While some research has found that concurrent training may hinder endurance and strength training adaptations, certain protocols can render benifits.
The training for a high-strength, high-power activity such as weightlifting or for a high-endurance, aerobic power event such as marathon running may seem uncomplicated when you are designing a single training program (Nader 2006). Traditionally, low-intensity endurance activities require light-resistance, high-repetition training. On the other hand, high-intensity, short-duration activities benefit from heavy-resistance, low-repetition training (Hickson 1980). Designing programs for sports and activities that demand a combination of strength and endurance training can be more complicated (Nader 2006). In exploring the concept of concurrent training, this article first reviews the adaptations involved in strength and endurance exercise; then looks at the relevant research, both negative and positive; and concludes with practical ideas on how to apply concurrent-training concepts to program design for recreational exercisers and athletes.
The most important adaptation of resistance training is muscular hypertrophy and, thus, increases in strength (National Strength and Conditioning Association [NSCA] 2000). Strength gains are first made within 6–8 weeks as several neuromuscular factors are enhanced: neural activation of the muscle improves; motor unit recruitment and synchronization become more efficient; and there is increased excitability of the alpha motor neurons. Later strength gains are seen with subsequent muscle fiber enlargement (NSCA 2000; Millett et al. 2002). Additionally, strength acquired through resistance training has been shown to enhance speed and anaerobic capacity (NSCA 2000).
Strength training has an immense effect on other performance variables as well. It can increase muscle endurance for high power output and maximal rate of force production, and it can improve a client’s vertical jump ability. In addition to hypertrophic effects, strength training elicits other muscular adaptations, such as decreases in capillary and mitochondrial density; increases in the amount of fast heavy-chain myosin; and conversion of type II muscle fiber subtypes to almost all type IIa muscle fibers (NSCA 2000).
Other physiological variables affected by strength training include enzymatic activity, metabolic energy stores, connective tissue and body composition. Enzymatic activity within the muscle increases for creatine kinase, myokinase and phosphofructokinase. Interestingly, these enzymes are important regulators of the two anaerobic energy systems mentioned previously. Strength training also raises the stores of ATP, creatine phosphate, glycogen and possibly triglycerides in muscle. Other proposed benefits include improvements in ligament and tendon strength, collagen content and bone density. Resistance training also decreases body fat percentage and increases fat free body mass ( NSCA 2000).
Endurance and strength training often present opposing adaptations ( Chtara et al. 2005; Glowacki et al. 2004). The main adaptation of endurance training is an increase in the muscle’s aerobic capacity. This allows an individual to perform a given exercise intensity for a prolonged period of time and at a higher maximal aerobic power, or VO2max, than was possible prior to training. Examples of endurance training include running, swimming, cycling and recreational sports such as basketball. According to the NSCA, endurance training increases cardiovascular power, but it does not increase muscular strength or size (NSCA 2000).
Endurance training may increase aerobic power and muscle endurance during low-power-output activities, but it has no effect on muscle strength in consort with maximal rate of force production, vertical jump, anaerobic power or sprint speed. Again, endurance training does not affect muscle size or fast heavy-chain myosin, but it does increase capillary and mitochondrial density in the muscle ( NSCA 2000).
Like strength training, endurance training has positive effects on creatine kinase and myokinase activity and raises the levels of stored ATP, creatine phosphate, glycogen and intramuscular triglycerides. Ligament and tendon strength improves, and—provided the activity is weight-bearing—bone density increases, in proportion to the load. Endurance training can also decrease body fat percentage (NSCA 2000).
Research has shown that simultaneous benefits in strength and endurance from concurrent training may not be possible. In fact, some studies, but not all, have shown that concurrent training may hinder training adaptations for both muscular endurance and strength (Glowacki et al. 2004; Hickson 1980; Leveritt et al. 1999; Nader 2006). Hickson (1980) determined that little or no benefit was possible for endurance athletes performing concurrent training and, in addition, concurrent training had detrimental effects for strength training athletes. Glowacki and others (2004) showed a significant increase in VO2peak with endurance training, but not with strength or concurrent training, demonstrating that resistance training might have a negative effect on endurance training. In this same study, the authors suggested that strength training could be superior to concurrent training when muscle power was the ultimate goal and that concurrent training might interfere with improvements in cardiovascular capacity (Glowacki et al. 2004).
Lactate threshold can be a marker of cardiovascular endurance, and researchers have studied threshold changes in relation to concurrent training. In trained endurance athletes, strength training may not have a negative effect on lactate threshold, but it is also unlikely to supply the necessary stimulus to have a positive effect (Jung 2003; Paavolainen et al. 1999). Unfit individuals, on the other hand, may actually see a benefit from strength training in addition to endurance training.
Other negative drawbacks of introducing strength training to an endurance regimen, besides the ineffectiveness in enhancing either strength or endurance (according to some studies), are body mass gains and possible injury (Jung 2003).
Acute fatigue is a possibility when performing endurance and strength training on the same day. The first bout of activity, whether it is endurance or strength training, can cause exhaustion that may carry over and compromise gains in the second bout of activity.Over time this cycle can lead to either no training adaptations or very minimal positive results compared with no first bout of activity (Chtara et al. 2005; Leveritt et al. 1999; Leveritt et al. 2003). The recovery time between bouts can also be a limiting factor when trying to obtain advantageous strength and endurance adaptations (Leveritt et al. 1999).
The neuromuscular adaptations that occur in fast- and slow-twitch motor units during endurance training may affect resistance training gains and hinder strength development. Enhancing the metabolic and contractile properties of slow-twitch motor units through endurance activity negatively impacts the function and recruitment of fast-twitch motor units, thereby affecting overall strength and power output development (Dudley & Fleck 1987). Endurance training may also reduce power, owing to the neuromuscular system’s decreased ability to generate force. Therefore, concurrent training may hinder strength development by impairing the neuromuscular system’s ability to recruit motor units efficiently (Dudley & Fleck 1987; Leveritt et al. 1999; Leveritt et al. 2003).
Overtraining refers to any physiological or psychological factor that leads to a decrease or no alteration in performance even when there is adequate training stimulus (Hickson 1980). Fatigue caused by overtraining can result from excessive training frequency, volume or intensity and also from lack of proper rest and recovery (NSCA 2000). Some researchers have suggested that individuals who perform concurrent endurance and strength training are more susceptible to overtraining than those who do strength or endurance training alone (Bell et al. 1991; Leveritt et al. 1999; Leveritt et al. 2003).
The body adapts to both endurance and strength training, but a conflict in the skeletal muscle can occur with concurrent training. Skeletal muscle will attempt to adapt to both training extremes, which can be hard or even impossible (Leveritt et al. 1999). The difficulty occurs because the muscle cannot positively adapt to two different demands from two different energy pathways during the same training session (Chtara et al. 2005). Research has illustrated that when endurance training is added to strength training, limitations in muscle hypertrophy can interfere with strength gains (Bell et al. 2000; Leveritt et al. 1999).
Concurrent training can alter endocrine responses by causing changes in the anabolic-catabolic hormone balance. Strength training supports anabolic processes, such as muscle hypertrophy, whereas endurance training is more catabolic in nature, thus hindering strength development (Leveritt et al. 1999). When the body functions in a catabolic state, cortisol levels increase above normal levels. With little or no change in anabolic hormone concentration (testosterone, growth hormone) in this catabolic state, overtraining is likely with concurrent training (Bell et al. 1991; Bell et al. 2000).
To be advantageous in an endurance regimen, strength training must affect certain variables responsible for running performance, such as VO2max, lactate threshold and running economy (Jung 2003). Research suggests that anaerobic factors and neuromuscular characteristics (the interaction between the nervous and muscular systems) may also play a role in endurance performance.
Chtara and others (2005) demonstrated positive results when combining endurance and strength exercises. They found that concurrent training produced improvements in aerobic capacity and endurance performance, especially when endurance training preceded strength training in the same session (Chtara et al. 2005). The increases in strength that McCarthy and others (1995) observed during concurrent training were similar to the gains they observed when strength training was performed separately. These researchers also demonstrated that combination strength and endurance training did not hinder increases in aerobic fitness that would also be produced by endurance training alone. They concluded that the increases in aerobic capacity might be related to the increase in fat-free mass and the muscle hypertrophy elicited by strength training and potentially to the short rest periods and high intensity of the strength program.
Paavolainen and colleagues (1999) determined that a combination of explosive strength training and endurance training significantly improved 5K running performance in endurance athletes. The progressions were made without changes in VO2max, suggesting increases in muscle power and running economy. Strength training may decrease ground contact time by improving the stretch-shortening cycle, thus enhancing running economy (Paavolainen et al. 1999). In a different study, researchers observed increases in running economy following concurrent strength and endurance training in well-trained triathletes (Millett et al. 2002).However, these improvements did not occur with endurance training alone, possibly because of a decrease in muscle power and negative changes in neuromuscular characteristics.
Muscle stiffness, degree of neural input to the muscles, motor unit synchronization and motor unit recruitment can all affect running economy (Jung 2003; Paavolainen et al. 1999). Millett and others (2002) concluded that the addition of heavy weight training to an endurance training program did not affect VO2 kinetics in well-trained triathletes. The researchers found that a greater number of fast-twitch motor units were recruited to compensate for the decrease in power output caused by the fatigued muscle. By improving muscular power and neuromuscular characteristics through strength training, an individual can positively affect his endurance through improved running economy (Jung 2003).
When training to improve endurance in recreational athletes, it is important to emphasize maximal force in a strength program (Hoff, Gran & Helgerud 2002). Strength training may not increase VO2max, but it can still be beneficial to an endurance athlete. Improvements in endurance training can occur with some type of strength training through benefits garnered from neuromuscular characteristics, running economy or anaerobic capacity (Jung 2003).
Frequency may be the most important factor when combining strength and endurance training (McCarthy et al. 1995). Limit the frequency of same-day concurrent training to no more than 3 days a week (Bell et al. 1991; Leveritt et al. 2003; McCarthy et al. 1995; McCarthy, Pozniak & Agre 2002). Always consider which mode is more important for the individual. For example, if a client’s goal is to increase endurance for a 10K road race, then endurance training should be performed prior to strength training, or on alternate days, so as not to compromise results. Most important, note that performing endurance and strength exercises on the same day may compromise strength development (Sale et al. 1990).
The intensity factor applies to both endurance and strength training. According to Chtara and others (2005), a minimum of 50% of VO2max must be attained to see cardiovascular advantages. Additionally, an intensity of 70% of heart rate reserve has been shown to elicit cardiovascular fitness without compromising strength development (McCarthy et al. 1995; McCarthy, Pozniak & Agre 2002). The recommended volume of strength training is 3 sets of 6–8 repetitions of eight lifting exercises with approximately 75 seconds of rest between sets and 2 minutes of rest between each lifting exercise (McCarthy et al. 1995; McCarthy, Pozniak & Agre 2002). For clients over the age of 50 years, 10–15 repetitions are recommended. Vary strength training intensity and volume by weight, number of repetitions, length of rest periods or number of sets. Perform strength training exercises at moderate to slow speed, through full range of motion and in a controlled manner, emphasizing both the concentric and eccentric phases (ACSM 2000).
The type of exercise used for endurance training can be any of the individual’s favorite exercises or activities. The greatest improvements in cardiorespiratory fitness are seen with exercise that involves large muscle groups; for example, walking, hiking, running, stair climbing, swimming, cycling, rowing or cross-country skiing (ACSM 2000). The type of exercise used for strength training might include circuit training, traditional strength training or explosive weight training. The workout should address the entire body; that is, the exercises should include squats, leg extensions, hamstring curls, bench presses, lat pull-downs, biceps curls, shoulder raises and abdominals work (Leveritt et al. 2003;McCarthy et al. 1995;McCarthy, Pozniak & Agre 2002).
In relation to an athlete’s sport performance, design a program that optimizes the training factors essential to the client’s sport (Glowacki et al. 2004). (See “Concurrent Training for a Recreational Triathlete” chart for an example). As a reminder, if cardiovascular fitness is the client’s main objective, make sure that, in a same-day workout, endurance training precedes strength training. Performed first, strength training can produce muscle fatigue, thus reducing the cardiovascular effectiveness of endurance training (Chtara et al. 2005).
According to McCarthy and colleagues (1995), 50 minutes a day of either strength or endurance training is sufficient to promote both aerobic and strength development (McCarthy, Pozniak & Agre 2002). The time, or duration, will depend on the activity’s intensity; for example, perform moderate-intensity activity for 30 minutes or more and high-intensity activity for 20–30 minutes. Increase duration gradually as the individual adapts to the training stimulus without fatigue or injury (ACSM 2000). (See the charts for guidelines on concurrent training, endurance training progression and strength training progression for recreational exercisers.)
Kriston Koepp, MS, CSCS, is an exercise physiologist and sports performance coach at Performance Edge Sports Training in Hayden, Idaho. She specializes in training high-school and college athletes of all sports.
Jeffrey M. Janot, PhD, is the technical editor of IDEA Fitness Journal. He is an assistant professor of human performance in the department of kinesiology at the University of Wisconsin–Eau Claire.
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