I have distilled this excellent article, from IDEA exercise journal, titled:

Muscle Activity and Body Position, by Jason R. Karp, PhD.

It answers the question: Do slight changes in body position affect muscle activity?

You bet they do! Read below.

Regarding: Abdominal Crunches

With all the abdominal training devices on the market, how can clients know which is best for their abs? The answer: you are probably best served sticking with the floor or mat. There has been enough research comparing commercial abdominal devices with traditional mat or floor crunches or sit-ups to conclude that traditional crunches are just as effective or better for eliciting abdominal muscle activity.

When you do crunches, lie on an unstable surface. The rectus abdominis (6-pack muscle) and external obliques (waist muscles) are more active during crunches performed on a stability ball with feet on the floor and back on the ball than during crunches done while lying on a stable surface.

Sternlicht et al. (2007) found that abdominal muscle activity was significantly greater during stability ball crunches than during floor crunches only when the ball was placed under the lower back. Placing the ball under the upper back, below the shoulder blades, actually resulted in less abdominal activity than doing crunches on the floor.

BOTTOM LINE:
To maximize how muscles are working, follow these guidelines:

Abdominals

Have your clients do crunches on a stability ball with the lower back on the ball and the upper body lifted off the ball.

For Other Muscles:

Quadriceps (Thighs)

Do squats.
Use whatever leg position, stance width and squat depth are right for your body, since all are equally effective.

Gluteus Maximus (Buttocks)

Do squats.
Use a wide stance and squat until your thighs are past being parallel to the floor; unless you have knee issues.

Chest

Bench press.
Use a hand-grip that is nearly double your shoulder width.

Push-up.
Try a narrow hand position with your hands together, under the center of your chest-plate, forming a diamond shape between your thumbs and index fingers.

Upper Back
Do the seated row machine.

Jason R. Karp has a PhD in exercise physiology and is director and coach of REVO2LT Running Team. He is a freelance writer, a fitness consultant and an adjunct professor at Grossmont College in El Cajon, California. Karp writes for running, coaching and fitness magazines and currently coaches and consults through his website, www.runcoachjason.com.

References:

Sternlicht, E., et al. 2005. Electromyographical analysis and comparison of selected abdominal training devices with a traditional crunch. The Journal of Strength and Conditioning Research, 19 (1), 157–62.

Sternlicht, E., et al. 2007. Electromyographic comparison of a stability ball crunch with a traditional crunch. The Journal of Strength and Conditioning Research, 21 (2), 506–509.

Stoutenberg, M., et al. 2005. The impact of foot position on electromyographical activity of the superficial quadriceps muscles during leg extension. The Journal of Strength and Conditioning Research, 19 (4), 931–38.

Vera-Garcia, F.J., Grenier, S.G., & McGill, S.M. 2000. Abdominal muscle response during curl-ups on both stable and labile surfaces. Physical Therapy, 80 (6), 564–69.

Willett, G.M., et al. 2001. Relative activity of abdominal muscles during commonly prescribed strengthening exercises. The Journal of Strength and Conditioning Research, 15 (4), 480–85.