Is it possible to adapt simultaneously to strength training and endurance training? Considerable research has been undertaken in an attempt to answer this question. While there is a preponderance of evidence that suggests that training on endurance compromises strength development, particularly when the same body part is involved, some studies have not found an ‘interference effect’ on strength development as a result of endurance training. The variation between the many studies however may be a result of differences in study design, duration of study, pre-test fitness of subjects, the mode and intensity of training, and strength assessment procedures (Leveritt et al. 2003).
The first study to discuss the interference effect of endurance training on strength development was by Hickson (1980). In this study subjects were assigned to one of three groups: training on strength and endurance simultaneously (SE group), training on strength only (S group) or training on endurance only (E group). The exercise regimen of the SE group combined the separate training of the S and E groups. Hickson’s finding was that the SE group increased in strength for first 6-7 weeks, levelled off in week 8 and then surprisingly lost strength in the final 2 weeks as Figure 3 shows. However the S group continued to improve in strength.
Hickson’s conclusion, therefore, was that at the upper limits of strength development, aerobic training inhibits or interferes with further increases in strength. This view is supported by several studies. Hennessy and Watson (1994) found that improvements in power (vertical jump) and speed (20m sprint time) developed only in a group engaging in strength training but not in a group engaging in combined strength and endurance training (running training 4 days per week). Hennessy and Watson concluded that a combination of strength and endurance training resulted in gains in upper body strength gains but compromised power and strength development in the lower body. This finding adds weight to the view that fatigue is an underlying cause of the interference effect and that it develops as a result of reduced recovery time between strength training sessions. The diminished recovery time may also result in reduced muscle-glycogen content (Nader, 2006).
But in studies where the interference effect is confirmed, subjects have engaged in one form of training or another almost daily. What about if training sessions are spaced more apart does this ease the fatigue effect? A study by Häkkinen and colleagues (2003) found that endurance training conducted simultaneously with strength training did not affect the development of maximal strength or hypertrophy. In this study a subject group performing strength training only (S group) was compared with a subject group performing strength and endurance training (SE group). Subjects in both groups trained only twice per week for 21 weeks. However the study did find a difference regarding the rate of force development (power). The S group improved in rate of force development but the SE group did not. This study raises further questions particularly for those involved in sports where explosive power is a necessity. In Weightlifting, there is a need to carefully consider the content of the training program in regard to speed of movement and power development.
Another reason put forward to explain the interference effect between endurance and strength training is that these two modes of training produce contradictory training stimuli (Sale, MacDougall, Jacobs and Garner, 1990; Bell et al. 1997) and conflicting physiological changes (Hennessy & Watson, 1994). For example, these modes of training are known to have a different effect on the composition of muscle in terms of fibre type and cross-sectional area. Strength training is known to cause hypertrophy particularly in Type IIa fast twitch muscle fibre whereas endurance training does not have this effect. Endurance athletes show a much higher distribution of Type 1 ‘slow twitch’ muscle fibre than do athletes that engaged purely in strength training. There is no evidence that combined strength and endurance training leads to both an increase in the proportion of Type 1 slow twitch muscle fibre and an increase in cross-sectional area of Type IIa fast twitch muscle fibre. Therefore, continual fluctuation between strength and endurance training provides confusing stimuli for adaptive change.
Differences in endocrine response to strength and endurance modes of training should also be taken into account. A study by Kraemer and colleagues (1995) found that subjects engaging in combined strength and endurance training (SE group) showed significant stepping up of levels of the hormone cortisol at each testing stage of the 12 week training period. In addition, the same subjects also showed a significant increase in testosterone in the final 4-week period of the study. Other groups engaging in strength training or endurance training only did not show this dramatic change in testosterone or cortisol, nor did the group that engaged in strength training of only the upper body while performing endurance training. The endocrine response to the combination of concurrent strength and endurance was considered by Kraemer and colleagues to be possible signs of overtraining. Cortisol and testosterone are both hormones that influence total muscle protein and muscle fibre adaptations. Cortisol stimulates conversion of proteins to carbohydrates and therefore is catabolic (degrades muscle protein), and thus negatively influences muscle fibre size, power and strength. Testosterone stimulates protein synthesis and is anabolic in nature. Therefore Kraemer and colleagues considered the rise in testosterone in the SE group in the last 4 weeks to be a compensatory mechanism needed to override the earlier catabolic environment created by high intensity concurrent strength and endurance training. The result that the group engaging in upper body strength plus running training did not show the same elevated cortisol response as the group that performed whole body strength plus running training lead Kraemer and colleagues to also conclude that endurance training appears to compromise strength improvement only when both modes of training are performed using the same musculature.
There may possibly be a gender difference. A study by Bell and colleagues (1997) found that concurrent endurance and strength training programs compromised strength development in women but not in men. A reason given for the gender difference was that in regard to concurrent endurance and strength training, the female subjects showed elevated urinary cortisol during the last 8 weeks whereas the men did not. The combined strength and endurance training (6 days per week) may have induced a catabolic state with reduced recovery time as compared to the 3 days per week strength group. Bell and colleagues cited earlier research by Tsai et al. (1991) that women may adapt differently to men in response to endurance training and, in fact, women may be prone to be hypercortisolic.
For the weightlifting coach, these studies provide evidence that the inclusion of regular high-intensity endurance exercise in the training regimen of the weightlifter is incompatible with long-term goals of strength development and hypertrophy. This will be of interest for those who advocate that it is possible for an athlete to excel in both Weightlifting and Crossfit. If indeed this happens, there is substantial argument that the athlete could go further in strength development if unhindered by prolific endurance training. However, if an athlete is determined to participate in both Weightlifting and Crossfit, it may be possible to design training programs that minimise the interference effect of strength and endurance training by either not training the same muscle group for endurance and strength on the same day, or by extending the recovery interval after endurance training before strength training.
Bell, G., Syrotuik, D., Socha, T., Maclean, I., & Quinney, H.A., (1997). Effect of strength training and concurrent strength and endurance training on strength, testosterone and cortisol. Journal of Strength and Conditioning Research, 11(1), 57-64
Häkkinen, K., Alen, M., Kraemer, W.J., Gorostiaga, E., Izquierdo, M., Rusko, H., Mikkola, J., Häkkinen, A., Valkeinen, H., Kaarakainen, E., Romu, S., Erola, V., Ahtiainen, J., Paavolainen, L., (2003). Neuromuscular adaptation s during concurrent strength and endurance training versus strength training. European Journal of Applied Physiology, 89(1), 42-52, doi:10.1007/s00421-002-0751-9
Hennessy, L.C., & Watson, W.S., (1994). The interference effects of training for strength and endurance simultaneously. Journal of Strength and Conditioning Research, 8(1), 12-19
Hickson, R.C. (1980). Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology, 45(2-3), 255-263
Kraemer, W.J., Patton, J.F., Gordon, S.E., Harman, E.A., Deschenes, M.R., Reynolds, K., Newton, R.U., Triplett, N.T., & Dziados, J.E., (1995). Compatibility of high-intensity strength and endurance training on hormonal and skeletal muscle adaptations. Journal of Applied Physiology, 78(3), 976-989
Leveritt, M., Abernethy, P.J., Barry, B., and Logan, P.A., (2003). Concurrent strength and endurance training: The influence of dependent variable selection. Journal of Strength and Conditioning Research, 17(3), 503-508
Sale, D.G., MacDougall, J.D., Jacob, I., & Garner, S., (1990). Interaction between concurrent strength and endurance. Journal Applied Physiology, 68(1), 260-270
Tsai, L., Johansson, C., Pousette, Å., Tegelman, R., Carlström, K., & Hemmingsson, P., (1991). Cortisol and androgen concentrations in female and male elite endurance athletes in relation to physical activity. European Journal of Applied Physiology, 63(3-4), 308-311