Effects of high-frequency initial pulses and posttetanic potentiation on power output of skeletal muscle

J. Appl. Physiol. 88: 35 40, 2000. Effects of high-frequency initial pulses and posttetanic potentiation on power output of skeletal muscle F. ABBATE, 1 A. J. SARGEANT, 1,2 P. W. L. VERDIJK, 1 AND A. DE HAAN 1 1 Institute for Fundamental and Clinical Human Movement Sciences, Faculty of Human Movement Sciences, Vrije Universiteit, 1081 BT Amsterdam, The Netherlands; and 2 Neuromuscular Biology Research Group, Manchester Metropolitan University, Alsager, ST7 2HL, United Kingdom Abbate, F., A. J. Sargeant, P. W. L. Verdijk, and A. de Haan Effects of high-frequency initial pulses and posttetanic potentiation on power output of skeletal muscle. J. Appl. Physiol. 88: 35 40, 2000. The effects of high-frequency initial pulses (HFIP) and posttetanic potentiation on mechanical power output during concentric contractions were examined in the in situ medial gastrocnemius of the rat with an intact origin on the femur and blood supply. Stimulation of the muscle was performed via the severed sciatic nerve. In the experiments, HFIP or the potentiating tetanus was followed by a stimulation of 80, 120, or 200 Hz. The results showed that both HFIP and the tetanus increased power output at high contraction velocities ( 75 mm/s) when followed by a train of 80 or 120 Hz (200 Hz resulted in no effects). Mechanical power output was increased maximally by HFIP to 120 and 168% by the tetanus. Furthermore, when HFIP or the tetanus were followed by a train of 80 Hz, the peak power in the power-velocity curve tended to be shifted to a higher velocity. excitation-contraction coupling; muscle performance; rate coding IT IS COMMONLY KNOWN that previous activity may affect the performance of skeletal muscle. Activation history may decrease muscle performance (fatigue) but also may result in an enhanced performance (potentiation, which is known as the increase in force output as a result of previous activation). Several types of potentiation have been described; two well-known examples are posttetanic potentiation (PTP) and potentiation induced by high-frequency initial pulses (HFIP). HFIP have been demonstrated to occur in vivo at the start of ballistic movements in many animals, including humans (see e.g., Ref. 16). Such HFIP are usually followed by stimuli of a much lower frequency. In fast motor units of freely moving rats, HFIP as high as 333 Hz, followed by a train of 85 Hz, have been reported (10). Previous studies on HFIP using mammalian skeletal muscle or motor units were mostly performed under isometric conditions. Two effects that can occur as a result of HFIP were revealed, i.e., an increase in peak force and an increase in the rate of force development (e.g., Ref. 6). The extent to which these two effects 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. appear depends on the frequency that follows the HFIP. If a train of a relatively low frequency follows HFIP, an increase in both peak force and rate of force development was obseved. However, if a train of a relatively high frequency followed HFIP, only an increase in the rate of force development was observed (6). Next, to different activation strategies, potentiation may also occur after repeated, usually short-lasting, contractions (staircase) and/or after one long contraction (tetanus), the latter known as PTP. For example, isometric twitch force following a tetanus has been shown to increase by 132% compared with the unpotentiated state (12). It should be noted, however, that PTP is primarily found in fast-twitch muscle. Although some reports have been made concerning PTP in cat soleus (4, 22), slow-twitch muscles are predominantly known to cause posttetanic depression (5). Because the effects of PTP in fast-twitch muscle have been reported to last several minutes, most investigations into the mechanisms have concentrated on long-lasting changes due to tetanic stimulation such as an increased phosphorylation rate of the myosin light chains. Indeed, Moore and Stull (17) showed that in mouse gastrocnemius muscle the increase in force correlated well with an increase in myosin light chain phosphorylation in the nonfatigued state. An augmented rate of phosphorylated myosin light chains could result in a higher force by increasing the number of cross bridges in the force generating state (23) or by prolonging the force generating state (19). If phosphorylation of the regulatory myosin light chains does prolong the force-generating state of cross bridges, it may explain why phosphorylation does not increase chemical energy usage of the muscle (1). If so, an increase in myosin light chain phosphorylation might be expected to increase the efficiency of the muscle during a contraction. As for HFIP potentiation, indications for an increased Ca 2 release have been found (9), although improved force transmission through augmented muscle stiffness has also been proposed (20). Together, HFIP and PTP thus represent a way to improve muscle performance during one or a series of submaximal contractions. Nevertheless, very little is known about the effects of potentiation on parameters important for in vivo movement such as mechanical power output or about whether potentiation s effects are velocity dependent. Moreover, most previous work on the effects of HFIP and PTP used low stimulation http://www.jap.org 8750-7587/00 $5.00 Copyright 2000 the American Physiological Society 35

36 POTENTIATION AND POWER OUTPUT OF SKELETAL MUSCLE frequencies (or twitches) after the potentiating stimulus to determine the extent of potentiation during isometric contractions. However, it has been shown that the stimulation frequency-force relationship for dynamic contractions is shifted to the right compared with isometric contractions (8). Consequently, higher stimulation frequencies are needed to obtain maximal forces during dynamic contractions. Thus any investigation into the effects of HFIP and PTP on maximal force and power output during dynamic contractions would need to include relatively high stimulation frequencies compared with the studies of isometric function. Therefore, the present study was set out to investigate the effects of HFIP and PTP on mechanical power output by using high stimulation frequencies and to determine whether the effects were velocity dependent. MATERIALS AND METHODS Muscle Preparation and Experimental Setup Experiments were performed on male Wistar rats (n 20; body mass 267 20 g) anesthetized with urethan (1.5 g/kg body mass ip). Supplemental injections of 0.63 g/kg body mass ip were given if necessary. The medial gastrocnemius muscle tendon complex was excised free of surrounding muscles without compromising the blood supply, as has been described previously (7). The animal was placed prone on a heated pad (35 C) with the femur of the operated leg clamped in a vertical position, and the muscle was held horizontally. The tendon was connected to a force transducer, which was part of an isovelocity-measuring system. The system is based on a motor-indexer combination, from which one revolution is electronically divided into 25,000 steps (Compumotor, Petaluma, CA). The stepping motor has a resonance of 120 Hz. This leads to sinusoidal disturbances of the force records during shortening contractions because the force transducer is mounted on the lever arm of the motor. The velocity of shortening, reported in millimeters per second, refers to the contraction velocity of the muscle-tendon complex. To prevent dehydration, the muscle was covered with a thin film of paraffin oil. The tendon was smeared with vaseline. Stimulation was performed through the severed sciatic nerve with only the branch to the medial gastrocnemius left unimpaired. In the experiments, each pulse was given with a current of 1 ma, which was 30% higher than that needed for maximal force development. Pulse duration was 0.05 ms. Motor movements and stimulation were computer controlled. Muscle temperature was controlled by a water-saturated airflow around the muscle of 34 0.5 C. A previous study has shown that muscle temperature was within 1 C of the temperature of the airflow (8). Optimum length for force was determined by using 150-ms tetani of 120 Hz. Peak force developed by the fresh muscle during tetanus stimulation at optimum length will be referred to hereafter as the control force. At the end of the experiments, rats were killed by cervical dislocation. HFIP To study the effects of HFIP on mechanical power output, two types of HFIP were used, i.e., two pulses of 200 Hz (D200) and three pulses of 400 Hz (T400). HFIP were immediately followed by a train of either 80, 120, or 200 Hz. The effects of HFIP were studied by comparing force data obtained by using HFIP with data obtained without HFIP at the start of electrical stimulation. Each contraction was followed by a 2-min rest. Each tenth contraction was followed by a tetanus (150 ms, 120 Hz) to verify the state of the muscle. If the developed force was 90% or less of the initial control force, no further contractions were performed in that muscle and the data of the previous 10 contractions were removed from the data set. PTP In a pilot study, it was estimated which protocol produced the highest twitch potentiation. It appeared that optimal potentiation was evoked by a tetanus of 1 s at 160 Hz. Potentiation remained constant for more than 10 s after the tetanus and had vanished after 15 min. In the experiments, the potentiating tetanus was followed by three concentric contractions, which followed one another every 2 s. Each set of three contractions was performed at the same shortening velocity. Each contraction was performed with a different (randomized) stimulation frequency of either 80, 120, or 200 Hz. After a set of contractions, a 15-min resting period was included to reduce fatigue and make sure that no potentiation was left. Before further experiments were performed, the state of the muscle was tested in a similar way as with HFIP potentiation. Isovelocity Contractions Concentric contractions were performed at either 0, 25, 50, 75, 100, 125, 150, or 200 mm/s in random order (estimated maximal velocity of the muscle 300 mm/s; Ref. 8). The dynamic contractions were performed by using shortening distances ranging from 3 [at velocity (v) 25 mm/s] to 6 mm (at v 200 mm/s). The muscle was first passively stretched by a third of the shortening distance. At that length, stimulation was started after 200 ms and force was allowed to reach the level that had previously been estimated as the force level at the shortening velocity. When that force level had been reached, the motor started and the muscle shortened (see Fig. 1, B, C, and D). These type of contractions are near isotonic and minimize the effects of the series elastic elements (see also Ref. 8). As the force generated during shortening is lower at high contraction velocities, the force that could be maintained during a contraction was reached sooner. Therefore, the duration of the phase in which force built up was less at high than at slow shortening velocities ( 30 ms at v 25 mm/s and 10 ms at v 200 mm/s). The results of this study were not affected by a possible decreased level of activation during high contraction velocities, since in the present setup maximal active state was reached very soon after the start of stimulation (8). Force was obtained directly from the force records and measured when the muscle passed tetanus optimum length (L o ). To minimize the effects of the sinusoidal disturbances in the signal, the force was measured between the minimum and maximum of an oscillation. An estimation of the passive force was made by performing a similar contraction without stimulation of the muscle 2 min before the contraction. The passive force was measured at L o and subtracted from the force measured during a contraction, which resulted in force (F). Power was calculated by multiplying force (F) and shortening velocity. Data Analysis To compare data among animals with different peak forces (range 9 11 N), all data were normalized to the mean peak force of 10 N. Normalized data were averaged, and SD was calculated. Data were tested for significant differences by a

POTENTIATION AND POWER OUTPUT OF SKELETAL MUSCLE 37 Fig. 1. Examples of digitized force traces (sample frequency 10,000 Hz) during isometric and dynamic contractions with 80-Hz stimulation. This example shows effects of potentiation induced by high-frequency initial pulses on force output during dynamic contractions. Contractions were isometric (A) or dynamic with shortening velocities (V) of 75 mm/s (B), 100 mm/s (C), and 150 mm/s (D). Muscles passed optimum length during shortening at t 70 ms (B), t 40 ms (C), and t 28 ms (D). Order of force traces from low to high: 80 Hz, 80 Hz preceded by a doublet of 200 Hz, 80 Hz preceded by a triplet of 400 Hz. Arrows in each panel indicate appropriate force scale. two-way ANOVA for repeated measurements. In case of significance, a Bonferroni post hoc test was used to locate the differences. P 0.05 was considered significant. RESULTS Effects of HFIP Force during isometric contractions. Examples of force traces are shown in Fig. 1A. In this example, the muscle was activated with an 80-Hz train or an 80-Hz train preceded by either a D200 or a T400. During these experiments, the total number of pulses was kept constant. Therefore, the duration of stimulation with HFIP is shorter than stimulation without HFIP. As can be seen, the D200 as well as the T400 led to an increase in the rate of force development, whereas peak force remained unchanged. Also, it was found that the T400 increased the rate of force development more than the D200. Similar results, although smaller, have been observed when the D200 or the T400 preceded a 120-Hz stimulation. However, no effects on the rate of force development or peak force were found when the T400 preceded a 200-Hz stimulation. Force during dynamic contractions. Similarly, force traces of concentric contractions with different shortening velocities are shown in Fig. 1, B D. As both the T400 and the D200 increased the rate of force development, the force that could approximately be maintained during concentric contractions was reached earlier. Therefore, to prevent an overshoot of the force signal, the duration of the isometric phase was reduced in the contractions with T400 and D200. In contrast to the absence of effects on peak force in isometric contractions, force output was increased during high-velocity concentric contractions when HFIP were followed by an 80- or 120-Hz train. The extent to which force increased was determined by the type of HFIP used (T400 led to higher increases than the D200) and the velocity of shortening. No increase in force during shortening was found if the T400 was followed by a 200-Hz train. Power output. Figure 2 shows the effects of HFIP on power output measured at L o. From the data, two effects of HFIP on power output can be distinguished. First, power output was increased at higher shortening velocities ( 75 mm/s) when an 80- or 120-Hz train followed HFIP, but not at slower velocities. In the experiments using the 80-Hz train, no clear peak in the power-velocity curve was observed [optimal velocity (V opt ) between 50 and 75 mm/s]. However, from curve fittings we observed that there was a tendency that V opt shifted to higher velocities after HFIP in the 80-Hz train contractions (see Fig. 2A). Second, the extent to which power output increased depended on the type of HFIP used (D200 or T400) but also on the frequency of the train that followed HFIP. Power output increased most (up to 120%) when HFIP was followed by a train of 80 Hz (see Fig. 2). The effects were lower (maximally 72%) when a train of 120 Hz followed HFIP and were absent in case of a 200-Hz train. PTP Force during isometric contractions. During isometric contractions, no effects on peak force were found for the frequencies used (80, 120, or 200 Hz). However, PTP led to an increase in the rate of force development

38 POTENTIATION AND POWER OUTPUT OF SKELETAL MUSCLE during 80- and 120-Hz stimulation. No effects on the rate of force development were found when a 200-Hz stimulation followed the potentiating tetanus. Force during concentric contraction. As with HFIP, the force that could approximately be maintained during concentric contractions was reached earlier because of an increased rate of force development. Thus, to prevent an overshoot in the force signals, the duration of the isometric phase was reduced. The output of the potentiated force during concentric contractions increased for 80 and 120 Hz. When a 200-Hz stimulation was used during concentric contractions, no increase was observed. Power output. The effects of PTP on power output during concentric contractions (Fig. 3) show many similarities with the effects of HFIP on power output (Fig. 2). For example, both PTP and HFIP increased power output at 80- and 120-Hz stimulation during high shortening velocities ( 75 mm/s), whereas no effects on power output were found at slow contraction velocities. Similarly, in case of 80-Hz stimulation, PTP tended to shift V opt toward higher contraction velocities after curve fitting. Furthermore, the extent to which PTP increased power output at higher contraction velocities depended on the stimulation frequency during a contraction. Again, as with HFIP, the highest increase in power output occurred with 80-Hz stimulation (maximal 168%), whereas no effects were found with 200-Hz stimulation. DISCUSSION The aim of the present investigation was to study the effects of HFIP and PTP on maximal mechanical power output and on the dependence of the effects on shortening velocity during concentric contractions. Although the types of potentiation studied were induced by different stimulation protocols, they affected mechanical power output in a remarkably similar way. 1) The results showed that the effects of both HFIP and PTP on force and power output during concentric contractions depended on the frequency of stimulation following the potentiating stimulus (D200, T400, or tetanus). The highest increase in power and force output occurred with an 80-Hz train following HFIP or the potentiating tetanus, whereas no effects were found with 200 Hz. Furthermore, in the case of HFIP potentiation, the extent to which power could be increased depended on the type of HFIP used (D200 or T400); i.e., the T400 led to higher increases than the D200, which was similar to the findings of Kwende et al. (13). 2) The effects of HFIP and PTP on power output were dependent on the velocity of shortening; i.e., increases in power output were found at high shortening velocities, whereas no effects were detected at low shortening Fig. 2. Effects of high-frequency initial pulses on power velocity curves at different stimulation frequencies. Average power-velocity data ( SD) are shown for 80 Hz (A; n 6), 120 Hz (B; n 6), and 200 Hz (C; n 4). Data are displayed for constant-frequency train (r), constant frequency preceded by a doublet of 200 Hz (s), and constant frequency preceded by a triplet of 400 Hz (m). *Significant difference from constant-frequency train (P 0.05).

POTENTIATION AND POWER OUTPUT OF SKELETAL MUSCLE 39 Fig. 3. Effects of posttetanic potentiation on power velocity curves at different frequencies. Average power/velocity data ( SD; n 6) are shown for 80 Hz (A), 120 Hz (B), and 200 Hz (C). Data are presented for control contractions (r) and posttetanic potentiation (s). * Significant difference from control contractions (P 0.05). velocities. This may be explained by the finding that the stimulation frequency-force relationship obtained during isometric contractions is shifted rightward during concentric contractions. Therefore, at a particular frequency, a lower relative force will be generated during dynamic contractions than during isometric contractions. For example, under the present experimental conditions, a stimulation of 120 Hz will, during isometric contractions, lead to 98% of maximal force. However, at a shortening velocity of 75 mm/s the muscle will generate only 75% of maximal force (8). It may, therefore, be possible at high shortening velocities to increase the force/power output at stimulation frequencies that will in the isometric state lead to (near) maximal forces by further increasing the stimulation frequency (8) or by PTP/HFIP potentiation. Similarly, other studies have shown that HFIP potentiation is most pronounced when excitation contraction coupling is suppressed, e.g., at short muscle lengths (21) and during low-frequency fatigue (2). It is known that an increase in the activation frequency augments the Ca 2 release, and, as the effects on power output are very similar, it may be speculated that HFIP potentiation is at least partly accounted for by a similar mechanism. The Ca 2 level during HFIP potentiation could be raised by 1) increased Ca 2 release per pulse as has already been shown to occur in barnacle muscle by Duchateau and Hainaut (9) and/or 2) by faster release than reaccumulation of Ca 2 by the sarcoplasmic reticulum due to the very short interval between the initial pulses. It should be indicated, however, that direct measurements of intracellular Ca 2 levels are necessary to study either hypothesis. An increased effect of Ca 2 on cross-bridge formation has been suggested as a mechanism of PTP (18). However, as the effects of PTP remain after complete relaxation of the muscle and even minutes after the potentiating tetanus, an increase in sarcolemmal Ca 2 concentration seems unlikely to account for the effects of PTP. It has also been hypothesized that PTP is induced by an increased level of myosin light chain phosphorylation (e.g., Refs. 14, 15, 18), which is thought to either increase the number of cross bridges in the force-generating state (23) and/or prolong the forcegenerating state of each cross bridge (19). The present data tend to support the first suggestion, as the effects of PTP were most pronounced at high shortening velocities where it is not possible for cross bridges to remain in the force-generating state for a long time. However, it is still possible that more force per cross bridge is generated in the potentiated state. It is presently unknown what the in vivo function of potentiation and its effects may be. In contrast to the present findings, Sandercock and Heckman (21) reported a decreasing effect of HFIP potentiation on force output when the shortening velocity was increased. It should be noted, however, that Sandercock and Heckman performed their experiments on a slow postural muscle (soleus) of the cat. Those findings and the results of the present study indicate that, although HFIP potentiation occurs in fast- as well as in slow-

40 POTENTIATION AND POWER OUTPUT OF SKELETAL MUSCLE twitch muscles, its function may differ for each muscle type. However, in the present study, HFIP and PTP both increased peak power and may have increased the velocity at which this occurred (V opt ) in the case of 80-Hz stimulation. Similar findings have been described previously when a comparison was made between the power output at several constant stimulation frequencies (8). These studies indicate that an increase in the stimulation frequency or potentiation of the muscle have similar effects on the power-velocity curve. Therefore, as was already suggested by Sweeney et al. (24), it may be that, in fast-twitch skeletal muscle, potentiation enables the muscle to develop (or maintain) a high power output at a lower stimulation frequency than that needed in the unpotentiated state. Indeed, in the present investigation, the power output of an 80-Hz stimulation preceded by the T400 is only slightly lower than the power output of a constant 120-Hz train. Patterns of decreasing activation frequency have also been described for single motor units during prolonged activations (up to 3 min, e.g., Refs. 3, 11) as well as during human voluntary contractions (up to 20 30 s, Ref. 16), during which force output was kept constant. It should be noted, however, that these patterns of decreasing activation were more gradual than the stepwise patterns used in this study. Although the mechanism by which the stimulation frequency is decreased in vivo is not clear, there may be several advantages to lowering the stimulation frequency while maintaining force/power output. First, fewer pulses are needed to keep power output constant, which may reduce the risk of high-frequency fatigue. This may be of great advantage, especially since higher stimulation frequencies are needed for optimal power output during fast concentric contractions. Second, reducing the number of pulses may also have energetic advantages for the muscle, as less Ca 2 needs to be actively reaccumulated by the sarcoplasmic reticulum. Summary The purpose of this study was to investigate the effects of potentiation on mechanical power output and whether the effects were dependent on shortening velocity. Potentiation was induced by either HFIP or a tetanus. It was shown that both forms of potentiation increased power output at high but not at low shortening velocities. Furthermore, the velocity at which peak power was reached in the potentiated state tended to be shifted toward higher shortening velocities. Similar effects on power output have already been reported by increasing the activation frequency; as suggested previously, it may therefore be speculated that a possible function for in vivo potentiation is to maintain a high power output while the activation frequency is lowered. However, additional work to test this hypothesis is needed. Address for reprint requests and other correspondence: F. Abbate, Vrije Universiteit, Institute for Fundamental and Clinical Human Movement Sciences, Faculty of Human Movement Sciences, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands (E-mail: F_Abbate@fbw.vu.nl). Received 8 February 1999; accepted in final form 3 September 1999. REFERENCES 1. Barsotti, R. J., and T. M. Buttler. Chemical energy usage and myosin light chain phosphorylation in mammalian skeletal muscle. J. Muscle Res. Cell Motil. 5: 45 64, 1984. 2. Binder-Macleod, S. A., and C. B. Barker. Use of a catchlike property of human skeletal muscle to reduce fatigue. 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