Back to reviewing some research literature here. The study I chose is a joint project between the University of Montreal and University of Poitiers, France, and concerns the effect of plyometric vs. weight training on the energy cost of running.
Source: Journal of Strength and Conditioning Research 24 (7) 1818-1825, 2010.
The energy cost of running refers to the effort needed to achieve a particular running performance. Or in other words is a measure of the effort and relative intensity needed to run a certain pace for a determined distance. The less effort it takes one to keep a pace the longer the runner can hold that pace, and so the better their performance will be.
In the introduction the authors cite various studies that looked at how runners improved their performance through plyometric training (explosive work such as depth jumps and rebound work. The present study took 35 trained endurance runners and divided them into plyo groups and a group that did strength training.
The strength training group did ONE session per week of from 3-6 sets of 8 repetitions of lower body squats in a smith machine (poor choice in my opinion) at a relatively high intensity in order to maximize peak force output. The other group did reactive rebound jumps from a 20-60cm box in order to improve power output. Both are fairly low volume, high intensity protocols.
Another group did no strength or power work, only endurance running.
Results: Both strength and power training groups improved the efficiency of the energy expenditure, with the depth jump group showing slightly better improvements. Results were better for the lower level runners than more experienced runners, which the authors hypothesize is because stronger, higher level runners need higher intensity and greater volume to affect their performance - which makes sense.
The bottom line is that doing some form of strength and power training, even only once a week, which is far from optimal according to other research out there on athletic training, does produce results. So if you are an endurance athlete looking to improve your performance then you should get on a strength & conditioning program designed according to your condition and needs.