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J Appl Physiol 98: 1243-1250, 2005. First published November 19, 2004; doi:10.1152/japplphysiol.00735.2004
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Postactivation potentiation influences differently the nonlinear summation of contractions in young and elderly adults

Stéphane Baudry, Malgorzata Klass, and Jacques Duchateau

Laboratory of Applied Biology, Université Libre de Bruxelles, Brussels, Belgium

Submitted 14 July 2004 ; accepted in final form 15 November 2004


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The force enhancement of a twitch after a maximal conditioning muscle contraction [i.e., postactivation potentiation (PAP)] is reduced with aging, but its influence on the summation of force in response to repetitive stimulation at different frequencies is not known. The purpose of this work was to compare the electrically evoked mechanical responses of the tibialis anterior muscle between young and elderly adults after a 6-s maximal voluntary contraction (MVC). The results showed that, immediately after the conditioning MVC, twitch torque and its maximal rate of development and relaxation were significantly enhanced in both groups, but the magnitude of potentiation was greater in young (148.0 ± 14.2, 123.7 ± 16.5, and 185.4 ± 36.5%, respectively) compared with elderly adults (87.4 ± 15.2, 63.8 ± 9.9, and 62.9 ± 11.0%, respectively). This age-related difference in potentiation of the twitch disappeared completely 1 min after the conditioning MVC. The potentiation of torque and speed-related parameters in response to two- and three-pulse trains, delivered at a constant interval of 10 ms (100 Hz), was less than for a single pulse for both groups. In young adults, the magnitude of PAP on the successive individual mechanical contributions within a train of stimuli declined progressively such that the third contribution did not differ significantly from the same contribution before the conditioning MVC. In contrast, the second and third contributions did not potentiate (P > 0.05) in elderly adults. Although these contributions did potentiate significantly at a lower frequency of stimulation (20 Hz) in the two groups, the difference in PAP between young and elderly adults still persisted. This overall attenuation of potentiation with aging, however, appears to have a moderate influence on the decrement of the muscular performance.

skeletal muscle; contractile properties; electrical stimulation

THE TORQUE OF AN ELECTRICALLY evoked muscle twitch (Pt) and its maximal rate of development (+dPt/dt) and relaxation (–dPt/dt) are transiently increased following a brief maximal voluntary contraction (MVC) (3, 15, 30, 37, 43). This phenomenon, called postactivation potentiation (PAP), is maximal immediately after the conditioning contraction and declines exponentially over time to disappear completely within 5–10 min of rest (1, 30). The most common explanation for PAP is phosphorylation of myosin regulatory light chains during the conditioning contraction. This mechanism increases the sensitivity of the contractile proteins to activation by the ionized calcium (Ca2+) that is released by the sarcoplasmic reticulum and thereby enhances the force of the twitch (14, 39). This potentiating effect is, however, not constant for the individual mechanical responses within a train of stimuli at high frequency (1, 10). Potentiation is maximal for the first evoked response within a train, and its magnitude declines progressively during the train. This observation suggested that a saturation process limits the extent of potentiation during repetitive activation of a muscle (1, 10, 32).

Aging is associated with a progressive loss of voluntary strength and slowing of the muscle contractile kinetics (42). The latter adaptation is due not only to a greater atrophy of type II fibers compared with type I fibers (22) but also to a general slowing of the contractile properties of both fast and slow muscle fibers (6). Some studies reported that the magnitude of twitch potentiation is less in elderly subjects compared with young subjects (16, 34, 42), although the importance of this difference on the summation of contractions is not known. This reduced capacity of potentiation with aging has been related to a selective atrophy of fast (type II) muscle fibers. It is indeed known that the type II fibers have a greater capacity to potentiate force compared with type I fibers (15, 39). In addition, the magnitude of PAP is negatively correlated with the preconditioning twitch torque-to-MVC ratio (15), a ratio that is usually lower in type II than in type I fibers (5, 15). Consequently, the shift toward an overall slowing of the muscle contractile properties in part explains the reduced capacity of potentiation in elderly adults.

Although the decrease in PAP magnitude with age is well documented (16, 34, 42), it is unclear how this adaptation influences the summation of mechanical responses to repetitive muscle activation and, consequently, the rate at which the force is developed (1, 9, 23, 32). This parameter is of importance, as the ability to exert a rapid rise in muscle force may reduce the incidence of falls due to impaired control of postural balance with increasing age. Therefore, this study was designed to compare in the tibialis anterior (TA) of young and elderly adults: 1) the extent of PAP on the twitch and its decay over time; and 2) the effect of PAP on the nonlinear summation of torque and its rate of development in response to repetitive muscle activation at various stimulation frequencies.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Ten young subjects (5 women and 5 men), aged between 23 and 47 yr (mean ± SD: 31.8 ± 3.1 yr), and 10 elderly adults (4 women and 6 men), aged between 70 and 85 yr (79.2 ± 1.7 yr), volunteered to participate in this study. They were well accustomed to the experimental procedure. None of the subjects presented signs of neurological disorders. Each subject gave informed consent before participation in this study. The experimental procedures were approved by the local Ethics Committee and were performed in accordance with the Helsinki Declaration.

Mechanical and electromyogram recordings.   Each subject sat on an adjustable chair in a slightly reclined position with the right foot strapped to a footplate of an ankle ergometer, as described previously (1). The plate was inclined at an angle of 45° to the floor, and the seat was adjusted so that the ankle joint was at 90°. The foot was held in place by a heel block and secured to the plate via two straps: one strap was placed around the foot 1–2 cm proximal to the metatarsophalangeal joint of the toe and another around the ankle. The isometric torque of either the dorsiflexor muscles during MVCs or the TA in response to electrical stimulation was recorded by a strain gauge transducer (sensitivity: 0.018 V/N·m; linear range: 0–200 N·m).

The surface electromyogram (EMG) was recorded from the TA by means of two silver disk electrodes (8 mm in diameter). The first electrode was placed over the muscle belly, at the proximal one-third of the distance between the neck of the fibula and the lateral malleolus. The second electrode was positioned 2 cm below the first one. The ground electrode (2 x 3-cm silver plate) was attached between the electrode of stimulation and the EMG recording electrodes. The EMG signals were amplified (x1,000) and filtered (10 Hz–5 kHz) by a custom-made differential amplifier. The torque and the EMG signals were simultaneously recorded on a computer at a sampling rate of 2 kHz and analyzed offline by using the AcqKnowledge data analysis software (model MP150; Biopac System, Santa Barbara, CA).

Stimulation procedure.   The TA was stimulated by rectangular electrical pulses (0.2 ms in duration) delivered through two silver disk electrodes (8 mm in diameter), with the cathode attached to the skin over the common peroneal nerve at the neck of the fibula and the anode fastened over the medial part of the tibia head. A digital timer (model 4030; Digitimer, Welwyn Garden City, UK) was used to trigger the custom-made stimulator that sent the stimulus to the peroneal nerve. Maximal electrical stimulation was determined by progressively increasing the intensity until the compound muscle action potential (M wave) and the corresponding torque reached a plateau. The level of stimulation was then set 10–20% above maximum. Care was taken to avoid activation of the peroneal muscles, and the absence of muscle activity was checked by palpation. The degree of muscle activation during voluntary contraction was assessed by the twitch-interpolation method (2). In this study, we stimulated the peroneal nerve with paired supramaximal pulses at 10-ms interval (100 Hz).

Experimental procedure.   Before the subjects performed the conditioning MVC, the responses to three single stimuli, to three two-pulse trains (PT2), and to three three-pulse trains (PT3), delivered with a constant interpulse interval of 10 ms (100 Hz), were recorded. Thereafter, the subject performed a 6-s MVC. Its duration was based on previous studies showing that maximum PAP occurs for maximal contractions of 5- to 10-s duration (30, 43). The post-MVC stimulation protocol consisted of one single stimulus, and one PT2 and PT3, each separated by 2–3 s, and delivered in the following sequence: 5 s after the MVC, every minute during the first 10 min, and every 5 min between the 10th and 20th min.

To assess the influence of the stimulation frequency on the magnitude of muscle potentiation, five young and five elderly adults took part in a second series of experiments. The general protocol was similar to the first series of experiments, but various interpulse intervals were tested. The frequencies used (100, 50, and 20 Hz) were chosen to cover the range of motor unit discharge rates during fast and slow MVCs (23).

Measurements.   The average torque value during the conditioning MVC was measured during 1 s at the contraction plateau before the paired supramaximal pulses were delivered. The size of the superimposed twitch response was expressed as a percentage of the one obtained in the resting muscle. This percentage was then subtracted from 100% to provide a quantitative measure of central activation. Peak torque of the single (Pt), double (PT2), or triple (PT3) responses and the corresponding contraction time (CT) and one-half relaxation time (RT1/2) from each contraction were measured. Maximal rate of torque development and relaxation were obtained from the first derivative of the force signal. The mechanical response to the second stimulation (C2) of PT2 was obtained by subtracting the response to the single stimulation (Pt or C1) from PT2. The third mechanical response (C3) was obtained by subtracting the response to PT2 from PT3. The extent of PAP was determined by computing the ratio between the size of the different mechanical responses recorded before and after the conditioning MVC. The peak-to-peak amplitude and peak-to-peak duration of the M wave were measured from the EMG signal.

Statistics.   The changes in the electrical and mechanical parameters before, immediately after, and during the 20 min of recovery were analyzed by means of a two-way ANOVA with repeated measures on one factor (time). When a significant main effect was found, Dunnett's post hoc test was used to identify the significant differences among the selected means. The comparisons of the electrical and mechanical parameters before and after the conditioning MVC from the different frequency trains were compared by ANOVA with repeated measures and Newman-Keuls post hoc test. P < 0.05 was considered significant. Otherwise specified, data are given as means ± SE.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
MVC and twitch characteristics in unpotentiated conditions.   The MVC torque was significantly (P < 0.05) greater in young adults (39.4 ± 2.4 N·m) compared with elderly adults (30.5 ± 2.8 N·m), whereas muscle activation, which was assessed by the interpolated twitch method, was maximal for the dorsiflexor muscles in all subjects of both groups. Before the conditioning MVC, Pt was greater (P < 0.001) for the elderly (2.4 ± 0.3 N·m) compared with the young adults (1.9 ± 0.3 N·m). The Pt-to-MVC ratio was, therefore, significantly (P < 0.05) lower for the young (0.04 ± 0.01) compared with the elderly adults (0.07 ± 0.01). The twitch CT and RT1/2 were significantly longer (P < 0.01) for the elderly (109.2 ± 3.8 and 82.2 ± 6 ms, respectively) compared with the young adults (63.8 ± 2.8 and 65.6 ± 2.5 ms, respectively; Table 1). Furthermore, elderly subjects had slower +dPt/dt compared with the young adults (P < 0.05). Except for the absolute peak torque, the pattern of these observations was similar for PT2 and PT3, but the effect on the contractile kinetics was more pronounced (Table 1). These mechanical adaptations were associated with a lower amplitude (P < 0.05) and longer duration of the M wave (P < 0.01) for the elderly (2.7 ± 0.8 mV and 5.1 ± 0.3 ms, respectively) compared with the young adults (4.9 ± 2.5 mV and 3.6 ± 0.2 ms, respectively).


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  		Table 1. Comparison of the mechanical responses of the tibialis anterior to single-, two-, and three-pulse trains, and contribution of the second and third responses before and immediately after a 6-s conditioning MVC in young and elderly subjects

 
Magnitude of twitch potentiation and time course decay.   Immediately after the MVC, the peak torque and the rate of torque development and relaxation of the different mechanical responses were significantly greater than the control values for both groups (Fig. 1), except for PT3 in the elderly subjects (Table 1). These changes occurred with no change in M-wave amplitude (P > 0.05). The magnitude of PAP, however, differed for the young and elderly subjects (Fig. 2). The potentiation of the twitch torque and the rate of torque development and relaxation were greater for young adults (148.0 ± 14.2, 123.7 ± 16.5, and 185.4 ± 36.5%, respectively) compared with elderly adults (87.4 ± 15.2, 63.8 ± 9.9, and 62.9 ± 11.0%, respectively) immediately after the conditioning MVC. In contrast, the twitch time course (CT and RT1/2) was not significantly altered by the conditioning MVC (Table 1). When the magnitude of Pt potentiation was plotted as a function of Pt/MVC in young and elderly subjects, the experimental data points were best fitted with a negative linear function (r = 0.60, P < 0.01; Fig. 3A). Similarly, Pt potentiation was also negatively correlated (r = 0.63, P < 0.01) with the twitch CT for the pre-MVC conditions (Fig. 3B).



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  		Fig. 1. Torque signals (top traces) and their corresponding first derivatives (bottom traces) in response to single-pulse (1) and two- (2) and three-pulse trains (3), in a young and an elderly adult. Traces recorded before and 5 s after a 6-s conditioning maximal voluntary contraction (MVC) are superimposed, and extents of postactivation potentiation (PAP) are indicated.

 


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  		Fig. 2. Time course of PAP decay for peak twitch torque (Pt; A), maximal rate of development (+dPt/dt; B), and maximal rate of relaxation (–dPt/dt; C) after a 6-s conditioning MVC in young (Y; {circ}) and elderly (E; {bullet}) adults. All data are expressed as means ± SE. *Significant difference between groups, P < 0.05.

 


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  		Fig. 3. A: relationship (y = –1,309.2x + 300.2; r = 0.60, P < 0.01) between the extent of twitch potentiation (% of preconditioning MVC) and Pt-to-MVC ratio for young ({circ}) and elderly ({bullet}) adults. B: relationship (y = –1.317x + 332.5; r = 0.63, P < 0.01) between the extent of twitch potentiation and the twitch contraction time for young and elderly adults.

 
After the conditioning MVC, the potentiation of Pt, +dPt/dt, and –dPt/dt exhibited an exponential decrease over time for the young and elderly adults (Fig. 2). The time constants for Pt, +dPt/dt, and –dPt/dt were 0.7, 1.0, and 0.8 min for young and 1.0, 2.5, and 1.6 min for elderly adults, respectively. For both groups, Pt returned to control values after 5 min, and +dPt/dt and –dPt/dt recovered their control values between 4 and 5 min (Fig. 2). Furthermore, the difference in PAP between the young and elderly subjects disappeared completely 1 min after the conditioning MVC.

Nonlinear summation of contractions in unpotentiated conditions.   The mechanical contributions during stimulation (100 Hz) for the PT2 and PT3 did not sum linearly (Fig. 4). In control conditions, the torque of each mechanical contribution and its +dPt/dt and –dPt/dt were larger (P < 0.01) for C2 compared with C1 in the young and elderly adults (Table 1). The comparison of C1 and C3 was not statistically different for elderly adults, whereas the peak torque of C3 and the rate of torque development and relaxation were greater (P < 0.01) than C1 for the young subjects. In contrast, there was no statistical difference between C2 and C3 for the young adults, whereas C3 was lower (P < 0.01) compared with C2 for elderly adults (Table 1). In addition, CT and RT1/2 displayed an overall increase in the successive contributions, with a similar enhancement in the two groups.



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  		Fig. 4. Torque signals (top traces) and their first derivatives (bottom traces) in response to a single pulse (C1) and the contribution of the second response (C2) in a two-pulse train and third contribution (C3) in a three-pulse train in a young and an elderly adult. Traces recorded before and 5 s after a 6-s conditioning MVC are superimposed, and extents of PAP are indicated.

 
Effect of PAP on the nonlinear summation of contractions.   The potentiation of the torque and the rate of torque development and relaxation in response to PT2 and PT3 were less compared with these parameters for the twitch in the young and elderly adults 5 s after the conditioning MVC (Fig. 1 and Table 1). These results were more pronounced for elderly adults compared with the young adults and were due to a progressive reduction in the contribution of potentiation to each successive electrical stimulus (Fig. 4). Opposite behavior was found, however, for C2 in the two groups; it potentiated by 18.2 ± 7.4% (P < 0.01) in young adults but decreased by 9.5 ± 5.0% in the elderly adults (Fig. 5). The torque of C3 did not potentiate immediately after the conditioning MVC for young or for elderly adults. However, 1 min after the MVC, it increased in both groups but reached statistical significance only for young adults (Fig. 5). Although quantitatively different, the maximal rate of torque development and relaxation showed similar trends (Table 1; Fig. 5). Furthermore, CT and RT1/2 of C2 were respectively increased and decreased for both groups, whereas these changes only occurred in young adults for C3. As shown by Fig. 5, the time course of PAP decay for C2 and C3 responses differed from those for C1. For the latter, maximal potentiation was reached immediately after the conditioning MVC (Fig. 2), whereas it occurred 1 min later for C2 and C3 responses.



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  		Fig. 5. Time course of PAP decay for torque (top panels), maximal rate of torque development (+dtorque/dt; middle panels), and maximal rate of torque relaxation (–dtorque/dt; bottom panels) for C2 and C3 after a 6-s conditioning MVC in young ({circ}) and elderly ({bullet}) adults. All data expressed as percentage of preconditioning values are means ± SE. Significant difference *from pre-MVC values and {dagger}between groups: P < 0.05.

 
Muscle potentiation at various frequencies of stimulation trains.   The mechanical responses to PT2 and PT3 decreased with the increase of the interpulse interval for the two groups, because the extent of torque summation was less at lower frequencies of stimulation (Table 2). In young subjects, the torque produced in response to PT2 was 88.9% (50 Hz) and 73.6% (20 Hz) of that recorded at 100 Hz. A similar torque level (91.4 and 75.7% for 50 and 20 Hz, respectively) was observed for the elderly subjects (Table 2). The differences in the magnitude of potentiation between groups observed for 100-Hz stimulation trains (see above) were still present at lower stimulation frequencies. However, as shown in Table 2, the reduction of the stimulation frequency increased the magnitude of C2 and C3 potentiation in both young and elderly subjects.


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  		Table 2. Effect of stimulation frequency on the extent of PAP for twitch, two- and three-pulse trains at various frequency, and the second and third contribution within PT2 and PT3, respectively, after a 6-s conditioning MVC in young and elderly subjects

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The present study confirms previous investigations (16, 34, 42) reporting that the magnitude of twitch potentiation is reduced with aging. Our results extend this finding by showing that the difference in PAP between young and elderly adults occurred immediately after the conditioning MVC, but this age-related difference disappeared completely 1 min later. Another novel observation of the present study is that the magnitude of potentiation within a short train of stimuli at a brief interpulse interval (10 ms) differed between the two groups. In young adults, the maximal potentiation of C1 and C2 occurred immediately after the conditioning MVC, whereas, for C3, it was observed 1 min later. In contrast, C2 and C3 were not potentiated in elderly subjects over the range of time investigated in this study. Because the difference in potentiation between young and elderly adults persisted, regardless of the stimulation frequency, but the potentiation of C2 and C3 increased at a lower frequency (20 Hz) for both groups, these observations indicate that a saturation process limits the magnitude of potentiation for the subsequent responses to a train of stimuli. At high frequency of stimulation (100 Hz), this attenuation occurred earlier in the train for elderly compared with young adults.

Muscle electromechanical properties in young and elderly subjects.   Our study showed a reduction in maximal voluntary strength without change in muscle activation with aging, and this is consistent with previous papers (8, 42, 44). Thus the age-related reduction in MVC torque of the elderly subjects is most likely due to a loss in muscle mass (e.g., sarcopenia; Refs. 13, 22, 35, 42). In addition, this study showed the age-related slowing of the contractile properties (19, 20, 42, 44), which is usually attributed to a general slowing of the contractile properties of the muscle fibers (6), greater atrophy of type II fibers compared with type I fibers (18, 22), alterations in excitation-contraction (E-C) coupling (33, 36), and a reduction in tendon stiffness (28). These speed-related changes are consistent with a greater Pt/MVC in elderly compared with young subjects. This ratio, which is usually lower in type II than in type I fibers (5) or in muscles with a great percentage of type II fibers (15), can be used as a rough estimate of the fiber-type distribution of a muscle. The relatively low Pt/MVC observed in the present work, compared with studies using the twitch-to-tetanus ratio (30), can be explained by an overestimation of the force recorded during the MVC due to the contribution of synergist muscles. These age-related adaptations of the contractile properties were accompanied by a decrease in M-wave amplitude (20, 42) that is attributed to an overall decline in the size of motor unit action potential (26), possibly due to alterations in Na+-K+ transport system through the cell membrane (21). Changes in the properties of the subcutaneous layers between the muscle and the recording electrodes (12), as well as a reduced synchronization between fiber action potentials due to a slowing of neuromuscular conduction (36, 38), may have also contributed to the decline in M-wave amplitude in elderly adults. The limited increase in M-wave duration with aging indicates, however, that the latter parameter does not play a major role.

Nonlinear summation of contractions in young and elderly subjects.   In both groups, the mechanical responses to trains of stimuli (PT2 and PT3) were significantly greater than the twitch (Tables 1 and 2; see also Ref. 23). When individual mechanical contributions in response to each stimulus were analyzed, the contractions due to repetitive stimulation did not sum linearly either in the young (1, 10, 24, 32) or in elderly adults. Before the conditioning MVC, the torque of C1 was significantly lower (P < 0.001) than those of C2 and C3 in young adults for high stimulation frequency (100 Hz), whereas no significant difference was observed between C1 and C3 in the elderly group (Table 1). A similar tendency was obtained for 50- and 20-Hz stimulation frequencies (Table 2). The greater torque produced by C2 and C3 compared with C1 is usually attributed to increased muscles stiffness (32) and greater release of Ca2+ from the sarcoplasmic reticulum during the successive stimuli (11, 32). The observation that C1 and C3 did not differ significantly in the elderly group suggests some adaptations in one or both of these mechanisms. Although the stiffness of the active fraction (cross bridges; myofibrils) of the series elastic component (SEC) of the muscle-tendon unit was found to increase with aging (29, 40), it has also been reported that the stiffness of the passive fraction of the SEC (tendon) is reduced (28). In addition to a slowing of cross-bridge cycling (17), a decrease in tendon stiffness could have contributed to the slowing of the rate of twitch torque development with aging. However, this adaptation should have produced its greatest effect on the first mechanical response of the train of stimuli compared with the subsequent responses, because the first response takes up most of the slack in the muscle SEC. As a consequence, the reduced potentiation of C2 and C3 (see below) is most likely not attributed to the differences in SEC but due to differences in the E-C coupling (33).

Effect of PAP on the nonlinear summation of contractions in young and elderly subjects.   The twitch torque potentiation was lower for the elderly adults compared with young adults when measured immediately after the conditioning MVC, and this is consistent with previous reports (16, 34, 42). Although less pronounced for C3, the torque of C2 and its rate of relaxation were more potentiated in young compared with elderly adults. This lower potentiation capacity in elderly has been attributed to a change in muscle fiber composition toward a slower fiber type (18, 22); PAP is greater in type II fibers (39) and in muscles composed of a high percentage of type II fibers (15, 37). These findings are indirectly supported by our observation of a negative correlation between the magnitude of PAP and the preconditioning twitch CT (r = 0.63, P < 0.01) or Pt/MVC (r = 0.60, P < 0.01). However, a novel finding from the present work is that the age-associated reduction in potentiation occurs immediately after the conditioning MVC but that, thereafter, the time course of PAP decay is very similar for elderly and young adults.

Possible mechanisms for PAP.   In the absence of change in muscle architecture after the conditioning contraction (24), the most likely mechanism for PAP is an increased Ca2+ sensitivity of the contractile proteins, because PAP is correlated with the phosphorylation of myosin light chains (25, 39). However, a saturation process (41) could explain the lower potentiation of C2 compared with C1 and the absence of potentiation in C3 immediately after the conditioning contraction at high stimulation frequency (50–100 Hz). This explanation is consistent with the time course of PAP in young adults because, 1 min after the conditioning MVC, torque potentiation was reduced (–98%) for C1, unchanged for C2, but increased (+25%) for C3 (Figs. 2 and 4). Furthermore, when a lower stimulation frequency (20 Hz) was used, C3 was also potentiated (Table 2). The ceiling effect of the mechanical responses observed during PAP in both groups, which occurs earlier in the train for the elderly, suggests a saturation process that limits the magnitude of muscle potentiation. This ceiling effect is also observed for the catchlike property (9) and may be due to saturation of either the cytosolic Ca2+ concentration ([Ca2+]c) or actin-myosin binding. It is also possible that a change in actin-myosin sensitivity to Ca2+ that is produced by the conditioning MVC could contribute to this ceiling process, because its effect is reduced at high [Ca2+]c (27, 31). This suggestion is consistent with the concept that the mechanisms for PAP and the nonlinear summation of contraction are not completely independent (4). The saturation in the summation of the successive contractions within the train of stimuli occurred earlier for the elderly adults, and, therefore, this suggests an alteration in E-C coupling with aging (33, 36). First, during repetitive stimulation, it was shown that the amount of Ca2+ release is reduced when [Ca2+]c is increased (11, 32). Second, Decostre and coworkers (7) observed an increase of [Ca2+]c after a brief tetanus (1 s) at a time at which twitch potentiation is maximal. Thus the reduced potentiation of C2 and C3 at high stimulation frequencies could be explained by the enhancement of [Ca2+]c following the first stimulus due to the conditioning MVC. Our observation that the ceiling effect is reached later during the train when the stimulation frequency decreased is consistent with this explanation. Moreover, the absence of potentiation of C2 and C3 at high stimulation frequencies in elderly adults could be related to a slowing of Ca2+ re-uptake by the sarcoplasmic reticulum (19, 21), which leads to increased [Ca2+]c and, consequently, reduces the Ca2+ release by the subsequent stimuli. It is, however, not excluded that elderly adults have a greater level of myosin light chain phosporylation in the resting state than do young adults, a condition that should induce a more rapid rise to saturating Ca2+.

In conclusion, the results of this study indicate that aging alters the potentiation capacity of the muscle. Because the individual mechanical contributions to a short train of stimuli were less influenced with repeated stimulations, it is suggested that a saturation process limits the potentiation of the successive responses. Although this mechanism occurs earlier in the train for elderly compared with young adults, the reduced potentiation with aging appears to have a moderate influence on the decrement of the muscular performance.


   GRANTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This study was supported by the European Community (Grant QLK6-CT-2001–00323) and the Fonds National de la Recherche Scientifique of Belgium.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
The authors are particularly grateful to Prof. K. Hainaut and Dr. S. Hunter for critical reading of this paper and to A. Deisser for assistance in the preparation of the manuscript.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. Duchateau, Laboratory of Applied Biology, Université Libre de Bruxelles, 28 Ave. P. Héger, CP 168, 1000 Brussels, Belgium (E-mail: jduchat{at}ulb.ac.be)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

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