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.

Squats

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.