Effects of Strength Training on Endurance Capacity in Top-Level Endurance Athletes

Scand J Med Sci Sports 2010: 20 (Suppl. 2): 39–47 & 2010 John Wiley & Sons A/S doi: 10.1111/j.1600-0838.2010.01197.x

Review Effects of strength training on endurance capacity in top-level endurance athletes

P. Aagaard1, J. L. Andersen2

1Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark, 2Institute of Sports Medicine Copenhagen, University of Copenhagen, Copenhagen, Denmark Corresponding author: Prof. Per Aagaard, PhD, Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark. Tel: 145 6550 3876, Fax: 145 6550 3480, E-mail: paagaard@ health.sdu.dk Accepted for publication 21 January 2010

The effect of concurrent strength (S) and endurance (E) ately trained individuals to elite top-level athletes. It is training on adaptive changes in aerobic capacity, endurance concluded that strength training can lead to enhanced performance, maximal muscle strength and muscle mor- long-term (430 min) and short-term (o15 min) endurance phology is equivocal. Some data suggest an attenuated capacity both in well-trained individuals and highly trained cardiovascular and musculoskeletal response to combined top-level endurance athletes, especially with the use of high- E and S training, while other data show unimpaired or even volume, heavy-resistance strength training protocols. The superior adaptation compared with either training regime enhancement in endurance capacity appears to involve alone. However, the effect of concurrent S and E training training-induced increases in the proportion of type IIA only rarely has been examined in top-level endurance muscle fibers as well as gains in maximal muscle strength athletes. This review describes the effect of concurrent SE (MVC) and rapid force characteristics (rate of force devel- training on short-term and long-term endurance perfor- opment), while likely also involving enhancements in neuro- mance in endurance-trained subjects, ranging from moder- muscular function.

Equivocal reports exist for the effect of concurrent have indicated that both short-term endurance capa- strength (S) and endurance (E) training on adaptive city (Hickson et al., 1980, 1988; Hoff et al., 1999, 2002; changes in aerobic capacity, endurance performance, Østera˚ s et al., 2002; Støren et al., 2008; Losnegaard maximal muscle strength and muscle morphology. et al., 2010) and long-term endurance capacity (Hick- Thus, previous studies have reported a diminished son et al., 1980, 1988; Marcinik et al., 1991) can be range of cardiovascular and musculoskeletal adapta- improved in response to strength training. In contrast, tion, respectively, when S and E training regimes were the effect of concurrent strength training on the endu- combined (Hickson et al., 1980; Dudley & Djamil, rance capacity during long-term exercise (430 min) 1985; Hunter et al., 1987; Nelson et al., 1990; Kraemer only very rarely has been addressed in top-level et al., 1995; Glowacki et al., 2004; Izquierdo et al., endurance athletes. 2005). In contrast, other data have suggested that concurrent endurance and strength (resistance) training can lead to similar cardiovascular or musculoske- Changes in short-term endurance capacity letal adaptations compared with either training regime alone (Bell et al., 1991; McCarthy et al., 1995, 2002; Concurrent SE training has been reported to lead to Izquierdo et al., 2005), or that concurrent endurance improved short-term (o15 min) endurance capacity and strength training may even increase endurance measured as an increased time to exhaustion during performance beyond that achieved by endurance treadmill running or ski ergometer testing in un- training alone (Hoff et al., 1999, 2002; Østera˚ s et al., trained subjects (Hickson et al., 1980), well-trained 2002; Støren et al., 2008; Rønnestad et al., 2010; recreational endurance athletes (Hickson et al., Losnegaard et al., 2010). However, the effect of con- 1988), well-to-highly trained cross-country skiers current S and E training on endurance capacity in (Hoff et al., 1999, 2002; Østera˚ s et al., 2002; Mikkola relation to muscle morphology, fiber type composi- et al., 2007b; Losnegaard et al., 2010) and well- tion and contractile muscle function have not been trained distance runners (Støren et al., 2008) and examined previously in top-level endurance athletes. competitive cyclists (Sunde et al., 2009). Using a Previous studies conducted in untrained individuals longitudinal comparative setting, the gain in short- and moderately to well-trained endurance athletes term endurance performance was observed to be

39 Aagaard & Andersen greater following concurrent SE training than E 400 training alone (Hoff et al., 1999, 2002; Støren et al., SE pre SE post 2008; Sunde et al., 2009). A few of these studies *** E pre examined competitive athletes (Hoff et al., 2002; 350 E post Østera˚ s et al., 2002; Sunde et al., 2009; Losnegaard et al., 2010); however, on a group basis, these individuals could not strictly be characterized as 300 absolute top-level endurance athletes since demon- strating a VO2max below 70 mL O2/min/kg. A border- line exception might be the study by Hoff et al. (2002) 250

who looked at male cross-country skiiers with an 45-min Time Trial performance (W) 0 Pre Post Pre Post average VO2max of 69–70 mL O2/min/kg (SD: 2.3). Early studies by Hickson et al. (1980) reported that SE E the time to exhaustion during treadmill running Fig. 1. Long-term endurance performance measured as and ergometer biking at 100% of their pretraining average biking power output ( Æ SD) during an all-out 45- min time trial in elite (National Team) road cycling athletes VO2max (o7 min) could be increased by 12 and 47%, pre- and post 16 weeks of concurrent strength and endurance respectively, in previously untrained individuals in training (SE) or endurance training alone (E). Average response to 10 weeks of strength training that left power (biking speed) increased 8% after SE training and VO2max unaffected. These data were among the first was not affected by E training alone. *Post4pre (Po0.05), to demonstrate that short-term endurance capacity **SE4E(Po0.01). Data adapted from Aagaard et al. can be positively affected by strength training. In a (2010). subsequent classical study, short-term endurance capacity increased by 11–13% during standardized ately trained individuals (i.e. Hickson et al., 1980, maximal treadmill running and cycling exercise (4–8 1988; Marcinik et al., 1991; Bishop et al., 1999), and min duration) in well-trained recreational endurance only a single study have involved highly trained athletes (VO2max 60 mL/min/kg) when a regime of endurance athletes (Bastiaans et al., 2001). In the multi-exercise heavy-resistance strength training was latter study, long-term endurance capacity (60-min performed concurrently with their regular endurance time trial) increased to a similar extent in well-trained training for 10 weeks (Hickson et al., 1988). Similarly, competitive cyclists in response to concurrent SE recent data obtained in well-trained male and female training or E training alone, respectively (Bastiaans runners (VO2max 56–61 mL/min/kg) showed 21% et al., 2001). An improved 5K running time increased time to exhaustion (5.6–6.8 min) during (18.3–17.8 min) was observed in well-trained runners treadmill running at a speed corresponding to 100% (VO2max of 68 mL/min/kg) when about 30% of their of VO2max when heavy-resistance strength training was normal running training was changed into explosive- added to the scheme of endurance training over an 8- type strength training; however, no measures of long- week period (Støren et al., 2008). term (430 min) endurance capacity were obtained Similar to the above findings in runners, adaptive (Paavolainen et al., 1999). changes in short-term endurance capacity with con- We recently examined the effect of strength train- current strength training have been demonstrated in ing on short/long-term endurance capacity, mechan- well-trained to highly trained cross-country skiers. ical muscle output, skeletal muscle fiber size, fiber Thus,strengthtrainingfor8weeksledtoanimproved type composition and muscle vascularization in top- time to exhaustion (from 6.5 to 10.2 min) during all- level endurance athletes, where a regime of heavy- out testing on a double poling ski ergometer at Wmax resistance strength training was found to result in (velocity at VO2max) in highly trained male cross- enhanced long-term endurance capacity (improved country skiers (VO2max 70 mL/min/kg) (Hoff et al., 45-min time trial performance) in highly trained 2002). Similar findings were reported for well-trained National Team cyclists (VO2max 71–75 mL O2/ female cross-country skiers (VO2max 55 mL/min/kg) min/kg) (Aagaard et al., 2007, 2010) (Fig. 1). These in response to 9 weeks of concurrent S and E training changes were accompanied by an increased propor- (Hoff et al., 1999). tion of type IIA fibers and reduced proportion of type IIX fibers, elevated maximal muscle strength (MVC) and increased rapid force capacity [elevated Changes in long-term endurance capacity with rate of force development (RFD)] (Aagaard et al., strength training 2007, 2010). Importantly, the regime of concurrent strength and endurance (SE) training did not result in The effect of strength training on long-term cellular muscle hypertrophy or reduced muscle (430 min) endurance performance almost exclu- capillary density in the group of highly trained sively have been examined in untrained-to-moder- endurance athletes (Aagaard et al., 2007, 2010). In

40 Strength training and endurance capacity accordance with these recent findings in top-level Thornell, 2000) and capillary density (cap/mm2) endurance athletes, improved long-term endurance (Hather et al., 1991; McCall et al., 1996; Green previously have been observed in untrained-to-mod- et al., 1998) appears to remain unchanged or even erately trained individuals following a prolonged increase despite the presence of cellular muscle hy- period of concurrent SE training (Hickson et al., pertrophy. The above findings suggest that resistance 1980, 1988; Marcinik et al., 1991; Izquierdo et al., exercise (strength training) may provide an effective 2005), with SE training producing greater improve- stimulus for angiogenesis, which may hold true even ments than E training alone (Hickson et al., 1980, for high-level endurance athletes despite that these 1988; Izquierdo et al., 2005). Interestingly, improved subjects are already characterized by a very high long-term endurance performance has failed to be degree of muscular vascularization. Importantly, demonstrated in studies using low-volume (o8 concurrent endurance and strength training can weeks duration) and/or low-intensity (o80% 1 RM diminish or fully blunt the muscle hypertrophy that loadings) strength training, which typically have normally occurs with strength training, while in- comprised use of a single exercise, low-training creases in maximal muscle strength are still observed intensity (o60% 1 RM) and/or short-duration ex- (Hickson et al., 1988; Kraemer et al., 1995; Bishop et ercise periods (Bishop et al., 1999; Bastiaans et al., al., 1999; Aagaard et al., 2007, 2010), the latter likely 2001; Levin et al., 2009). Taken together, the avail- as a result of neuromuscular adaptation (Aagaard, able data suggest that a high muscle loading inten- 2003). This gain in mechanical muscle output in the sity (85–95% 1 RM) and/or a large volume of absence of cellular hypertrophy may also result in strength training need to be performed before a enhanced short-term and long-term endurance per- benefit on long-term endurance performance can be formance in highly trained endurance athletes. achieved. Importantly, maximal ( 85% 1 RM) (Hoff et al., 1999, 2002; Aagaard et al., 2007, 2010; Støren et al., 2008; Rønnestad et al., 2010; Sunde et Adaptive mechanisms al., 2009) and/or explosive-type (Paavolainen et al., 1999; Mikkola et al., 2007a, b) strength training The likely candidates for the observed improvement appears to be more advantageous in endurance in long-term endurance capacity comprise an in- athletes (well trained to top level) than the hyper- creased proportion of type IIA muscle fibers (Aa- trofic type of training. gaard et al., 2007, 2010) that are less fatigable and yet Notably, concurrent S and E training can lead to highly capable of producing high contractile power elevated maximal muscle strength even in the absence (Bottinelli et al., 1999). In addition, concurrent SE of muscle fiber hypertrophy (Hickson et al., 1988; training led to substantial gains in maximal muscle Bishop et al., 1999; Aagaard et al., 2007, 2010) or strength (MVC) and rapid force capacity (RFD) in gains in anatomical muscle CSA (Losnegaard et al., top-level endurance athletes (National Team cy- 2010). The latter finding is especially important to clists), which are likely to have contributed to the top-level endurance athletes, who typically intend to observed increase in long-term endurance capacity avoid gains in muscle mass, as elevated muscle mass manifested by a 7% increased mean Watt production is thought to be detrimental for an optimal endur- in a standardized 45-min time trial performed in the ance capacity within endurance sports where muscle lab (Aagaard et al., 2007, 2010). Concurrent SE forces are generated to support the body mass training in competitive cyclists has failed previously against gravity (i.e. running, cycling). Also, cellular to produce greater gains in long-term endurance hypertrophy effects are often undesirable in the top- capacity (1-h lab time trial) than endurance training level endurance athlete because an increased single alone (Bastiaans et al., 2001). muscle fiber area would lead to an increased diffusion A similar lack of improved long-term endurance distance from the exterior to the interior (central capacity was observed in well-trained cyclists/triath- portions) of the muscle cell, thereby potentially letes (30-km time trial) in response to 6 weeks of compromising the transport of glucose and free fatty strength training using a mix of strength and power acid (FFA) from the capillary bed into the muscle exercises (Levin et al., 2009), and in well-trained cells as well as potentially leading to a reduced female cyclists (1-h cycling) after 12 weeks of low- removal of excessive heat production from the work- volume (single exercise) strength training (Bishop ing muscle(s), altogether resulting in impaired long- et al., 1999). However, as discussed above, these term endurance capacity. negative findings may have been caused by the use Longitudinal data obtained in previously un- of short-term (9 weeks), low-resistance (i.e. ‘‘power’’ trained individuals suggest that resistance exercise type) strength training (Bastiaans et al., 2001), rather (i.e. strength training) per se may provide a stimulus than long-term (412 weeks), heavy-resistance train- for capillary neoformation (Hather et al., 1991; ing (Aagaard et al., 2007, 2010) that appears to be McCall et al., 1996; Green et al., 1998; Kadi & more optimal for eliciting neuromuscular adaptation

41 Aagaard & Andersen (Aagaard, 2003) and inducing shifts in fiber type It is likely that the involvement of heavy-resistance composition (Andersen & Aagaard, 2000). Addition- exercise over a prolonged period of time in highly ally, the lack of adaptation in long-term endurance trained endurance athletes may evoke changes in neural capacity could arise from a short duration of training function that will contribute to the observed gain in (Levin et al., 2009), and/or a low volume of strength maximal muscle strength and rapid force capacity, training performed (Bishop et al., 1999). respectively (Aagaard, 2003). Thus, a high volume of For cycling athletes, the increase in maximal muscle heavy-resistance and/or explosive-type strength training strength observed following concurrent SE training previously have been shown to induce increased neuro- means that when producing a pedal thrust force of a muscular activity during maximal muscle contraction certain magnitude (corresponding to a given Watt efforts that resulted in an elevated MVC and RFD production for a given pedaling cadency), this will (Ha¨ kkinen et al., 1998; Van Cutsem et al., 1998; represent a reduced relative load (relative to max), Aagaard et al., 2002a; Del Balso & Cafarelli, 2007). which in turn may have contributed to the observed This improvement in neural function may comprise enhancement in long-term endurance performance spinal as well as supraspinal adaptation mechanisms (Hickson et al., 1988; Aagaard et al., 2007, 2010; (Aagaard et al., 2002b; Del Balso & Cafarelli, 2007; Rønnestad et al., 2010). Further, an increase in rapid Duclay et al., 2008). Interestingly, neuromuscular ac- force capacity (RFD) also was observed following tivity (VL muscle) recorded during MVC appeared to concurrent SE training in top-level cyclists (Aagaard remain unchanged following 21 weeks of low-frequency et al., 2007, 2010), which may also contribute to a concurrent SE training in untrained individuals (Ha¨ k- reduced degree of muscle fiber exhaustion for a given kinen et al., 2003), which suggests that a relatively high cycle power output during long-term endurance (time volume of S training may be necessary to induce trial) events. Thus, the observed increase in RFD neuromuscular adaptations in response to concurrent following SE training would enable cyclists to more training regimes. rapidly produce pedal force and thereby allow for a more prolonged relaxation phase in each pedal revolution. Similar arguments have been forwarded to Changes in economy of movement explain the improved short-term performance and economy observed in high-level cross-country skiers Studies conducted in untrained individuals (Loveless after concurrent SE training (Hoff et al., 1999), et al., 2005), well-to-highly trained runners (Paavo- namely that the observed training-induced increase lainen et al., 1999; Østera˚ s et al., 2002; Støren et al., in RFD would allow for a shorter propulsion phase 2008), cross-country skiers (Hoff et al., 1999, 2002; for a given overall power output, hence allowing for Mikkola et al., 2007b), triathletes (Millet et al. 2002), an extended muscle relaxation phase. The prolonged and moderately to well-trained cyclists (Rønnestad muscle relaxation phase would reduce the time of et al., 2010; Sunde et al., 2009) have reported that contraction-induced muscle occlusion, and hence in- concurrent SE training or S training alone can lead to crease the time of muscle perfusion given the pro- improved economy of movement compared with E longed relaxation phase, thereby increasing the mean training alone, although not always a uniform find- capillary transit time (MTT). Thus, a prolonged ing (Aagaard et al., 2007, 2010; Mikkola et al., muscle relaxation phase potentially would allow for 2007a). Cycling economy did not improve in top- an increased MTT per pedal revolution. In turn, due level (National Team) cyclists following 16 weeks of to the relatively large molecular size of FFAs, it has concurrent SE training (Aagaard et al., 2007, 2010). been suggested that an increased MTT could enable It is possible, however, that cycling economy may an increased diffusion of FFA into the muscle cells, already be highly optimized in top-level cyclists and hence potentially sparing the rate of muscle glycogen therefore highly difficult to improve, at least within breakdown and thereby delaying the onset of muscle weeks and months of training. In support of this fatigue (Kiens et al., 1993). Further, a longer MTT notion, net and delta cycling efficiency during graded potentially could lead to an enhanced removal of cycle ergometer testing remained unaffected in a metabolites produced by the contracting muscle group of competitive cyclists in response to 9 weeks fibers, which might contribute to the enhanced long- of concurrent SE training (Bastiaans et al., 2001). term endurance performance that was observed in In a recent study conducted in well-trained cyclists top-level athletes in response to concurrent SE train- (VO2max 66–70 mL O2/min/kg), Raastad and col- ing (Aagaard et al., 2007, 2010). Because currently leagues demonstrated that cycling economy was based on theoretical speculations, future experiments improved to a greater extent by concurrent SE should examine the potential interaction between training than E training alone during the final hour concurrent strength-endurance training and altered of a 185-min long cycling test, which was accompa- muscle perfusion dynamics during endurance exercise nied by a reduced rise in heart rate and blood lactate, (longer MTT). respectively (Rønnestad et al., 2010) (Fig. 2).

42 Strength training and endurance capacity

# §

À Fig. 2. Oxygen uptake (VO2; top), heart rate (HR; mid) and blood lactate concentration ([La ]; bottom) during 180 min of cycling at 44% of baseline Wmax before (Pre) and after (Post) 12 weeks of concurrent endurance (cycling) and strength training (E1S; left) and endurance (cycling) training alone (E; right). Wmax was calculated as the mean power output during the final 2 min of a short-term all-out incremental VO2max test. *Post different from pre (Po0.05), relative changes (pre-to-post training) different between E1S and E at 120–180 min (#, Po0.01) and 60–120 min (§, Po0.01). Data adapted from Rønnestad et al. (2010). Furthermore, all-out cycling performance measured during a 5-min max test conducted immediately at the end of the 185-min cycling was substantially improved following SE training while unaffected by E training alone (7% improved average power production), sug- gesting that sprint capacity in the final phase of a race can be enhanced by strength training (Rønnestad et al., 2010) (Fig. 3). Likewise, cycling economy was improved following 8 weeks of concurrent cycling and strength training in well-trained competitive cyclists Fig. 3. Mean power output during a 5-min all-out trial performed following 185 min of cycling at 44% of baseline (VO2max of 58–64 mL O2/min/kg) (Sunde et al., 2009), Wmax before (Pre) and after (Post) 12 weeks of concurrent while similar findings were demonstrated in moder- endurance (cycling) and strength training (E1S) or endur- ately to well-trained individuals (VO2max 52– ance (cycling) training alone (E). *Post different from pre # 62 mL O2/min/kg) after 12 weeks of single-mode (Po0.01). Difference between E1S and E in relative change strength training (Hansen et al., 2007). from pre-test to post-test (Po0.01). Data adapted from Rønnestad et al. (2010).

Lack of muscle fiber hypertrophy with concurrent type IIA fiber proportions (increased % area) in the training in endurance athletes quadriceps (VL) muscle, which increased from 26 to 34% while remaining unchanged with E training As mentioned above, 16 weeks of concurrent SE alone (Aagaard et al., 2007, 2010) (Fig. 4). The training in National Team cyclists resulted in altered elevated percentage of type IIA fibers appeared to

43 Aagaard & Andersen occur at the expense of reduced type IIX area. creational endurance athletes after an extensive re- Notably, no signs of quadriceps muscle fiber hyper- gime of concurrent SE training. In accordance, no trophy were detected, despite the prolonged period of signs of muscle fiber hypertrophy were observed in a heavy-resistance strength training, and muscle fiber group of moderately trained female cyclists in re- capillarization also remained unchanged with either sponse to a protocol of low-volume strength training SE or E training (Aagaard et al., 2007, 2010). (Bishop et al., 1999) while anatomical quadriceps In a classical study, Hickson et al. (1988) were also muscle CSA remained unaltered after 12 weeks of unable to demonstrate any measurable muscle fiber concurrent SE training in elite cross-country skiers hypertrophy (VL muscle) in moderately trained re- (Losnegaard et al., 2010). Recent studies have shown that distinct cell signaling events involving the Akt/ 80 mTOR or AMPK pathways appear to become acti- vated by resistance or endurance training, respectively (Atherton et al., 2005), and that inhibitory 60 cross-talk exists from one pathway to the other (Nader 2006; Baar 2006). Consequently, the endur- * ance training stimuli delivered to the muscle cells 40 during concurrent SE training may effectively blunt the muscle hypertrophy response that is normally 20 observed in response to heavy-resistance strength training alone. In support of this notion, absent or (*) reduced muscle hypertrophy may occur in response 0 to concurrent strength and endurance training in I IIC IIA IIAX IIX I IIC IIA IIAX IIX untrained to moderately trained individuals (Krae- Pre training Post training mer et al., 1995; Bell et al., 2000), although not a Fig. 4. Muscle fiber type distribution based on muscle fiber universal finding (McCarthy et al., 2002). area obtained in the quadriceps muscle (VL) pre and post 16 In further support of an atrophy stimulus provided weeks of concurrent strength and endurance training in a by endurance training, a reduction in muscle fiber group of male elite (National Team) cycling athletes. *Type ( ) CSA (atrophy) has been observed following intensive IIA: post4pre (Po0.05), * trend for type IIX: postopre (P 5 0.08). No changes were observed in cycling athletes endurance training (Terados et al., 1986; Ratzin who did not perform strength training. Data adapted from Jackson et al., 1990; Kraemer et al., 1995; Harber Aagaard et al. (2010). et al., 2004; Trappe et al., 2006). From a muscle

Strength training

Improved neural function ↑ % type IIA fibers ↑ tendon stiffness

↑ MVC ↑ RFD ↑ Economy of movement ↑ Capillary MTT

Short-term endurance Long-term endurance performance (0-15 min) performance (30-180 min)

Fig. 5. Proposed mechanisms by which short-term and long-term endurance performance can be increased in well-trained to highly trained endurance athletes by the addition of strength training to ongoing endurance training. MVC, maximal muscle strength; RFD, rate of force development (Dforce/Dtime) or rapid muscle strength; MTT, mean transit time; economy of movement: oxygen uptake rate (VO2) when moving a certain distance at a certain speed, a reduced VO2 reflects an improved economy. Full lines indicate a stimulatory effect, and dotted lines indicate proposed potential interactions that await experimental verification with concurrent strength and endurance training. Question mark indicates a potential effect that awaits general experimental verification (see text for more details).

44 Strength training and endurance capacity perfusion perspective, this adaptation could be appear to negatively affect muscle perfusion capacity highly important, because it results in an elevated (capillary density), at least when strength training is capillary to muscle fiber CSA ratio, which in turn performed for up to 20 weeks. facilitates O2 delivery and FFA uptake into the muscle cell due to the reduced diffusion distance. In turn, an elevated FFA uptake would result in a Conclusions reduced rate of glycogen breakdown, which poten- Experimental data demonstrate that strength train- tially would lead to an enhanced endurance perfor- ing can lead to enhanced long-term (430 min) and mance (prolonged time to exhaustion). Overall, these short-term (o15 min) endurance capacity both in data suggest that concurrent strength and endurance well-trained individuals and highly trained top-level training evoke concurrent opposing stimuli for cel- endurance athletes, especially (but not exclusively) lular hypertrophy and atrophy, respectively. In con- when high-volume, heavy-resistance strength train- sequence, no muscle fiber hypertrophy and no signs ing protocols are applied. As summarized in Fig. 5, of a reduction in capillary density have been ob- the enhancement in long-term endurance capacity served when strength and endurance training are appears to involve training-induced increases in the combined (Hickson et al., 1988; Bell et al., 2000; proportion of type IIA muscle fibers as well as gains Aagaard et al., 2007, 2010). It should be noted that in in maximal muscle strength (MVC) and rapid force previously untrained individuals, strength training characteristics (RFD), while also likely involving per se appears to increase the number of capillaries enhanced neuromuscular function. per fiber (Hather et al., 1991; McCall et al., 1996; Green et al., 1998; Kadi & Thornell, 2000) or result Key words: Aerobic capacity, concurrent training in unchanged capillarization (Luhti et al., 1986; adaptions, muscle, neuromuscular function. Tesch et al., 1990; Bell et al., 2000). Likewise, capillary density (cap/mm2) appears to remain unchanged (Hather et al., 1991; McCall Acknowledgements et al., 1996; Green et al., 1998). Muscle capillarization was not improved (nor compromised) when We like to thank coworkers Michael Kjær, Morten Bennekou, concurrent strength training was performed in elite Benny Larsson, Hanne Overgaard, Jens Olesen, Regina Cra- endurance athletes characterized by a high initial meri and Peter S. Magnusson from the Institute of Sports 2 Medicine Copenhagen (ISMC), Bispebjerg Hospital, Univer- capillary density ( 6–700 cap/mm , 7–8 cap/fiber) sity of Copenhagen. (Aagaard et al., 2009). Thus, concurrent strength Conflicts of interest: The authors have no potential conflicts training in top-level endurance athletes does not of interest.

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