137, 1997.
Anodal block and high-frequency fatigue are unlikely. High-freqeuncy
fatige may become an issue using a 100-pps train at very long
durations...i.e, seconds, or at much higher frequencies.
Greater responses after 5 second MVCs than after 3 second ones may indeed
be due to greater potentiation. We used to potentiate the muscle using
MVCs. If I am not mistaken, pilot work for those studies suggested that
10 s long contractions were needed to maximally potentiate the muscle.
Additionally, present work in our laboratory is investigating the time
dependant development of potentiation. Please see the following for an
interesting perspective on potentiation mechanisms. Sweeney et al., Am.
J. Physiol. 264 (cell physiol. 33):C1085-C1095, 1993.
I hope some of this information is helpful.
Sincerely,
Samuel C.K. Lee, PhD, PT
Interdiciplinary Program in Biomechanics and Movement Sciences
University of Delaware
I'm not sure what the mechanism could be. Perhaps an acute
increase in tissue compliance (due to the MVCs) occurs during
both protocols but greater potentiation in the short-rest protocol
prevents the twitch from decreasing. You could repeat the long rest
protocol at different joint angles to investigate this possibility.
Cliff Klein
Ph.D candidate
Kinesiology, University of Western Ontario
London, Ontario
Thanks to those who replied to my questions
regarding shrinking twitches. Here is the original question and the
replies;
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, remai n
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 Tony, I am not on
Biomech-L, but a colleage has forwarded your interestingquery to me. I
work with Dr. Stuart Binder-Macleod and some of yourfindings may be
consistent with findings from our laboratory. Please seeresponses
interspersed below. First, recent work suggests that the twitch
interpolation technique is notsufficient to assess maximum quadriceps
activation. Please see Kent-Braunand Le Blanc, Muscle and Nerve,
19:861-869,1996 among others.Additionally, work in our lab that is in
preparation for submissionsuggests a 13-pulse, 100pps train was the easiest
tolerated by oursubjects and best assesses completeness of activation
(typically we use600 microsecond pulses). We find that the additional pulses
are neededbecause of the time dependent rate of rise of force in response
tostimulation.Second, it does not sound that the intent of your MVC
protocols are toavoid fatiuge. Anecdotally, we find that if we want to
avoid fatigue, wewait 5 minutes between MVC attempts and limit are attempts
to 4 or 5 atthe maximum. It sounds however, that you are studing
fatigue and centralactivation?Third, the time occurance of your
attenuated twitch responses areinteresting. We find that after fatigue
is produced with a modifedBurke's fatigue protocol, recovery responses to
low-frequency trainsvs. high frequencies arethe most attenuated at the 2
minute recovery period. That is we seeinital recovery of responses,
then maximum depression at 2 minutes, andthen further recovery. Our
observations support others in that there aretwo components to LFF...i.e, a
rapidly recovering metabolite-dependentportion and a slow-recovering
component. Please see Binder-Macleod andRuss. J. Appl. Physiol.
86(4):1337-1346, 1999. and look for Russ andBinder-Macleod Muscle and
Nerve in Press, August 1999. Part of the phenomena you may
be observing may also be related to thecatchlike property of skeletal muscle
that would be elicitited by yourdoublet pulses. In fresh,
unpotentiatied muscle, and in fatigued muscle,doublet initiated trains are
highly effective in augmenting peak force andforce-time integrals when
compared to comparable consant-frequency trains.In fresh, but highly
potentiated muscle, no difference in peak force orforce-time integrals are
generally observed. Please see Bevan et al., J.Physiol., 449:85-108,
1992. You might find that the discussion sectionsof some of our work
may be interesting to you, for example, Binder-Macleodand Lee, J. Appl.
Physiol 80(6):2051-2059, 1996; Binder-Macleod, Lee, etal. J. Neurophysiol.
79:1858-1868, 1998; Binder-Macleod, Lee, Baadte,Arch. Phys. Med. and Rehabil
78:1129-1137, 1997.Anodal block and high-frequency fatigue are
unlikely. High-freqeuncyfatige may become an issue using a 100-pps
train at very longdurations...i.e, seconds, or at much higher
frequencies.Greater responses after 5 second MVCs than after 3 second
ones may indeedbe due to greater potentiation. We used to potentiate
the muscle usingMVCs. If I am not mistaken, pilot work for those
studies suggested that10 s long contractions were needed to maximally
potentiate the muscle.Additionally, present work in our laboratory is
investigating the timedependant development of potentiation. Please
see the following for aninteresting perspective on potentiation
mechanisms. Sweeney et al., Am.J. Physiol. 264 (cell physiol.
33):C1085-C1095, 1993.I hope some of this information is
helpful.Sincerely,Samuel C.K. Lee, PhD, PTInterdiciplinary Program
in Biomechanics and Movement SciencesUniversity of Delaware
I'm not sure what the mechanism could be.
Perhaps an acute increase in tissue compliance (due to the MVCs) occurs
during both protocols but greater potentiation in the short-rest protocol
prevents the twitch from decreasing. You could repeat the long rest
protocol at different joint angles to investigate this
possibility.Cliff KleinPh.D candidateKinesiology, University of
Western OntarioLondon, Ontario