Endurance training works one of two physiological energy production systems in our body; the aerobic cardio respiratory system, while resistance or strength training works our anaerobic energy system. Energy is used primarily in the muscle fibers, often referred to as the slow twitch and fast twitch fibers. The aerobic system trains our type I muscle fibers, which are more densely packed with mitochondria which utilize oxygen to make ATP. Meanwhile, the type II resistance fibers create ATP in the absence of oxygen by splitting molecules. Since many sports and other events rely more heavily on one type of fiber more than the other, athletes and the general public often train for that specificity. Most the population is composed of each fiber type by a 50:50 split, but specificity in training can shift the ratio of fibers either way slightly.
What happens when an endurance runner trains not just for their endurance abilities but also for strength simultaneously?
There have been multiple studies on this topic, with some studies suggesting that training simultaneously with both strength and endurance takes away from the optimal performance of one to improve the other, meaning you can be highly trained for endurance or just moderately trained for both strength and endurance, or vice versa. However, in opposition to many of those studies, others have tweaked the study method in finding how both can be trained for the benefit of improving upon an already trained ability. That is to say, if a runner was to train aerobically and then perform strength exercises used in running, would they improve? In the Journal of Strength and Conditioning, one of many studies was conducted on well trained endurance runners on how the addition of strength training to endurance training would affect stride ability. Groups were assigned to a periodized sport specific strength conditioning program with endurance (strength exercises changed each week), a consistent sport specific strength conditioning program with endurance (same workout), and an endurance only group. They found that the combination of periodized strength conditioning program with endurance training greatly resisted fatigue in overall strides than both the other groups. The exercises were sport specific to running, including squats, calf raises, hamstrings, and others. Many studies done on cyclists, soccer players, and rowers suggest the same findings.
There are several things to note on how this training would optimize performance. Strength was done at sub maximal weights and moderate repetitions – never to fatigue. This type of training will train the muscles for more endurance prolonged use, while enhancing strength. In the case of the runners the addition of strength increases the threshold of fatigue that the muscles endure by improving power. More muscular strength in the leg muscles contribute to greater power in each stride the runner takes. With that said, the studies in which competitors improved both modes of exercise were all trained with sport specific muscle groups. For instance a competing runner would not want to bulk their upper body like a rower might; it would only take away from their running ability. Studies among the general population show that those who want to improve health should train both modes for better overall conditioning. Strength and endurance training does not seem to negate one or the other for improving health and rehabilitation, but for those with an athletic specific goal, remaining sport specific is key.