Resistance training increases demands on many of the tissues in your body: muscles, tendons, ligaments, bones, etc.  Increasing these tissues' ability to withstand greater forces, especially those forces repeated thousands or millions of times (as in every time your foot hits the ground during running), potentially decreases the risk of injury.

 

In any activity in life, bad reps are bound to happen (we all bite our tongue occasionally, even though we've been chewing since infancy), but increasing your resiliency can help mitigate the damage done when this happens.

 

Every task requires a certain energy demand affected by a number of factors. In running, for instance, these factors include: cardiovascular fitness, stride length, stride frequency, training age, soft tissue/joint integrity, and many others. Resistance training increases your MOBILITY, which we will define here as “the ability to actively access a given range of motion”...this is very different that FLEXIBILITY, which is just the ability to move a joint to a certain range.  Just because the joint is capable of getting there, doesn't mean it has any useful functional capacity at that point. 

 

Lifting through a full range of joint motion decreases your energy demands during activities that might use only a partial range, because you're asking your body to access a lesser percentage of its total useful range. 

 

Think of it this way: if you can easily perform a full range lunge (long stride, back knee almost to the ground, good posture) without much effort, your energy demand decreases while running, when you're accessing only a fraction of that full range. Someone who has difficulty moving outside of their running gait is going to fatigue faster and have a much harder time maintaining speeds that are demanding versus the person who has joint control in a wider range of motion.