Concurrent Strength and Endurance Training

     You know exercise can change your body. Through exercise, one can change parameters such as body composition, endurance capacity, maximal strength, efficiency, etc. The basis behind these changes is that the stress of exercise causes adaptations to occur in the body. The rule of specificity tell us these adaptations are specific to the training stimulus (Hickson).  For example, the most effective way to increase a muscle’s maximum strength would be to specifically train that muscle through short-duration, high intensity resistance training, not through prolonged, low resistance repetitive contractions. Conversely, endurance based athletes like marathon runners or long distance cyclists, should train specifically for their endurance events by training over prolonged amounts of time with a lower sustainable resistance or work rate. With this said, how should athletes in events that require a mixture of strength and endurance train? Employing a training regime combining strength and endurance training may yield the best results, but research indicates that for maximum strength gains, the two types of training should not be done in the same session, or even the same day.
     First, let’s discuss these two types of training. Strength training is generally an activity of short-duration done a very high or maximal intensity. These exercises, such as weight training or plyometrics, increase one’s capacity to do high-intensity, high-resistance work. Some athletes rely more on high-intensity, high-resistance work than others. For example, the general goal of football players, powerlifters, and sprinters is to generate as much force as possible for short periods of time. Following the rule of training specificity, these athletes should train specifically for their respective high intensity event that requires rapid, high-force generation and the best way to do this is by strength training. Through strength training, athletes improve their strength by learning how to recruit, or signal, more motor units. Additionally, those motor units recruited at high thresholds trigger larger, faster, higher force generating fibers. Further, by recruiting more motor units, more fibers are recruited, more fibers contribute to the effort and more force can be generated by the muscle(s). Strength training also results in muscle cell hypertrophy. Hypertrophy is an increase in muscle size or volume. This increases the number of contractile proteins in the muscle cells and enables the muscles to generate more force.
     At the opposite end of the spectrum, endurance athletes do not rely primarily on high-power movements. Instead an endurance athlete’s performance relies strongly on his aerobic capacity. This is the ability of one’s body to take in and use oxygen. An increase in endurance performance may result from an increase in VO2max, or the maximal rate that oxygen can be taken up and delivered to the muscle cells. Improvements in maximal muscular force generation will not play much of a role in endurance events.  Adaptations from endurance training include increased heart stroke volume, increased mitochondrial and capillary density as well as increased aerobic enzyme content in muscle cells. These adaptations directly increase the body’s ability to deliver oxygen and produce ATP aerobically.
     But what about events in the middle of the strength-endurance exercise continuum? Sports such as short to middle distance running (400m - 5000m), track cycling, and swimming, rely on both strength and endurance adaptations. Logically, one might think that combining the two types of training would yield favorable results; and he would be right to a degree. But recently research has shown that through concurrent strength and endurance training, athletes do not get the same benefits as they would if they separated their strength workouts from their endurance based workouts. One study found that same day, compared to alternate day, training impaired strength development in one repetition leg press tests. The authors of the study found that both the subjects that alternated strength and endurance training and the subjects that did concurrent training sessions increased their one repetition leg press max, but the group that alternated days had a significantly greater increase. 
     The authors offered a few theories that could account for this phenomenon. One theory was that they believed the quality or the volume of the strength training session was reduced following endurance training. The reduction in quality was due to already fatigued muscles. Alternatively, the volume of the strength training session was also reduced when it was done prior to endurance training because the athletes were anticipating a strenuous endurance training session and did not want to fatigue themselves before a long and difficult training bout (Sale et al).
     Another proposed theory for the inhibition of strength gains through concurrent training is the theory of overtraining.  Overtraining occurs when the volume and intensity of an individual's exercise exceeds their recovery capacity. It is generally characterized by a decrease in athletic performance. One study showed strength declined during the ninth and tenth weeks of concurrent training. The author believed the mechanism behind the decrease was overtraining because the participants were training 80 minutes per day (Hickson). Deeply connected with the overtraining theory is the theory of low muscle glycogen.   Much like overtraining, low muscle glycogen results from the inability of glycogen stores to replenish themselves from session to session. Chronically low glycogen stores could impair subsequent workouts, especially high intensity exercise. Athletes training more strenuously or frequently are more likely to experience overtraining or glycogen depletion and these states will impair recovery from exercise sessions reducing the favorable adaptations (Nader).
     Dudley and Djamil found that concurrent training reduced the magnitude of the increase in muscular strength in high-velocity low-force contractions, but did not alter the magnitude of the increase in high-force low-velocity contractions. Previous studies had proven high-speed low-resistance exercises would increase muscle strength at that same high speed. Here, the combination of training negatively affected the said adaptation. This means that concurrent training did not affect low-speed strength, but decreased the favorable adaptation to high-speed contractions. This adaptation would not be favorable to a middle distance, combined strength and endurance athlete because these athletes rely on fast explosive movements. Dudley and Djamil’s proposed theory was that the resistance component of concurrent training negatively affects the adaptation that would normally be seen with high intensity endurance training on a neural level (Dudley, Djamil). But it should be noted this proposal has not been proven and there is little evidence to suggest that endurance training negatively affects recruitment of fast-fatigable motor units (Nader, 2006).
     Another, and what I find to be the most interesting mechanism behind the incompatibility of concurrent training occurs on a molecular level. Biochemical and genetic based studies have revealed that muscle cells have specific cellular regulatory process that can be induced or hindered by certain forms of exercise. Resistance and endurance training activate different signal pathways that initiate different responses from muscle cells. Studies show that acute resistance exercise activates a signaling pathway that in turn increases protein synthesis (see figure 1). This increase in protein synthesis leads to hypertrophy of muscle cells thus increasing the force they can generate. Conversely, aerobic exercise activates another signal pathway that increases metabolic adaptations favorable to endurance exercise.  Endurance exercise stimulates AMPK, an important energy regulator in skeletal muscle (figure 2). Recent studies show that the two signal pathways have antagonistic effects on each other. This means that the activation of AMPK from aerobic exercise inhibits protein synthesis (Nader).
Figure 1. Resistance training activates IGF/Calcineurin Pathway which activate mTOR and leading to hypertrophy. Slide from Morris 2012.
Figure 2. Decreases in Glycogen, CP, and ATP (from endurance training) activates AMPK and inhibits mTOR - inhibiting hypertrophy. Slide from Morris 2012.

     There is a number hypotheses as to why concurrent training may result in less than optimal adaptations for strength gains and possibly aerobic efficiency. Regardless of the varying hypotheses, all of the studies have one thing in common: that concurrent strength and endurance training is not as effective as separated training when the goal is improving strength, hypertrophy, or rate of force development. After reviewing this research, it appears that it would be most beneficial for the athletes participating in events that combine strength and endurance components to arrange their workouts into separate sessions. This way they can reap the benefits of both their strength and endurance training.
     But while there is substantial evidence to suggest that aerobic training inhibits hypertrophy, there is still little evidence demonstrating that endurance training negatively effects motor unit recruitment when done concurrently with strength training. Yes, Dudley and Djamil found that force of high speed contractions was limited -- but this study also recruited 14 untrained college aged women. Hakkinen found that rate of force development was limited, but here again this study recruited a number of untrained women. In the end, I believe the jury is still out on concurrent training. While it may inhibit strength gains from hypertrophy, I still think improvements can be made in the weight room through improved recruitment. Additionally, by recruiting these fast-fatigable motor units and fast twitching fibers again and again -- are they not being trained? Can they not improve their glycolytic and/or aerobic capacities? After all, once the slower muscle fibers become fatigued, won't the faster fibers will be recruited to get the job done?
     What the evidence tells us: If your goals are hypertrophy and strength gains -- stick to the weight room. If your goal is improved strength, power, and middle/long distance running performance -- perhaps separate sessions (and periodization?) are the way to go. Additionally, there has been much debate over whether distance runners should lift low weight with high repetitions or high weigh twith low repetitions. The bottom line, the heavier the weight, the greater force output required, the more motor units recruited. I leave you with a Flotrack video of Galen Rupp in the weightroom: Rupp WOW Rupp's results speak for themselves.

Further reading:


Dudley, Gary A., and Rusdan Djamil. "Incompatability of Endurance- and Strength-Training Modes of Exe

rcise." Journal of Applied Physiology 59.5 (1985): 1446-51. Web. 8 Nov. 2010. <>.

Hickson, Robert C. "Interference of Strength Development by Simultaneously Training for Strength and Endurance." European Journal of Applied Physiology 45 (1980): 255-63. Web. 17 Nov. 2010. <>.

Nader, Gustavo. "Concurrent Strength and Endurance Training: From Molecules to Men." Medicine & Science in Sports & Exercise 38.11 (2006): 1965-70. Web. 4 Nov. 2010. <>.  

Sale, D.G., et al. "Comparison of two regimens of concurrent strength and endurance training."Medicine & Science in Sports & Exercise 22.3 (1989): 348-56. Web. 5 Nov. 2010 < msse/Abstract/1990/06000/Comparison_of_two_regimens_of_concurrent_strength.12.aspx>.


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