Low-frequency fatigue, post-tetanic potentiation and their interaction at different muscle lengths following eccentric exercise
1 Institute for Fundamental and Clinical Human Movement Sciences, Vrije
Universiteit, Van der Boechorststraat 9, 1081 BT Amsterdam, The
Netherlands
2 Integrated Biomedical Engineering for Restoration of Human Function,
Instituut voor Biomedische Technologie, Faculteit Construerende Wetenschappen,
Universiteit Twente, Postbus 217, 7500 AE Enschede, The Netherlands
3 Institute for Biophysical and Clinical Research into Human Movement,
Manchester Metropolitan University, Crewe & Alsager Faculty, Cheshire ST7
2HL, UK
* Author for correspondence (e-mail: j.rijkelijkhuizen{at}vumc.nl)
Accepted 11 October 2004
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Summary |
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Key words: force-length characteristics, maximal stimulation, submaximal stimulation.
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Introduction |
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Up to now, LFF has been studied mainly at one particular muscle length (for
instance optimum length) to understand the mechanisms. However, there is a
prominent influence of muscle length on the stimulation frequency-force curve
with a shift to higher frequencies at shorter muscle lengths
(De Haan et al., 2003;
Rack and Westbury, 1969
;
Roszek et al., 1994
). This
means that at shorter muscle lengths, higher stimulation frequencies are
necessary to obtain the same percentage of the maximum force. Because of the
length-dependence of the stimulation frequency-force relationship, it is
conceivable that the extent of LFF differs with muscle length. Thus, the first
aim of the study was to investigate the manifestation of LFF at different
muscle lengths. Based on length-dependent differences in Ca2+
sensitivity, it was expected that the effects of LFF would be more pronounced
at shorter muscle lengths.
LFF is present after different types of exercise, but is most pronounced
after eccentric exercise (Edwards et al.,
1981; Rijkelijkhuizen et al.,
2003
). Therefore, the effects of a series of eccentric
contractions on the force-length characteristics for maximal (stimulation
frequency of 200 Hz) and submaximal (60 Hz) stimulation were investigated in
this study. Any variation of LFF with muscle length can be deduced from these
relations. Consequently, by comparing the pre-exercise 60:200 Hz force ratios
with the post-exercise values, the extent of LFF can be quantified.
Stimulation frequency-force relationships are highly affected by
potentiation, which progressively enhances force at lower stimulation
frequencies (e.g. MacIntosh and Willis,
2000). Since the submaximal forces are affected in LFF, we were
interested to find out whether PTP could counteract the effects of LFF.
Potentiation of force by previous activation is caused by increased rates of
phosphorylation of the myosin light chains (MLCs) (e.g.
Manning and Stull, 1979
;
Moore and Stull, 1984
),
leading to an increased sensitivity to Ca2+. Therefore, it might be
expected that potentiation can counteract the effects of the reduced
Ca2+ release found during LFF. Thus, the second aim of this study
was to compare LFF in non-potentiated muscles with LFF in muscles potentiated
by a previous tetanic contraction. This is particularly interesting because
muscles in vivo are likely to be active more often in a (more or
less) potentiated state than in a non-potentiated state. It was hypothesised
that PTP can (partly) compensate for LFF. Because potentiation (when expressed
as potentiated force relative to non-potentiated force) is muscle length
dependent, being higher at short muscle lengths
(Rassier and MacIntosh, 2002
;
Roszek et al., 1994
;
Wallinga-de Jonge et al.,
1980
), we investigated how the extent of LFF was expressed across
different muscle lengths both in a potentiated and a non-potentiated
condition.
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Materials and methods |
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Muscle preparation and experimental set-up
During surgery as well as during the experiment, the animal was placed
prone on a heated pad of 35°C to prevent hypothermia. The GM muscle-tendon
complex of the right leg (N=8) was dissected free of surrounding
skin, connective tissue and other muscles. This means that most, if not all,
effects of extra- and intermuscular force transmission (e.g.
Huijing, 1999) were excluded.
The muscle origin and the blood supply remained intact. The sciatic nerve was
cut as proximally as possible within the upper leg. All distal branches of
this nerve were cut except the branch innervating the GM.
The femur was clamped vertically past the edge of the heated pad and the
muscle was positioned horizontally (see also
De Haan et al., 1989a). The
distal tendon with a piece of the calcaneal bone was connected to a force
transducer. The sciatic nerve was placed on a bipolar electrode used for
stimulation. Muscle temperature was controlled by a water-saturated airflow of
33°C around the muscle. The force transducer (custom made, compliance 8
µm N-1, resolution 0.005 N) used was part of an isovelocity
measuring system. The force transducer was mounted on the lever arm of a
servomotor. Acceleration, velocity, start length, (onset of) movement, (onset
of) stimulation, stimulation frequency and duration of the muscle contractions
were computer controlled. Stimulation current was 1 mA with a pulse width of
0.05 ms for maximal stimulation of all fibres. The data (force, length and
stimulation pattern) were AD converted with a sample frequency of 1000 Hz and
stored on disc. After the experiments, the rats were killed by cervical
dislocation.
Experimental muscle length
Tetanic optimum muscle length for maximal force
(L0,200Hz) was determined using 200 ms tetani with a
stimulation frequency of 200 Hz. Other muscle lengths were expressed relative
to L0,200Hz. After lengthening or shortening the muscle to
the desired length, the measurements started with recording of passive muscle
force during 100 ms in which time the muscle-tendon complex adjusted to the
new length. The passive force measured after the contraction was subtracted
from recorded force to obtain active force.
Pre-exercise force-length characteristics, potentiation and 60:200 Hz force ratios
Following determination of L0,200Hz, force-length data
for maximal stimulation were obtained by imposing tetani at a stimulation
frequency of 200 Hz. The duration of each contraction was 200 ms, which was
sufficient to reach a force plateau. Contractions were performed at nine
lengths in random order (range: 4 mm below to 4 mm above
L0,200Hz with 1.0 mm increments). Time between
contractions was 2 min, which was enough time to avoid fatigue. After this set
of contractions, 20 min of rest (duration determined in pilot experiments) was
included to make sure that no potentiation of the previous contractions was
left. Subsequently, force-length data for submaximal stimulation were obtained
at the same nine muscle lengths but with a stimulation frequency of 60 Hz and
a pulse train of 500 ms, which time was needed to reach a force plateau at
this frequency of stimulation.
To study the effect of PTP, the muscle was potentiated with an isometric
tetanus (duration 800 ms, stimulation frequency 200 Hz). Pilot experiments
showed that such tetanic contractions yield a high level of potentiation
without significant fatigue (see also
Abbate et al., 2000).
Potentiation remained constant for more than 20 s and had vanished after 15
min. Directly after the potentiating tetanus, the force-length relationship
was determined again for a stimulation frequency of 60 Hz to study the effects
of potentiation on the (submaximal) 60 Hz force. Time between the nine
contractions was 2 s to maintain potentiation. The contractions were applied
in random order to minimise effects due to changes in potentiation and/or
fatigue. Using the measurements at stimulation frequencies of 60 Hz and 200
Hz, the 60:200 Hz force ratio was calculated at each muscle length (both in
the non-potentiated and in the potentiated condition).
Eccentric exercise
After 10 min rest, a series of 40 eccentric contractions was performed
within 14 s. Each contraction was performed with a velocity of 20 mm
s-1, lasted 70 ms and was induced by 10 stimulation pulses applied
at a frequency of 150 Hz. Pilot experiments had indicated that a stimulation
frequency of 150 Hz was sufficient to obtain maximal activation for eccentric
contractions and that a stimulation duration of 70 ms resulted in an eccentric
force that did not exceed maximal isometric force of the preparation. During
the eccentric contractions, the stimulation started simultaneously with
lengthening of the muscle (i.e. without any prior isometric phase). In these
dynamic conditions, the range of movement was from -3.5 to +0.5 mm, where 0 mm
indicates L0,200Hz. The peak force during the stretch was
reached at approximately L0,200Hz-2 mm and relaxation was
completed during the stretch phase. Fig.
1 shows typical examples of eccentric contractions as performed in
this protocol. By avoiding stretching to non-physiologically long muscle
lengths we aimed to prevent severe force loss due to the eccentric
exercise.
|
Post-exercise force-length characteristics, potentiation and 60:200 Hz force ratios
Because of possible changes in optimum length and their effects on the
60:200 Hz force ratio, L0,200Hz was determined again after
the eccentric series of contractions with a few isometric contractions at a
stimulation frequency of 200 Hz. Due to the eccentric series of contractions,
L0,200Hz had changed and therefore the muscle length was
set at the new (i.e. 1.0 mm longer) length. In the period between 40 and 60
min after the exercise, force-length data were obtained using a stimulation
frequency of 200 Hz. After an additional 20 min of rest (i.e. 80 min after the
exercise), force-length data were collected using a stimulation frequency of
60 Hz with 2 min rest in between contractions. Subsequently, the muscle was
potentiated with a potentiating tetanus similar as in the pre-fatigue
condition. Tubman et al.
(1996) showed that in in
situ rat GM muscle, MLC phosphorylation was significantly lower in
fatigued muscles than in fresh muscles after the same tetanic contraction,
while the extent of PTP was similar. Directly after the potentiating tetanus,
the force-length data were collected at a stimulation frequency of 60 Hz, with
2 s in between contractions.
To quantify LFF, 60:200 Hz force ratios were calculated at all muscle
lengths studied and compared with the pre-exercise values both in the
potentiated and the non-potentiated condition. A decrease of the 60:200 Hz
force ratio indicated the presence of LFF. LFF was quantified in a period of
40-80 min after the cessation of the fatiguing exercise because muscle
metabolites causing short-term fatigue were expected to have returned to their
pre-exercise values (De Haan et al.,
1989b) and pilot experiments had indicated that LFF had fully
developed by that time.
Statistics
All values are described as mean ± standard error of the mean
(S.E.M.). Analyses of variance (ANOVA) for
repeated measures on one or two factors (`condition' and/or muscle length)
were used to determine statistical differences in force or 60:200 Hz ratio.
The factor `condition' consisted of fur levels: pre-exercise,
pre-exercise-PTP, post-exercise, post-exercise-PTP. If significant main
effects or interaction effects were observed, Bonferroni
post-hoc tests were performed. The level of significance was
0.05.
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Results |
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Submaximal isometric force (60 Hz)
Force-length characteristics at 60 Hz (with and without potentiation) are
illustrated for the pre- and post-exercise conditions
(Fig. 3A). Note the different
shape of the force-length relationship (levelling off of force at shorter
lengths) at 60 Hz in the non-potentiated muscle post-exercise compared with
the pre-exercise condition. In the pre- and post-exercise condition, optimum
muscle length for force at 60 Hz without potentiation was approximately 2 mm
longer than at 200 Hz (L0,200Hz+2 mm). Potentiation
shifted optimum muscle length for 60 Hz force to
L0,200Hz+1 mm. Significant main effects of condition
(exercise and/or PTP) and muscle length on submaximal force were indicated, as
well as an interaction effect (all P<0.01). Post-hoc
comparison revealed that the curves differed significantly in each of the four
conditions. Hence, the effects of exercise and PTP were different for the
various muscle lengths but, in general, exercise reduced force at 60 Hz
whereas PTP raised the 60 Hz force.
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When the post-exercise values are expressed relative to the pre-exercise values (Fig. 3B), the relative force loss as a result of the series of eccentric contractions at each muscle length is visible. Significant main effects were detected for PTP (P<0.01) and for muscle length as well as a significant interaction of the effects of PTP and muscle length. At most muscle lengths (L0,200Hz-2 mm to L0,200Hz+3 mm), the force loss in the potentiated condition was much less than in the non-potentiated condition. In the non-potentiated condition, the 60 Hz force decreased most near L0,200Hz. At a muscle length of L0,200Hz-1 mm, the force at 60 Hz had decreased by 55.1±4.3%. In the potentiated state, however, the influence of muscle length on 60 Hz force was similar to the influence on maximal force (at 200 Hz) (compare Figs 3B and 2B). The force loss was most pronounced at short muscle lengths (53.5±3.8% at L0,200Hz-4 mm). Thus, length-effects were different in non-potentiated and potentiated conditions.
Post-tetanic potentiation
The effect of the potentiating tetanus (800 ms, 200 Hz) increased
submaximal force at most of the studied muscle lengths
(Fig. 3A). To evaluate the
quantitative effects of eccentric exercise on potentiation and the
force-length characteristics, we calculated the difference between the 60 Hz
force-length curve in the potentiated condition and the curve in the
non-potentiated condition (Fig.
4). Due to eccentric exercise, optimal potentiation was found at a
longer muscle length (a shift from L0,200Hz-3 mm to
L0,200Hz-1 mm). Significance was obtained for a main
effect of muscle length, as well as for the interaction of exercise and muscle
length effects (P<0.01), whereas the main effect of exercise was
not significant (P>0.05). Thus, different effects of muscle length
were found in the pre-exercise compared with the post-exercise condition. On
average, PTP had similar absolute effects pre- and post-exercise. However,
post-exercise PTP was significantly lower at short lengths but higher at long
muscle lengths than pre-exercise PTP.
|
Pre- and post-exercise 60:200 Hz force ratios
In all four conditions (pre-exercise, pre-exercise-PTP, post-exercise,
post-exercise-PTP), 60:200 Hz force ratios were calculated for all muscle
lengths studied (Fig. 5A). This
ratio was used to quantify LFF. A significant main effect of condition
(exercise and/or PTP) (P<0.01) was present. Furthermore, a
significant main effect of muscle length was found, and an interaction between
effects of muscle length and condition (all P<0.01). This means
that the effects of exercise and PTP depend on which length is considered.
Exercise decreased the 60:200 Hz force ratio whereas PTP increased the 60:200
Hz force ratio significantly at most (see below) muscle lengths. The decrease
of the ratio as a result of the eccentric exercise indicated the presence of
LFF. LFF was particularly evident in the non-potentiated condition; in the
potentiated condition, no significant main effect of exercise was found when
the pre-exercise values were compared with the post-exercise values, but
merely a significant interaction between effects of exercise and muscle
length, indicating that the effect of exercise was different for shorter
compared with longer muscle lengths.
|
Additionally, post-exercise ratios were expressed as a percentage of the pre-exercise ratios (for the non-potentiated and for the potentiated condition) (Fig. 5B). This shows the effect of PTP on the 60:200 Hz force ratios pre- and post-exercise at all muscle lengths. ANOVA indicated significant effects of PTP and muscle length as well as a significant interaction effect (all P<0.01). Thus, the deviation of 100% of the 60:200 Hz force ratio in the potentiated condition was smaller, indicating significantly less LFF, than in the non-potentiated condition at most muscle lengths. In the non-potentiated condition, LFF was most pronounced at muscle lengths near L0,200Hz; the 60:200 Hz force ratio had decreased to 54.6±5.9% of the pre-exercise ratio at L0,200Hz-1 mm. In the potentiated condition, LFF was only present at shorter muscle lengths; LFF increased with decreasing muscle lengths (the 60:200 Hz force ratio had decreased to 64.1±5.2% of the pre-exercise ratio at L0,200Hz-4 mm), indicated by the significant length effect (Fig. 5B).
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Discussion |
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Several aspects may underlie changes in muscle function following eccentric
exercise: First, muscle damage (A): for instance, regions of lengthened
sarcomeres associated with Z-line streaming are mentioned as a result of
eccentric exercise (Armstrong et al.,
1983; Friden et al.,
1983
). It has been suggested that the Z-lines may be disrupted due
to eccentric contractions, leading to affected muscle fibres
(Friden et al., 1983
). Second,
an impairment of the excitation-contraction coupling resulting in reduced
Ca2+ release (B) is associated with eccentric exercise but is also
marked as the main factor causing LFF
(Edwards et al., 1977
;
Hill et al., 2001
;
Westerblad et al., 1993
).
Third, deformed series-elastic elements, such as deformed myotendinous
attachments or an increased compliance of the tendon (C;
Jones et al., 1989
;
Lieber et al., 1991
), might
play a role in the changed muscle function after eccentric exercise. Damaged
myotendinous attachments are associated with muscle damage (A) and will
therefore not be discussed separately. An increased compliance of the tendon
may lead to changes in the stimulation frequency-force relationship
(Jones et al., 1989
) and may
therefore be a reason for the force losses found in the present study. The
results of the present study will be discussed below, taking these three
aspects into account.
Reduction of force at maximal stimulation in relation to muscle length
The series of eccentric contractions in the present study resulted in a
decrease in force at maximal as well as at submaximal stimulation with
significant effects of muscle length. Force at maximal stimulation was
affected less near L0,200Hz than at shorter and longer
muscle lengths (Fig. 2). It is
possible that the force loss at maximal stimulation is caused mainly by
mechanical damage (A) possibly in combination with a more compliant tendon (C)
and not by reduced tetanic Ca2+ concentrations (B), since reduced
Ca2+ levels are known to result in large changes in force at low
stimulation frequencies but only in small changes in force at maximal
stimulation (Westerblad et al.,
1993).
The length-dependence of the force loss may be related to a changed serial
distribution of sarcomere lengths caused by muscle fibre damage (A). It has
been suggested that eccentric exercise may result in regions of non-functional
lengthened sarcomeres (Armstrong et al.,
1983; Friden et al.,
1983
), which may have two consequences. First, upon activation of
the muscle-tendon complex, the functional sarcomeres need to shorten
relatively more to stretch the series-elastic components. As a result, the
optimum length will shift to a longer length. After the eccentric contractions
in our experiment, we observed a shift of L0,200Hz to a 1
mm longer length, for which we have corrected. Second, the functional fibre
length will be shorter, which may lead to relatively more force loss at the
longest and shortest muscle lengths studied, as we observed
(Fig. 2B).
Reduction of submaximal force in relation to muscle length
Force loss at maximal stimulation was minimal near
L0,200Hz. By contrast, force loss at submaximal
stimulation in the non-potentiated condition was maximal near
L0,200Hz. Since the characteristic of LFF is a greater
force loss at lower frequencies than at higher frequencies, and the cause is
found in a disturbance of the E-C coupling
(Edwards et al., 1977), it
could be expected that this decrease in submaximal force is the result of a
disturbance in E-C coupling (B) leading to a decreased tetanic Ca2+
concentration, possibly in combination with muscle damage (A) and a more
compliant tendon (C). In the potentiated condition, the loss of submaximal
force was much less, and the length effects were similar as for force
production at maximal stimulation: minimal force loss was found near
L0,200Hz. Since PTP is thought to have a greater effect in
situations of lower Ca2+ concentrations, the effects of a
disturbance of E-C coupling by low Ca2+ concentrations (B) should
be smaller following PTP. Mechanical damage (A) possibly in combination with a
more compliant tendon (C) might be the main factors causing the submaximal
force loss in the potentiated condition.
Low-frequency fatigue in relation to muscle length
The manifestation of LFF at different muscle lengths was deduced from the
force-length relationships at maximal and submaximal stimulation. A decrease
of the 60:200 Hz ratio as a result of the eccentric exercise indicated the
presence of LFF. A decrease of this ratio was present mainly in the
non-potentiated condition; significantly less LFF occurred in the potentiated
condition. In both the non-potentiated and the potentiated condition, no LFF
was present at longer muscle lengths and LFF increased with decreasing muscle
length. This may be related to a reduced Ca2+ release (B) in
combination with length-dependent Ca2+ sensitivity. A lower
Ca2+ release can result in a greater loss of submaximal force at
shorter muscle lengths because the Ca2+ sensitivity of muscle is
lower at those lengths (e.g. Stienen et
al., 1985). However, in non-potentiated muscle, LFF was less at
the shortest muscle lengths studied (Fig.
5B). This is the result of the different shape of the force-length
relationship (levelling off of force at shorter lengths) at 60 Hz in the
non-potentiated muscle post-exercise (Fig.
3A) compared with the pre-exercise condition. The levelling off of
force at shorter lengths in a force-length relationship is probably caused by
a distribution of sarcomere lengths, which is always present in a muscle
(Huijing, 1996
). Therefore,
force-length relationships in general show a levelling off when approaching
low force levels at short lengths. In rat GM muscle, the different
distribution of sarcomere lengths may be intensified by a distribution of
optimum lengths of the proximal and distal fibres. The proximal muscle part of
the rat GM consists predominantly of fast oxidative fibres, the distal part of
fast glycolytic fibres (De Ruiter et al.,
1995
). In addition, it is known that optimum length of the fast
glycolytic part is about 1.5 mm higher than of the fast oxidative part
(De Ruiter et al., 1995
).
Therefore, at short muscle lengths the fast oxidative fibres are at higher
relative length than the fast glycolytic fibres and will contribute more to
the total force. Additionally, fast oxidative fibres are less susceptible to
LFF (Rijkelijkhuizen et al.,
2003
) and will therefore be able to exert relatively more force
than fast glycolytic fibres in the presence of LFF.
In the potentiated condition, a more linear relationship between the extent of LFF and muscle length was found, with increasing LFF occurring with decreasing muscle lengths. PTP reduced LFF significantly and the effects on maximal and potentiated submaximal force showed the same length dependence. It should be noted that the absence of LFF at certain muscle lengths does not imply that there were no effects of the eccentric exercise but merely that the force loss at maximal and submaximal stimulation was similar at those muscle lengths, resulting in an unchanged 60:200 Hz force ratio.
Counteracting effects of LFF and PTP
Potentiation led to a reduction in LFF over a wide range of muscle lengths
(Fig. 5). Potentiation of force
by previous activation is caused by increased levels of phosphorylation of
MLCs (e.g. Manning and Stull,
1979; Moore and Stull,
1984
). It has been proposed that MLC phosphorylation causes
individual myosin heads to swing out from the myosin back-bone
(Sweeney et al., 1993
),
thereby bringing the actin binding site of the myosin head in close proximity
to the actin filament. This is thought to permit a faster rate of engagement
of cross-bridges on activation. A faster rate of engagement of cross-bridges,
with no change in rate of dissociation will result in more cross-bridges in
the force-generating state during contraction at a given level of activation
(Ca2+ bound to troponin). Thus, potentiation could be explained by
an increased fraction of cross-bridges in the force-generating state
(Sweeney and Stull, 1990
) or a
prolonged force-generating state of the cross-bridges
(Patel et al., 1998
).
Potentiation probably decreases LFF after eccentric exercise by increasing
Ca2+ sensitivity, which would counteract the effects of the reduced
Ca2+ release (B). Therefore, when muscles are potentiated, it may
seem as if no LFF is present. However, the reduced Ca2+ release may
still be present in the muscle but the effect (LFF) may be counteracted by an
increased Ca2+ sensitivity induced by potentiation.
The increased Ca2+ sensitivity caused by PTP may be length
dependent. For the descending limb of the force-length relation, the present
study found a curve with a negative slope relating PTP and muscle length
(Fig. 4). In the pre-exercise
condition, the negative slope was present at muscle lengths of
L0,200Hz-3 mm and longer. However, on the ascending limb
of the force-length relationship in the post-exercise condition, the
relationship between PTP and muscle length showed a positive slope, indicating
less PTP at shorter muscle lengths. It has been suggested that the mechanism
for the length dependence of potentiation may be related to length dependence
of activation (Rassier, 2000).
At long muscle lengths, the affinity among the actin binding site and the
myosin head is higher because the interfilament spacing is small. This results
in a higher rate of attachment while the rate of detachment is not affected.
Thus, stretching the muscle as well as MLC phosphorylation during repetitive
stimulation causes an increase in Ca2+ sensitivity. Therefore,
muscles that are active at a longer length are already `potentiated' due to a
stretch-induced increase in Ca2+ sensitivity, and the effects of
MLC phosphorylation are likely to be smaller. However, in the present study,
at the shortest muscle lengths studied in the post-exercise condition,
potentiation was less than at the muscle lengths just below
L0,200Hz. This deviation may be related to the earlier
mentioned levelling off of the force-length curve at 60 Hz in the
post-exercise condition.
Thus, the increasing Ca2+ sensitivity caused by PTP seems to be length-dependent with a small effect at long length where interfilament spacing is small, an optimal effect at lengths near optimum length, and a decreasing effect at shorter lengths where interfilament spacing is high. The present study showed that the extent of LFF increased with decreasing muscle length when muscles were potentiated, indicating that the length effect of the submaximal force loss as a consequence of the reduced Ca2+ release may be strong(er) than the increase of submaximal force as a result of the increase in Ca2+ sensitivity induced by a potentiating tetanus.
The functional relevance of LFF for in vivo functioning muscles
When LFF is present, force is considerably decreased at submaximal
stimulation frequencies and recovery of this phenomenon can take up to 24 h
(Edwards et al., 1977). During
in vivo action, motoneurones usually fire at relatively low (for rat
muscle: <60 Hz) frequencies (Hennig and
Lømo, 1987
). Therefore, LFF may have large effects on
in vivo performance and important consequences for muscle control.
The central nervous system has to increase muscle activation to prevent a loss
of force output. The present study showed that PTP counteracts LFF mainly at
long muscle lengths, probably by increasing the sensitivity for
Ca2+, and therefore compensating for the effect of the reduced
Ca2+ release, which is causing LFF. Since muscles are potentiated
quickly during repeated activity, the functional significance of LFF in a
potentiated muscle may be most relevant at shorter muscle lengths during
in vivo activities. In conclusion, this study showed that the
manifestation of LFF as a result of a series of eccentric contractions varied
when measured at different muscle lengths. PTP fully counteracted the effects
of eccentric exercise at long muscle lengths but LFF was still observed at the
shortest muscle lengths studied.
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