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1 This article was downloaded by: [Tartu University Library] On: 17 January 2011 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK European Journal of Sport Science Publication details, including instructions for authors and subscription information: Comparison of twitch contractile properties of plantarflexor muscles in Nordic combined athletes, cross-country skiers, and sedentary men Jaan Ereline a ; Helena Gapeyeva a ; Mati Pääsuke a a Institute of Exercise Biology and Physiotherapy, University of Tartu, Tartu, Estonia Online publication date: 17 January 2011 To cite this Article Ereline, Jaan, Gapeyeva, Helena and Pääsuke, Mati(2011) 'Comparison of twitch contractile properties of plantarflexor muscles in Nordic combined athletes, cross-country skiers, and sedentary men', European Journal of Sport Science, 11: 1, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 European Journal of Sport Science, January 2011; 11(1): 6167 ORIGINAL ARTICLE Comparison of twitch contractile properties of plantarflexor muscles in Nordic combined athletes, cross-country skiers, and sedentary men JAAN ERELINE, HELENA GAPEYEVA, & MATI PÄÄSUKE Institute of Exercise Biology and Physiotherapy, University of Tartu, Tartu, Estonia Downloaded By: [Tartu University Library] At: 15:56 17 January 2011 Abstract The purpose of this study was to compare twitch contractile properties of skeletal muscles in male athletes who train for power and endurance simultaneously (Nordic combined athletes) with athletes who train for endurance (cross-country skiers) and sedentary individuals. Ten Nordic combined athletes, 13 cross-country skiers, and 14 sedentary males aged 2026 years participated. To determine the contractile properties of the plantarflexor muscles during isometric twitch, the posterior tibial nerve in the popliteal fossa was stimulated by supramaximal square wave pulses of 1 ms duration. Twitch peak force, maximal rates of force development and relaxation, contraction and half-relaxation times were measured. The percentage increase in twitch peak force after a 5-s maximal voluntary contraction (MVC) was taken as an indicator of postactivation potentiation. Nordic combined athletes had a significantly greater twitch post-activation potentiation and rate of force development and shorter contraction time than the other two groups (PB0.05). They also had a greater (PB0.05) twitch peak force than cross-country skiers. No significant differences in measured twitch contraction characteristics were found in cross-country skiers and sedentary males. We conclude that the twitch contractile properties of the plantarflexor muscles differed markedly in athletes who train for power and endurance simultaneously compared with athletes who predominantly train for endurance. As an indicator of long-term adaptation to simultaneous power and endurance training, increased twitch force-generation and potentiation capacity, and shortening of twitch contraction times in the plantarflexor muscles were observed in Nordic combined athletes. Keywords: Contractile properties, post-activation potentiation, power training, endurance training, isometric contraction Introduction Exercise training can induce different processes of adaptation in the neuromuscular system through changes in neural control as well as morphology of the skeletal muscles. Several longitudinal studies have reported changes in the expression of contractile proteins (Li, Schlumberger, Wirth, Schmidtbleicher, & Steinacker, 2003; Putman, Xu, Gillies, MacLean, & Bell, 2004) and neural adaptations, such as alterations in motor units activation (Enoka, 1997; Van Cutsem, Duchateau, & Hainaut, 1998) or decreased antagonist co-contraction (Carolan & Cafarelli, 1992), after different training programmes. However, to analyse long-term training adaptations in humans, cross-sectional studies have generally been used. Some of these experiments have focused on differences in the neuromuscular system of powertrained and endurance-trained athletes (Kyröläinen & Komi, 1994; Lattier, Millet, Maffiuletti, Babault, & Lepers, 2003; Maffiuletti et al., 2001; Sleivert, Backus, & Wenger, 1995), illustrating that systematic exercise training induces specific processes of adaptation in the neuromuscular system depending on the type of physical activity performed. The measurement of twitch contractile properties of athletes muscles has been used for the analysis of specificity adaptation of the neuromuscular system to various types of systematic training (Alway, MacDougall, & Sale, 1989; Lattier et al., 2003; Maffiuletti et al., 2001; Pääsuke, Ereline & Gapeyeva, 1998, 1999; Sale, Upton, McComas, & MacDougall, 1983; Sleivert et al., 1995). By using electrically evoked supramaximal isometric twitch characteristics, the contractile properties of human skeletal muscles, independent of control and activation by the nervous system, can be determined. Correspondence: J. Ereline, Institute of Exercise Biology and Physiotherapy, University of Tartu, 5 Jakobi St., Tartu, Estonia. ereline@ut.ee ISSN print/issn online # 2011 European College of Sport Science DOI: /

3 Downloaded By: [Tartu University Library] At: 15:56 17 January J. Ereline et al. Twitch peak force is a direct indicator of the evoked force-generation capacity of muscles, which is more exposed in fast-twitch (type II) than slow-twitch (type I) muscle fibres (Alway et al., 1989). The timecourse of isometric twitches is thought to be highly dependent on the kinetics of excitationcontraction coupling, including intracellular Ca 2 movements (Klug, Leberer, Leisner, Simoneau, & Pette, 1988). The twitch rate of force development has rarely been used as an indicator of contraction speed, which depends largely on the rate of formation of crossbridges between myosin and actin (Lewis, Al-Almood, & Rosendorff, 1989). Two main factors have been described as responsible for the duration and rate of relaxation of twitches: sarcoplasmic reticulum Ca 2 uptake and rate of cross-bridge kinetics (Westerbladt, Lännegren, & Allen, 1997). Twitch contractile properties have been shown to differ in heavy resistance-trained athletes compared with sedentary individuals (Sale et al., 1983) and in power-trained athletes compared with endurancetrained athletes and sedentary individuals (Lattier et al., 2003; Maffiuletti et al., 2001; Pääsuke et al., 1998, 1999). Our previous research indicates that power training induces a more evident increase of force-generating capacity and speed of contraction and relaxation in plantarflexor muscles than endurance training (Pääsuke et al., 1998, 1999). Similar results have been obtained by others (Alway, MacDougall, Sale, Sutton, & McComas, 1988; Maffiuletti et al., 2001). However, no studies have investigated twitch contractile properties of skeletal muscles in athletes who combine power and endurance training simultaneously as is the case with Nordic combined athletes. Twitch contraction force is increased after a brief maximal voluntary contraction (MVC). This enhancement is called post-activation potentiation (Brown & Loeb, 1998; Moore & Stull, 1984; Sweeney, Bowman, & Stull, 1993). The most accepted mechanism underlying post-activation potentiation is a phosphorylation of myosin regulatory light chains during the conditioning contraction, which renders actin-myosin more sensitive to Ca 2 in the subsequent twitch (Grange, Vandenboom, & Houston, 1993; Sweeney & Stull, 1990; Yang, Stull, Levine, & Sweeney, 1998). Only one cross-sectional study assessed post-activation potentiation in endurance-trained athletes and sedentary individuals (Hamada, Sale, & MacDougall, 2000). The results showed that twitch post-activation potentiation was greater in endurance-trained athletes than sedentary controls only for the muscles trained, suggesting that the enhanced post-activation potentiation in endurance athletes was more likely the result of training adaptations than genetic endowment. Nevertheless, little is known about the influence of simultaneous power and endurance training on the twitch potentiation capacity of human skeletal muscles. The purpose of this study was to compare the twitch contractile characteristics of skeletal muscle and post-activation potentiation of elite male athletes who train for power and endurance simultaneously (Nordic combined athletes) with athletes who train predominantly for endurance (cross-country skiers). We also included a control group of sedentary individuals. Recordings were performed on the plantarflexor muscles, which are involved in many working and sports activities, including power and endurance events. Methods Participants Three groups of young men aged 1926 years were studied: Nordic combined athletes (n 10), crosscountry skiers (n 13), and sedentary (n 14) university students without a history of regular participation in physical activity. The athletes were members of national teams. Their training experience was 711 years. The group of Nordic combined athletes included four individuals who had participated in Winter Olympic Games and six individuals who were national-class athletes with experience of international competition. Two cross-country skiers were Winter Olympic Games medal winners, whereas the other cross-country skiers had participated regularly in international competitions. All participants were informed of the procedures and the purpose of the study and their written informed consent was obtained. The study received the approval of the University of Tartu Ethics Committee for Human Studies. The anthropometric characteristics of the participants are presented in Table I. Participants were given instructions 2448 h before data collection, and the testing of isometric MVC force of the plantarflexor muscles and electrical stimulation procedures were demonstrated. This was followed by a practice session to familiarize the participants with the procedures. Each participant s dominant leg was determined based on kicking preference. The participants refrained from caffeine for 24 h before the experiment. One day before the study, the participants were asked not to undertake any training. Test procedures During the experiment, the participants were seated on a custom-made dynamometer chair with the dominant leg (usually the right leg) flexed at a knee angle of 908 and mounted inside a metal frame

4 Table I. Anthropometric data, maximal voluntary contraction (MVC) force of the plantarflexor muscles, and MVC force relative to body mass (MVC force/bm) in athletes and sedentary controls (mean9s) Variable Nordic combined athletes (n10) Cross-country skiers (n13) Sedentary controls (n14) Age (years) Height (m) Body mass (kg) * # BMI (kg m 2 ) MVC force (N) * # MVC force/bm (N kg 1 ) * # *PB0.05 compared with Nordic combined athletes. # PB0.05 compared with cross-country skiers. BMIbody mass index. Contractile properties of plantarflexor muscles 63 Downloaded By: [Tartu University Library] At: 15:56 17 January 2011 (Pääsuke, Ereline, Gapeyeva, Sirkel, & Sander, 2000). The foot was strapped to an aluminium foot plate. The inclination of the foot could be altered by rotating the foot plate about an axis that corresponded to that of the ankle joint (i.e. the medial malleolus). The ankle was dorsiflexed to 208. This angle was associated with maximal voluntary and stimulated torques and corresponded to the optimal muscle length (Sale, Quinlan, Marsh, McComas, & Belanger, 1982). An adjustable pad held down the distal part of the thigh. Torques acting on the foot plate were sensed by a standard straingauge transducer connected with the foot plate by a rigid bar. The electrical signals from the strain-gauge transducers were amplified and displayed with a special amplifier. The strain-gauge transducer system signal was linear from 10 to 1600 N. The point of application of force to the foot plate was located on articulation regions between the metatarsus and ossa digitorum pedis. The force signals were sampled at a frequency of 1 khz and stored on a hard disk for further analysis. To determine the contractile properties of the plantarflexor muscles during isometric twitch, the posterior tibial nerve was stimulated through a pair of 2 mm thick, self-adhesive surface electrodes (Medicompex SA, Ecublens, Switzerland). Before attaching the stimulating electrodes, electrode gel was applied to the contact surface, and the underlying skin was prepared by shaving, sanding, and rubbing with isopropyl alcohol. The cathode (55 cm) was placed over the tibial nerve in the popliteal fossa and the anode (510 cm) was placed under the posterior-medial side of the thigh. Supramaximal square wave pulses of 1 ms duration were delivered from an isolated voltage stimulator (Medicor MG-440, Budapest, Hungary). The evoked compound action potential (M-wave) of the soleus muscle was recorded using bipolar (20 mm inter-electrode distance) electromyography (EMG) electrodes (Beckman miniature skin electrodes). The electrodes were placed longitudinally on the belly of the soleus muscle after the skin was cleaned using alcohol swabs and abraded lightly with fine sandpaper. As a reference electrode, a self-adhesive surface electrode (Medicompex SA, 5 10 cm) was placed over the proximal part of the triceps surae muscle between the stimulating and recording electrodes. The EMG signals were amplified and displayed using a standard preamplifier (Medicor MG-440, Budapest, Hungary) with the frequency band ranging from 1 Hz to 1 khz. These signals were sampled at 1 khz. On reporting to the laboratory, the participant sat resting for about 25 min before the dominant leg was placed in the apparatus. The rest period minimized any potentiation effect from walking to the laboratory. A maximal resting twitch was elicited by delivering a series of single stimuli of increasing intensity until a plateau of M-wave amplitude was obtained. During isometric twitch recording, the stimulus intensity varied from approximately 25 V to supramaximal in increments of 3050% ( V). First, three supramaximal isometric twitches of the plantarflexor muscles at rest with an interval of 15 s were elicited. Two minutes after the last resting twitch was recorded, the participant was instructed to make a MVC for 5 s and then to relax. Postactivation twitch was elicited within 2 s after the onset of relaxation. Two minutes after the postactivation twitch was recorded, the participant performed three isometric MVCs of the plantarflexor muscles. The joint position was the same as for previous twitch measurements. The participant was instructed to push the foot plate as forcefully as possible for 23 s. Strong verbal encouragement and visual feedback were used to motivate the participants. The greatest force of the three maximal efforts was taken as the isometric MVC force. Rest periods of 2 min were allowed between trials. The MVC force was calculated in relation to the body mass of each participant. Skin temperature of the tested muscle group was controlled and maintained at 358C with an infrared lamp. The following characteristics of resting isometric twitch were calculated: peak force, the highest value of isometric force production; contraction time, the time to twitch maximal force; half-relaxation time,

5 Downloaded By: [Tartu University Library] At: 15:56 17 January J. Ereline et al. the time of half of the decline in twitch maximal force; maximal rate of force development, the first derivate of the development of force (df/dt), and maximal rate of relaxation, the first derivate of the decline of force (df/dt). The percentage increase in post-activation twitch peak force in relation to resting twitch was taken as an indicator of postactivation potentiation. Figure 1 provides an example of forcetime curves of isometric twitches recorded in the present study at rest and in the potentiated state for each of the three groups studied. Statistical analysis Data are reported as means and standard errors of the mean (s x ). One-way analysis of variance (ANOVA) followed by Scheffé post hoc comparisons was used to test for differences between groups. Pearson correlation was used to assess the relationship between post-activation potentiation of twitch force, and resting twitch contraction time and halfrelaxation time. Lowest level of statistical significance was set at PB0.05. Results Body mass was greater (P B0.05) in sedentary individuals than in Nordic combined athletes and cross-country skiers (Table 1). There were no significant differences between the groups with respect to height and body mass index. The Nordic combined athletes and cross-country skiers had greater (P B0.05) MVC force and MVC force relative to body mass than sedentary controls, whereas no significant differences in these characteristics were observed between the two groups of athletes. The Nordic combined athletes had a significantly greater (P B0.05) resting twitch peak force compared with cross-country skiers and sedentary controls, while this parameter did not differ significantly in cross-country skiers and sedentary controls (Figure 2A). Twitch post-activation potentiation was significantly greater (P B0.05) in Nordic combined athletes than in sedentary individuals and crosscountry skiers (Figure 2B). Twitch contraction time was shorter in Nordic combined athletes compared with the other two groups, while it did not differ significantly between cross-country skiers and sedentary controls (Figure 3A). Nordic combined athletes had shorter (P B0.05) twitch half-relaxation time than sedentary controls, while there were no significant differences in this parameter between Nordic combined athletes and cross-country skiers, and between the crosscountry skiers and sedentary controls (Figure 3B). The Nordic combined athletes had a significantly greater (P B0.05) resting twitch maximal rate of force development than cross-country skiers and sedentary controls, while this parameter did not differ significantly between cross-country skiers and sedentary controls (Figure 4A). Resting twitch maximal rate of relaxation did not differ significantly among the groups (Figure 4B). Table II provides the correlation coefficients between twitch post-activation potentiation and resting twitch contraction time and half-relaxation time separately in Nordic combined athletes, crosscountry skiers, and sedentary subjects. In crosscountry skiers, twitch post-activation potentiation correlated negatively (P B0.01) with resting twitch contraction time (r 0.67). No significant correlations (P 0.05) were observed between twitch post-activation potentiation and time-course characteristics of resting twitch in Nordic combined athletes and sedentary controls. Figure 1. An example of the typical forcetime curves of isometric twitches at rest (continuous line) and in the potentiated (dashed line) state in Nordic combined athletes (A), cross-country skiers (B), and sedentary controls (C). Figure 2. (A) Resting twitch peak force (PF) and (B) postactivation potentiation in Nordic combined athletes (NCA, n 10), cross-country skiers (CCS, n13), and sedentary controls (SED, n14) (mean9s). *PB0.05; **PB0.01.

6 Contractile properties of plantarflexor muscles 65 Table II. Correlation coefficients between postactivation potentiation (PAP) of twitch force and resting twitch contraction (CT) and half-relaxation (HRT) times in athletes and sedentary controls Parameters PAP Nordic combined athletes (n10) CT 0.34 HRT 0.26 Cross-country skiers (n13) CT 0.67** HRT 0.14 Sedentary controls (n14) CT 0.30 HRT 0.47 **PB Downloaded By: [Tartu University Library] At: 15:56 17 January 2011 Figure 3. (A) Resting twitch contraction time (CT) and (B) halfrelaxation time (HRT) in Nordic combined athletes (NCA, n 10), cross-country skiers (CCS, n13), and sedentary controls (SED, n14) (mean9s). *PB0.05. Discussion To determine whether long-term simultaneous power and endurance training leads to different electrically evoked twitch contractile characteristics of plantarflexor muscles compared with long-term endurance training, we compared elite Nordic combined athletes and cross-country skiers. The Figure 4. (A) Resting twitch maximal rate of force development (RFD) and (B) maximal rate of relaxation (RR) in Nordic combined athletes (NCA, n10), cross-country skiers (CCS, n13), and sedentary controls (SED, n14) (mean9s). *PB 0.05; **PB0.01. main findings of this study were that: (1) twitch contractile properties of plantarflexor muscles differed significantly in Nordic combined athletes and cross-country skiers; (2) Nordic combined athletes had greater twitch rate of force development and twitch post-activation potentiation, and shorter resting twitch contraction time and half-relaxation time than sedentary controls, while these characteristics did not differ significantly between crosscountry skiers and sedentary controls. In agreement with our hypothesis, we observed a greater evoked twitch force-generating capacity in the plantarflexor muscles of Nordic combined athletes than cross-country skiers and sedentary individuals. Nordic combined includes two different sports events ski jumping and cross-country skiing and the athletes have to combine strategies of both disciplines into one training schedule. Training in Nordic combined requires special explosive-type strength (power) exercises coupled with endurance exercises for the lower extremities. A greater twitch peak force and rate of force development in powertrained athletes compared with endurance-trained athletes and sedentary individuals have been reported previously (Lattier et al., 2003; Maffiuletti et al., 2001; Pääsuke et al., 1998, 1999). Several factors can contribute to the increased twitch force in power-trained athletes muscles. High-level power-trained athletes have a greater number of fast-twitch fibres in their muscles than endurancetrained athletes (Costill, Fink, & Pollock, 1976). Some studies have shown selective hypertrophy of fast-twitch fibres after systematic strength/power training (Alway et al., 1988; Moore & Stull, 1984). It has also been suggested that longitudinal power training causes changes in excitationcontraction coupling and contractile apparatus of the muscle fibres that can affect their force-generating capacity (Duchateau & Hainaut, 1984).

7 Downloaded By: [Tartu University Library] At: 15:56 17 January J. Ereline et al. The isometric twitch force production can be enhanced by preceding contractile activity, such as occurs with post-activation potentiation. The most likely mechanism responsible for post-activation potentiation is the phosphorylation of myosin regulatory light chains during the conditioning contraction that increases sensitivity of actin-myosin to Ca 2 released by the sarcoplasmic reticulum (Grange et al., 1993; Sweeney & Stull 1990; Yang et al., 1998). The results of the present study show that twitch post-activation potentiation was greater in Nordic combined athletes than in cross-country skiers and sedentary individuals, with no significant difference between cross-country skiers and the sedentary group. Our previous studies indicated a greater twitch post-activation potentiation in plantarflexor muscles in power-trained athletes (sprinters and jumpers) than endurance-trained athletes (long-distance runners) and sedentary individuals (Pääsuke et al., 1998, Pääsuke, Ereline, Gapeyeva, & Torop, 2002). Hamada et al. (2000) observed that the magnitude of twitch post-activation potentiation was increased in endurance- and strength-trained athletes compared with sedentary individuals only for the trained muscles, suggesting the enhancement in post-activation potentiation could be explained by specific neuromuscular adaptation induced by training. Power training can enhance twitch postactivation potentiation by an increased ability to activate the muscles during MVC. An increased ability to activate high threshold motor units consisting of fast-twitch muscle fibres should increase twitch post-activation potentiation because fasttwitch fibres show greater post-activation potentiation than slow-twitch fibres (Hamada, Sale, MacDougall, & Tarnopolsky, 2003). Post-activation potentiation is often associated with a shortening of twitch contraction time and half-relaxation time (Hamada et al., 2000, 2003). In the present study, twitch post-activation potentiation was moderately negatively correlated with resting twitch contraction time in cross-country skiers, whereas no significant correlations were observed between post-activation potentiation and time-course characteristics of resting twitch in Nordic combined athletes and sedentary individuals. It was hypothesized that in endurance athletes the correlation between postactivation potentiation and twitch contraction time may indicate training adaptations in slow-twitch muscle fibres (Hamada et al., 2003). Nordic combined athletes had a shorter resting twitch contraction time than cross-country skiers and sedentary individuals, and a shorter halfrelaxation time than sedentary individuals, while these characteristics did not differ between crosscountry skiers and the sedentary group. Our previous studies indicated a shorter contraction time and half-relaxation time in power-trained athletes (sprinters and jumpers) and endurance-trained athletes (long-distance runners) compared with sedentary individuals, with no significant differences between the two groups of athletes (Pääsuke et al., 1999). On a muscle fibre level, the time-course of isometric twitches is probably highly dependent on the kinetics of excitation-contraction coupling mechanisms, including intracellular calcium movements (Klug et al., 1988; Kugelberg & Thornell, 1983). The shortened twitch contraction and halfrelaxation time of Nordic combined athletes muscles noted in the present study may indicate increased efficiency in sarcoplasmic reticulum function. Twitch contraction time has been reported to be closely related to development of the sarcoplasmic reticulum (Josephson, 1975), Ca 2 release and sequestration rate, and with Ca 2 concentration in the interfibrillar area (Salviati, Sorenson, & Eastwood, 1982), whereas twitch relaxation time has been connected with the efficiency of sarcoplasmic reticulum for re-uptake of Ca 2 (Westerbladt et al., 1997). One factor that could also affect twitch contractile characteristics is musculo-tendinous compliance, often referred to as the series elastic component. The series elastic component consists of both passive (tendon) and active (cross-bridges) elements (Fukashiro, Itoh, Ichinose, & Fukunaga, 1995). The time-course and rate of force and relaxation development of isometric twitches depend on the properties of both components. Conclusion In summary, our results indicate that the twitch contractile properties of the plantarflexor muscles differ markedly in athletes who train for power and endurance simultaneously (Nordic combined athletes) compared with athletes who train predominantly for endurance (cross-country skiers). As indicators of the long-term adaptation to simultaneous power and endurance training, we observed an increased twitch force-generation and potentiation capacity, and shortening of twitch contraction time of the plantarflexor muscles in Nordic combined athletes. Twitch post-activation potentiation was correlated with resting twitch contraction time in cross-country skiers, whereas no significant relationship was observed between post-activation potentiation and the time-course characteristics of resting twitch in Nordic combined athletes or sedentary controls. More detailed research is necessary comparing the twitch contractile characteristics of skeletal muscles in elite athletes who train for power and endurance simultaneously with athletes who train predominantly for power or endurance, to better understand the neuromuscular adaptation of

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