INTRODUCTION
Conflicts among coaches exist regarding the role of strength training for endurance athletes despite over 25 years of research supporting its efficacy and application (34-36,46,47,58,64,65,67,71,82). Historically, resistance and endurance training have been viewed as training modalities at opposite ends of a continuum with divergent adaptations (17,41). In a recent meta-analysis, Wilson et al. (92) reported an inverse relationship between frequency and duration of endurance training and subsequent changes in hypertrophy, strength, and power. Alternatively, strength training has been shown to have a positive effect on endurance performance (46,49,51,65,73). Previous research reports that concurrent strength and endurance training can increase endurance performance in high-level athletes to a greater extent than endurance training alone (46,47,58,64,65,82). The interference effects between strength and endurance training are outside the scope of this review and have been discussed extensively in previous studies (23,24,44,54,92). Endurance in sport has been defined as the ability to maintain or repeat a given force or power output (80). Endurance training can be further subdivided into low-intensity exercise endurance (LIEE) and high-intensity exercise endurance (HIEE). LIEE can be defined as long-duration endurance activities or the ability to sustain or to repeat low-intensity exercise. HIEE can be defined as the ability to sustain or to repeat high-intensity exercise and has been associated with sustained activities of ≤2 minutes (80). Competitive endurance athletes need more than enhanced aerobic power (
;)
) and LIEE (34). Requirements for endurance athletes should also include muscular strength, anaerobic power, and HIEE (34-36,46,58,68,82). Furthermore, strength training has been shown to positively influence both LIEE and HIEE across a spectrum of endurance events with greater effects observed in HIEE (34,46,50,58,82,83).
Strength can be defined as the ability to produce force (76). Strength is a skill, which can be expressed in a magnitude of 0–100% (80). In the current endurance literature, 2 primary forms of strength training have been investigated: maximal, high-force, low-velocity, strength training (HFLV) and explosive, low-force, high-velocity strength training (LFHV). Previous studies have examined the effectiveness of concurrent endurance and circuit resistance training, but have demonstrated inferior results (49,73,84). Maximum strength can be defined as the maximal amount of force a muscle or group of muscles can exert against an external resistance and corresponds with the high-force, low-velocity portion of the concentric force-velocity relationship (15,81). The term “explosive strength training” has been used in previous studies in reference to low-force, high-velocity training (0–60% 1 repetition maximum [RM] loads) with maximal movement intent (59,69). The use of this terminology is misleading, as explosive strength (alternatively defined as rate of force development [RFD] or power output) (15,81), can be developed across a continuum of loads (0–100% 1RM) (19).
In fact, HFLV training has been shown to elicit improvements in explosive ability (measured as power output) across a larger spectrum of loads compared with LFHV training in weak subjects (20). The ability to improve power output across a larger spectrum of loads, among other reasons, likely explains why HFLV and endurance training may provide superior alterations in endurance performance compared with concurrent LFHV and endurance training for weak endurance athletes (9,60,73). Thus, explosive strength training performed in previous research on endurance performance is alternatively defined here as LFHV training.
Previous research on untrained and recreationally trained individuals has demonstrated that concurrent strength and endurance training can augment LIEE and HIEE, aerobic power, maximal strength, muscle morphology, and body composition (27,28,32,34,42,47,49,71,85). There is also research demonstrating HFLV and LFHV strength training enhance performance in high-level endurance athletes (58,59,61,64,66,72,73). In a recent review of the literature, Beattie et al. (10) reviewed results from 26 studies examining the effects of strength training on endurance performance of well-trained athletes (10). Their findings showed that strength training is effective for improving movement economy, velocity at
(v
), power output at
(w
), maximal anaerobic running test velocity (VMART), and time trial performance, and suggested that HFLV strength be developed before LFHV strength in endurance athletes with limited strength training experience. Considering these findings, this article focuses primarily on studies examining the effects of HFLV and LFHV strength training on HIEE and LIEE of moderate to high-level endurance athletes. The purpose of this review is twofold: to elucidate the utility of resistance training for endurance athletes, and provide the practitioner with evidenced-based periodization strategies for concurrent strength and endurance training for competitive endurance athletes.
EFFECTS OF STRENGTH TRAINING ON ENDURANCE PERFORMANCE AND UNDERLYING MECHANISMS
CONCURRENT HFLV STRENGTH AND ENDURANCE TRAINING
In one of the earliest studies examining the effects of concurrent strength and endurance training on HIEE, Hickson et al. (34) had moderately trained (
: 60 mL−1·kg−1·min−1) runners and cyclists perform 10 weeks of endurance and HFLV strength training (>80% 1RM). They reported improvements in treadmill running (13%) and ergometer biking (11%) to exhaustion at maximal work rates (4–8 minutes) and cycling to exhaustion (20%) at 80%
(33). In addition, there were no statistical changes in muscle fiber cross-sectional area or thigh girth, although 1RM leg strength increased on average by 30%, which suggests primarily neural contributions.
In a more recent investigation by Aagaard et al. (2), highly trained national team cyclists (
: 71–75 mL−1·kg−1·min−1) performed HFLV strength training (mostly 5–6RM loads) for 16 weeks concurrently with regular endurance training. The strength and endurance training group improved average power output and total distance covered in a 45-minute cycling test (8%), whereas the endurance only group did not. Concomitant increases were found for maximal voluntary isometric contraction (MVIC) of the knee extensors (12%), peak RFD (20%), mean power output in 5 minutes of all-out cycling (3–4%), and mean power output during a 45-minute time trial (8%) with no changes in muscle fiber area, capillarization, and
in the strength and endurance training group (2). The superior LIEE performance in the strength and endurance training group may have been due to a shift in vastus lateralis muscle fiber type from type IIx to type IIa. Increased MVIC as a result of strength training may enhance HIEE and LIEE by decreasing the relative external resistance, which reduces the number of motor units required to produce a given amount of force (13). In addition, improvements in RFD contributed to the improved LIEE performance by reducing time to reach peak concentric forces necessary to produce the desired movement and increasing the length of the eccentric phase lending to greater muscle perfusion and longer capillary mean transit times (1,86).
Strength training has been reported to increase musculotendinous unit stiffness (21,43,51,87). This results in an enhanced ability to store elastic energy in the series and parallel elastic component during eccentric muscle actions, which in turn increases concentric muscle force. This is thought to be one of the reasons why improvements in running economy (35,51,81), cycling economy (8,12,66,83), and cross-country skiing economy (35,36,58) have been observed after a period of HFLV strength and endurance training. However, not all studies show improvements in movement economy as a result of strength training (2,9,14,45,46,64) possibly due to differences in training variables (e.g., mode, volume-load, frequency, duration) and subjects' training status. For example, elite athletes who already possess a high level of efficiency may not further improve movement economy with strength training (64).
The superior performance changes with heavier strength training may be attributed to greater increases in musculotendinous unit stiffness, greater recruitment of high-threshold motor units, and greater capacity to store and release elastic energy, which lead to a right and upward shift in the force-velocity and force-power relationships (58). This does not preclude LFHV strength training for endurance athletes because, although the loads used are typically lighter, there are notable improvements in RFD, which has been linked to greater movement economy and enhanced LIEE and HIEE performance (59,72,85,88).
Furthermore, an increase in musculotendinous stiffness has a greater application to running than cycling because of the greater contribution of the stretch-shortening cycle (69). In contrast, strength training induced improvements on cycling economy and performance are more evident at the end of long cycling tests. Rønnestad et al. (66) found that cycling economy improved more during the final hour of a 185-minute cycling test as a result of HFLV strength with endurance training (3 × 4–10RM, 2 d/wk for 12 weeks) compared with endurance training alone (66). They also reported reduced HR and blood lactate concentrations during the final hour in the strength-trained group. In addition, the subjects completed a 5-minute sprint at the end of the 180 minutes during which the strength-trained group improved average power production, but the endurance only group did not. This “anaerobic reserve” may have been due to increased contractile strength of type I fibers delaying the contribution of the less economical type II fibers, sparing them for the sprint finish (69). The “anaerobic reserve” for late race sprint performance may also be explained by the sparing of substrate stores within a muscle. Goreham et al. (25) reported that 12 weeks of HFLV strength with endurance training (3 × 6–8RM, 3 d/wk for 12 weeks) resulted in greater muscle phosphocreatine and glycogen content, and lower lactate concentrations at the end of a 30-minute cycle at 72%
compared with endurance training alone (25).
Previous research has shown that endurance athletes who strength train with loads >70% 1RM exhibit larger changes in movement economy and endurance performance than endurance athletes who strength train with lighter loads (36,48,73). Sedano et al. (73) reported greater magnitudes of change in running economy, countermovement jump (CMJ) height, v
, and 3-km time trial performance after 12 weeks of heavy strength training (>70% 1RM) compared with lighter strength training (<40% 1RM) and a control group (circuit training) in Spanish national level runners (
>65 mL−1·kg−1·min−1) (73). Similarly, Guglielmo et al. (26) found that after only 4 weeks of concurrent running and heavy strength training (3–5 sets at 6RM loads), there were larger magnitudes of change in running economy, 1RM strength, and CMJ height compared with concurrent running and lighter strength training (3–5 sets at 12RM loads) in middle- and long-distance runners (
∼61.9 mL−1·kg−1·min−1) who competed at the regional and national level (26). Mixed results have been reported for effects of HFLV strength training on cycling economy; however, there are still improvements in LIEE and HIEE performance (2,64,65,83). Furthermore, previous findings suggest that HFLV strength training may exhibit its effects most prominently during high-intensity bouts of endurance events (9,49,66), although there is evidence to the contrary (23,45) (Table 1).
Table 1-a: Effects of concurrent HFLV strength training and endurance training on HIEE and LIEE
Table 1-b: Effects of concurrent HFLV strength training and endurance training on HIEE and LIEE
CONCURRENT LFHV STRENGTH AND ENDURANCE TRAINING
LFHV strength training has been reported to elicit improvements in HIEE and LIEE performance (59,74,85); however, research conclusions are mixed (7,45,60) (Table 2). Paavolainen et al. (59) found that LFHV strength training (<40% 1RM) improved 5k run time, running economy, VMART, 20-m sprint speed, and distance covered on a 5-jump test in male cross-country runners (
∼64.4 mL−1·kg−1·min−1), whereas no changes in these measures were observed in the endurance only group (59). The LFHV strength training involved various plyometric exercises and short sprints (20–100 m), which suggests that although the absolute barbell loads during strength training sessions were relatively light, the forces placed on the musculoskeletal system were much larger than to what the athletes were previously accustomed.
Table 2: Effects of concurrent LFHV strength training and endurance training on HIEE and LIEE
Considering that gaining body mass is a concern for endurance athletes, one of the purported benefits of LFHV strength training is the lower degree of muscle hypertrophy compared with HFLV strength training (28,70), but still being able to achieve increases in strength. In addition, increased muscle fiber cross-sectional area (CSA), maximal strength, and power output can be diminished or fully blunted by concurrent strength and endurance training with the degree of decrement depending on the mode, frequency, and duration of endurance training (2,14,34,42). Rønnestad et al. (64) found that increased CSA of the quadriceps muscle was associated with increased peak power output and cycling time trial performance after combined heavy strength training (twice/week, 3 × 4–10RM) and endurance training in well-trained cyclists without a noticeable change in body mass (64). Therefore, altering the strength to body mass ratio should be more of a concern for endurance athletes than body mass alone. Furthermore, although increases in “nonfunctional” hypertrophy may be detrimental to performance (3,86,91), increases in task specific hypertrophy may be an important factor in enhancing endurance performance, as the typical ectomorphic endurance athlete is unlikely to gain significant amounts of hypertrophy through strength training (89).
It has been suggested that LFHV strength training may provide an additive effect to those elicited by HFLV strength training on HIEE and LIEE performance (85). Taipale et al. (85) tested this hypothesis by dividing endurance runners into 3 groups (LFHV, HFLV, combination) and found no differences between groups in measures of strength (1RM), power (CMJ height), and endurance performance (85). Considering there is a delay between when a training stimulus is implemented and the subsequent effects on performance (90), a sequenced approach may be more appropriate than trying to improve strength, power, and endurance simultaneously (15,81).
TRAINING THEORY
THE TRAINING PROCESS
The primary goals of any successful training program are to reduce the likelihood of injury and optimize performance (81). Before designing a training program, however, the coach and the athlete must understand that training is a comprehensive process that harmonizes a myriad of factors to foster athlete development. Figure 1 depicts some of these factors that affect athletic performance. Therefore, the sport coaches, strength and conditioning staff, and sports medicine professionals each play an important role within their own disciplines to contribute to an athlete's development. In addition, the management of external stressors in the athlete's daily life is also an important component in the optimization of performance. To achieve this objective, however, training variables must be integrated in a sequence over the course of the training process (79). The training process is traditionally organized into 3 basic levels: macrocycles, mesocycles, and microcycles (79). A macrocycle is a long-duration training cycle, typically classified as 12 months of training, which are composed of multiple mesocycles. Mesocycles are moderate-length periods of training, which can focus on developing specific fitness characteristics within the macrocycle. Finally, each mesocycle is composed of shorter training periods referred to as microcycles (79). The tool used to structure each phase of training within a macrocycle is referred to as an “annual training plan” (15).
Figure 1: Factors affecting athletic performance, modified from Stone et al. (
81). Adapted, with permission, from M.H. Stone, M. Stone, and W.A. Sands, 2007,
Principles and practice of resistance training (Champaign, IL: Human Kinetics), 203.
More specifically, the annual plan is a long-term training template used to guide the coach and athlete in the design and implementation of various training phases (15). The annual training plan can be separated into phases: the general preparatory phase, competitive phase, peak phase, and active rest (Figure 2). For a detailed description of each phase, the reader is encouraged to examine the work of Bompa and Haff (15).
Figure 2: Cross-country runner macrocycle.
PERIODIZATION
To reduce the likelihood of injury and maximize athletic performance, strength and conditioning professionals should organize training adaptations in a logical manner to minimize fatigue and highlight technical and fitness characteristics (e.g., strength, speed, endurance, etc.) at precise times of the training year “to increase the potential to achieve specific performance goals” (79). This process of chronologically manipulating physiological adaptations is referred to as periodization. Although varying definitions of this term have been proposed, periodization has been most recently defined as, “The strategic manipulation of an athlete's preparedness through the employment of sequenced training phases defined by cycles and stages of workload” (22). Furthermore, if the training stimuli are sequenced appropriately, each phase of training will enhance or “potentiate” the next training phase (15,79,81). This concept, referred to as phase potentiation, is essential in the development of endurance-specific performance characteristics.
THE IMPORTANCE OF POWER IN ENDURANCE SPORTS
The development of high-power outputs and high RFDs are vital to success in most sporting events (76) and can differentiate levels of athletic performance (5,6,29). Maximal power output and RFD have conventionally been viewed as fitness characteristics that are less important for endurance sports. This is misguided, however, because there is evidence indicating that average power output over the course of a long-distance race and maximal power output during the final sprint may be critical factors determining the outcome of the event (56,58,80).
THE IMPORTANCE OF STRENGTH IN THE DEVELOPMENT OF POWER
Power is defined as “the rate of doing work” (40) and is quantitatively expressed as power = force × velocity (55). Therefore, an athlete can either achieve greater power outputs by increasing the force production or by increasing the shortening velocity capabilities of skeletal muscle. It is important to note, however, that skeletal muscle shortening velocities are limited by the activity of myosin ATPase, which ultimately dictates the rate of cross-bridge cycling through ATP dissociation (57). Accordingly, this elucidates the vital role of maximal strength in the development of power (76). Simply put, an increased ability to produce force provides the athlete with the opportunity to enhance power production.
TRAINING SEQUENCING FOR THE ENDURANCE ATHLETE
SEQUENCE AND DURATION OF TRAINING PHASES
Originally proposed by Stone et al. (78), strength and power should be developed by cycling 4 distinct phases of training: strength-endurance, basic strength, strength, and power (78). This model of strength and power development, in addition to the concept of phase potentiation, has since been supported by further evidence (30,52,93) and is also referred to as block periodization (38) or the conjugate-sequencing system (90). A 4-week training phase has been previously suggested, using the first 3 weeks to progressively load the athlete, and the final week as an unloading period to modulate recovery (15,63,81). Although the duration of the phase is dependent on the relative training intensity, training volume, time of the season, needs of the athlete, and other external factors. Regardless of the length of each training cycle, however, it is important for practitioners to remember that the rate of decay, or involution of training effects, seems to be directly proportional to the length of the training period (81,94). Consequently, proper sequencing of training phases with appropriate durations will enhance fitness characteristics from prior stages of training and make them more resilient to decay. In addition, the subsequent training phase can be redirected to focus on another fitness characteristic to further the athlete's preparedness and dissipate accumulated fatigue from the previous training cycle (81).
Although there are a number of schematics to choose from when manipulating these variables, a traditional model fits the previously described sequence of strength and power development (79,81). During the general preparation phase, higher volumes of strength training should be used to enhance work capacity and increase lean body mass (15). Despite concerns over increases in body mass, for many endurance athletes, the general preparation phase is one of the few times during the annual plan where small increases in muscle hypertrophy can be achieved. This in turn will potentiate gains in maximal strength and power in subsequent phases of training. As the athlete progresses from the general preparation period to the specific preparation and competition phases of the macrocycle, strength training volume is progressively diminished while training intensity increases, as strength and power become the primary fitness characteristics of interest, respectively (38). Before a culminating event in the competitive season (e.g., championship race), the peaking phase or taper requires “a reduction of the training load during a variable period of time, in an attempt to reduce the physiological and psychological stress of daily training and optimize performance” (53). After the peaking phase, the athlete transitions into the off-season with a period of active rest consisting of recreational activities in which both intensity and volume are reduced and recovery is the objective (81).
TRAINING VOLUME AND INTENSITY
The selection of appropriate training volumes and intensities within each training phase is vital in the facilitation of the desired physiological response. For endurance athletes with limited strength training experience, a traditional model is appropriate (79,81). These athletes should begin with building a neuromuscular base using HFLV strength training, and after a certain strength level is achieved, LFHV strength training can then be implemented (10). This is supported by evidence indicating that among well-trained athletes, LFHV is necessary to make further alterations in the high-velocity end of the force-velocity curve (30,77). Thus, HFLV and LFHV strength training are both important components in the endurance athlete's strength and conditioning program provided they are included at the appropriate time and in the correct sequence (Figure 2).
Regarding high-level endurance athletes, however, the use of a traditional model with a single peaking phase is often impractical, as most athletes will compete in multiple significant events throughout the course of a competitive season. Accordingly, manipulating volume and intensity to produce specific physiological adaptations must coincide with this competitive schedule (80). Unlike the traditional model, after the athlete completes the peaking phase and competes in a key event of the season, further planning will be necessary to prepare the athlete for future competitions of importance (80,81). More specifically, if adequate time exists before the next major event, strength training volume may be increased to re-establish strength levels (63,79). Conversely, if time is insufficient, strength training volume should be increased cautiously to avoid undue fatigue before the next contest (63,80,81).
EXERCISE SELECTION FOR THE ENDURANCE ATHLETE
When selecting exercises for specific phases of training, it is important for practitioners and athletes to consider the transfer of training effect. That is, the degree of performance adaptation that can result from a training exercise (11,81). Therefore, choosing exercises with similar movement patterns and kinetic parameters (e.g., peak force, RFD, acceleration, etc.) will result in a greater transfer to performance (11). Although some endurance sport movements have both closed and open kinetic chain sequences, in movements such as running, closed kinetic chain exercises should be prioritized as they have been suggested to require greater levels of intermuscular coordination (76) and result in greater performance enhancement compared with open chain movements (62,75). Traditional squatting and weightlifting movements are primary examples. Moreover, squat strength has been strongly correlated to athletic movements that require relatively high-velocity, high-power outputs and RFD (6,18). Weightlifting exercises and their derivatives have also shown a strong transfer of training to such movements as well (4,16,37). Practically, these exercises may assist with passing an opponent, enhancing movement economy, increasing average power output, and sprinting the final 100 m of a race (56,58,80). Considering the essential role that these exercises play in the development of strength and power and subsequent effects on HIEE and LIEE, squatting and weightlifting movements should be staples throughout the training year for endurance athletes.
PRACTICAL APPLICATIONS
Previous research on concurrent training for endurance athletes suggests that maximum strength is associated with endurance factors, a relationship that is likely stronger for HIEE activities than for LIEE. HFLV strength training can affect increases in HIEE and LIEE through increasing peak force and RFD. LFHV strength training has also been reported to elicit improvements in HIEE and LIEE performance, however, not all studies agree. When considering findings from studies examining changes in endurance performance and related measures after strength training, it seems that concurrent HFLV strength and endurance training may provide superior results compared with LFHV strength and endurance training for relatively weak endurance athletes. For endurance athletes with more strength training experience, a sequenced approach (e.g., block periodized model) may be more appropriate than trying to improve strength, power, and endurance simultaneously.
A limitation to the current research exists in the design and implementation of training protocols. Some studies comparing different strength training modalities fail to control for differences in strength and endurance training volume between experimental conditions. Another limitation, only controlled for in a few studies, is the addition of strength training without a simultaneous reduction in the volume of endurance training. Practically, if applied in an athletic setting, this could result in poor fatigue management and an increased risk of overtraining syndrome. The implementation of an annual training plan where endurance and strength training variables are carefully manipulated will maximize athletic performance while reducing injury risk by more appropriately managing training volume. Future research should examine the effectiveness of monitoring programs in determining when to manipulate training variables throughout a macrocycle and the subsequent effects on endurance performance.
ACKNOWLEDGMENTS
The authors thank Jacob Goodin for his corrections and editorial comments in preparation of this article.
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