There have been studies which showed that simultaneous endurance and strength training produce interference effects. The effect is a loss of strength but not of endurance.
An early (1980) study (Hickson RC. Interference of strength development by simultaneously training for strength and endurance. Eur J Appl Physiol Occup Physiol. 1980;45(2-3):255-63.) stated:
The purpose of this study was to determine how individuals adapt to a combination of strength and endurance training as compared to the adaptations produced by either strength or endurance training separately.
There were three exercise groups: a strength group (S) that exercised 30–40 min . day-1, 5 days . week-1, and endurance group (E) that exercised 40 min . day-1, 6 days . week-1; and an S and E group that performed the same daily exercise regimens as the S and E groups.
After 10 weeks of training, VO2max increased approx. 25% when measured during bicycle exercise and 20% when measured during treadmill exercise in both E, and S and E groups. No increase in VO2max was observed in the S group.
There was a consistent rate of development of leg-strength by the S group throughout the training, whereas the E group did not show any appreciable gains in strength. The rate of strength improvement by the S and E group was similar to the S group for the first 7 weeks of training, but subsequently leveled off and declined during the 9th and 10th weeks.
These findings demonstrate that simultaneously training for S and E will result in a reduced capacity to develop strength, but will not affect the magnitude of increase in VO2max.
The graph in Hickson shows the interference effect on strength training. The strength alone group had continued strength gains. The Strength plus Endurance group increased in strength but then declined.
Another study (1985) showed similar interference effects (Dudley GA, Djamil R. Incompatibility of endurance- and strength-training modes of exercise. J Appl Physiol (1985). 1985 Nov;59(5):1446-51.).
Another study (Gustavo A Nader. Concurrent strength and endurance training: from molecules to man. Med Sci Sports Exerc. 2006 Nov;38(11):1965-70.) (PDF) described the interference effect as:
situations when strength and endurance training are performed simultaneously, a potential interference in strength development takes place, making such a combination seemingly incompatible
The paper described a number of possible causes for the interference effect.
At the molecular level, there seems to be an explanation for the interference of strength development during concurrent training; it is now clear that different forms of exercise induce antagonistic intracellular signaling mechanisms that, in turn, could have a negative impact on the muscle’s adaptive response to this particular form of training. That is, activation of AMPK by endurance exercise may inhibit signaling to the protein-synthesis machinery by inhibiting the activity of mTOR and its downstream targets.
One of the possible reasons is the reduction of glycogen stores from both modes of training. However, this assumes that endurance training is performed at a heart rate level which oxidizes carbohydrates. If endurance training is done in the aerobic zone then carbohydrate stores are not reduced.
This raises the question “What is Endurance Training?”