Here's some study's done that show no significant difference in doing 1 set per exercise vs. multiple sets. Whole writeup is found
here
"A low volume program, one set of each exercise, results in increases in muscle size and function similar to programs with two to four times as much volume."
Hass et. al. (2000) compared the effects of one set verses three sets in experienced recreational weightlifters. Both groups significantly improved muscular fitness and body composition during the 13 week study. Interestingly, no significant differences were found between groups for any of the test variables; including muscular strength, muscular endurance, and body composition.
During the 1980s, we conducted a much larger research project comparing the Nautilus principle of single-set strength training with two and three sets of strength exercise (Westcott, Greenberger, & Milius, 1989). The 77 subjects were experienced strength trainees who agreed to participate in a ten-week program of bar dips and chin-ups. All of the subjects were pretested for the maximum number of bar dips and chin-ups they could perform with proper technique. The subjects were divided into three training groups. Group One performed one set of bar dips and chin-ups, Group Two performed two sets of bar dips and chin-ups, and Group Three performed three sets of bar dips and chin-ups, three days per week throughout the study. The only difference between the three training groups was the number of sets performed during each exercise session.
The final study by the Pollock group (Medicine and Science in Sports and Exercise. Supplement 30(5): S115, 1998) addresses the training experience issue. As you'll recall, some have suggested that experienced trainers might benefit from higher volume. In other words, after you've been training for a while, you need increased volume to continue progressing - more is better. According to this study, those people should think anew.
The researchers recruited 40 adults who had been performing one set to muscular fatigue, using nine exercises, for a minimum of one year; average training time was six years. The participants were randomly assigned to either a one-set or three-set group; both groups did 8-12 reps to failure three days per week for 13 weeks.
Both groups significantly increased their one-rep maximum strength and endurance. There was no significant difference in the gains made by the two groups in the leg extension, leg curl, bench press, overhead press and arm curl. The researchers concluded:
"These data indicate that 1 set of [resistance training] is equally as beneficial as 3 sets in experienced resistance trained adults."
Another research group, K.L. Ostrowski and colleagues, tested "the effect of weight training volume on hormonal output and muscular size and function" in experienced trainers. (Journal of Strength and Conditioning Research. 11(3): 148-154, 1997) Thirty-five males, with one to four years weight-training experience, were assigned to one of three training groups: one-set, two-sets, or four sets. All participants did what I would call a periodized routine; they changed the rep range every few weeks. They did free-weight exercises four times a week for ten weeks using 12 reps maximum (week 1-4), 7 reps max (week 5-7) and 9 reps (week 8-10). All sets were performed to muscular fatigue with three minutes rest between sets. The only difference between the three programs was the number of sets.
As in the Pollock group studies, no significant differences in results were found. The authors concluded: "...A low volume program ... [one set of each exercise] ... results in increases in muscle size and function similar to programs with two to four times as much volume."