Running economy, yes! VO2max, no!
A 1988 study by Dr. R.C. Hickson and colleagues at the University of Illinois at Chicago investigated whether adding strength training to an endurance training program would improve endurance performance. The investigators had 8 well-trained cyclists and runners add weight-training to their exercise programs. The subjects performed 3-5 sets of 4 leg exercises, 3 times per week for 10 weeks. Leg strength increased by 27% for parallel squats, 37% for knee extension, and 25% for knee flexion, over the 10 weeks. The weightlifting, however, did not result in any change in the subjects' maximal oxygen consumption (VO2 max).
Short-term endurance was measured by having the subjects cycle and run as hard as possible for 4-8 minutes. When re-tested after weight-training, the subjects had increased their time to exhaustion by 11% during cycling and 13% during running. Similarly, the length of time that the subjects were able to cycle at 80% of VO2 max increased from 71 minutes before weight-training to 85 minutes after weight-training.
The effect of the weight-training on the subjects' running performances, however, were less clear-cut. Two of the 8 subjects were injured from the heavy-resistance training, and were unable to complete the post-weight-training 10 K run. Of the other 6 subjects, their 10 K times improved from an average of 42:27 before the weight-training to 41:43 after weight-training. While this improvement was not statistically significant, it does suggest that the weight-lifting had some positive impact on running performance.
A 1995 study, published in Medicine and Science in Sports and Exercise, found similar results concerning VO2 max. Dr. John McCarthy and colleagues at the University of Wisconsin-Madison Biodynamics Laboratory, found no increase in peak VO2 with the addition of strength training to an endurance training program. In this study, sedentary adult males were divided into 3 groups. One group strength trained, a second group endurance trained, and a third "combined" group both strength and endurance trained. Peak VO2 on the cycle ergometer, increased by 18% in the endurance-trained group and by 16% in the combined group. Neither this study nor the University of Illinois study, therefore, found an increase in max VO2 or peak VO2 with the addition of strength training to an endurance-training program.
A 1994 study from the Department of Kinesiology at the University of New Hampshire, however, strongly supports weight-lifting for distance runners. Exercise physiologist Ron Johnston found improvements in running economy after a 10-week weight-training program. In this study, 12 trained female distance runners were split into groups for 10 weeks of training. The experimental (Run & Lift) group continued to run, and added a strength training regimen consisting of 14 exercises working the upper body, abdominals, and legs, 3 times per week. The control (Run) group just ran.
The Run & Lift group improved their upper body strength by 24% and leg strength by 34%. VO2 max and lactate threshold VO2 did not change in either group, which is consistent with the results of the studies already mentioned. The most interesting finding in the UNH study was that running economy improved significantly in the Run & Lift group, but did not change in the Run group. Running economy improved by over 2% at the 3 running speeds used in the study. Johnston explains, "Strength-training improves running economy either due to a reduction in wasted motion, or because stronger legs allow runners to rely more heavily on their more economical slow-twitch muscle fibers." Johnston is no idle observer of the sport, having won the 1994 Maine 50-miler.
http://www.pfitzinger.com/labreports/weights.shtml