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Training Focus: Maximize Muscle Activation for Strength Gains

by Jason Karp, PhD on Feb 19, 2009

Do slight changes in body position affect muscle activation during strength exercises? The only way to truly know which muscles are used during an exercise is to measure their electrical activity with an electromyogram (EMG), the skeletal muscle equivalent of an electrocardiogram for your heart. Well, guess what? Scientists have done just that. Let’s take a look at how different body positions affect muscle activity during some common weight training exercises.



Leg Extensions

Comparing three leg positions (medially rotated, laterally rotated and neutral) during a set of 8 repetitions of leg extensions on a leg extension machine, Stoutenberg et al. (2005) found that medially rotating the legs by pointing the toes inward targeted the vastus lateralis and vastus medialis muscles, whereas laterally rotating the legs by pointing the toes outward increased rectus femoris activity.

Using isometric contractions on an isokinetic leg extension machine (a machine on which you can do isometric contractions against a nonmoving leg pad) at three knee joint angles (90, 150 and 175 degrees), Signorile et al. (1995) found that when the legs were held almost straight (175 degrees), a position often used to stabilize the knee joint during the early stages of rehabilitation, medially rotating the legs produced the greatest quadriceps activity. However, these researchers suggested that in the later stages of rehab, a neutral foot position at a 90-degree angle might provide the most effective condition for rehabilitation following knee injury, since this angle and foot position caused the greatest amount of quadriceps muscle activation.


With squats, Boyden, Kingman & Dyson (2000) found that changing leg position by pointing the toes in, out or straight had no effect on quadriceps activity. McCaw and Melrose (1999) found that stance width did not affect quadriceps activity either but did influence adductor longus and gluteus maximus activity, with a wide stance (140% of shoulder width) eliciting greater activity in these two muscles than a shoulder width or narrow (75% of shoulder width) stance. Comparing partial, parallel and full squat depths, Caterisano et al. (2002) found that the gluteus maximus, but not the vastus lateralis, vastus medialis or biceps femoris, became more active as squat depth increases.

Bench Press

Mainly thought of as a chest and triceps exercise, the bench press is also great for the deltoids. Clemons and Aaron (1997) found that the triceps and anterior head of the deltoid were the most active muscles during the bench press (compared with the pectoralis major and biceps). The researchers found that the widest grip tested (190% of shoulder width) elicited greater muscle activity in all four muscles than narrower grips (shoulder width and 130% of shoulder width). For the best overall training stimulus, clients should use a grip that is almost double their shoulder width.

For analysis of other exercises and additional “how-to” suggestions, along with a full reference list, please see the full article in the January issue of IDEA Fitness Journal or read it online in IDEA's Library.

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About the Author

Jason Karp, PhD

Jason Karp, PhD IDEA Author/Presenter

Dr. Jason Karp is a nationally-recognized running and fitness coach and owner of Run-Fit. As one of America’s foremost running experts and the 2011 IDEA National Personal Trainer of the Year, he has been profiled and interviewed in a number of publications. A rare combination of education and experience, he holds three degrees in exercise science, including a Ph.D. in exercise physiology from Indiana University and a master’s degree from the world-famous Human Performance Laboratory at the University of Calgary. No other running coach or personal trainer has the combination of credentials, education, and experience. A prolific writer who is more widely published than anyone in the fitness or coaching industries, he has more than 200 articles published in numerous international running, coaching, and fitness magazines and scientific journals, is the author of five books, including Running for Women and Running a Marathon For Dummies (of the internationally-known For Dummies brand), and is a frequent speaker at international fitness and coaching conferences. A former high school and college cross country and track coach, Jason is a nationally-certified running coach through USA Track & Field, has taught USA Track & Field’s highest level coaching certification, has led elite coaching camps at the U.S. Olympic Training Center, and is sponsored by PowerBar and Brooks. He has been a runner since sixth grade and was a member of the silver-medal winning U.S. Masters Team at the 2013 World Maccabiah Games in Israel.