Many bodybuilders regard the dumbbell press as an isolation exercise which they can use specifically to train their pectorals and to deactivate other muscle groups that are needed for other chest exercises. This idea is only partly true, Norwegian sports scientists reported in 2011 in the Journal of Sports Sciences.
The Norwegians got twelve trained male students to do bench presses with a barbell, bench presses on a Smith machine, and dumbbell presses, all at maximal weight. Just to be sure we’re all on the same page, the latter exercise is shown above. The researchers attached electrodes to the participants’ pectorals, delts, biceps and triceps so that they could measure how hard the muscles had to work.
The figure below shows that the dumbbell chest press stimulated the biceps more strongly than the regular bench press and the bench press done on the Smith machine.
The dumbbell press activated the shoulder muscles the same amount as the regular bench press and the bench press on the Smith machine did. At the same time, the dumbbell chest press activated the triceps less than the other two exercises did.
A comparison of muscle activity and 1-RM strength of three chest-press exercises with different stability requirements
The purpose of this study was to compare one-repetition maximum (1-RM) and muscle activity in three chest-press exercises with different stability requirements (Smith machine, barbell, and dumbbells). Twelve healthy, resistance-trained males (age 22.7 ± 1.7 years, body mass 78.6 ± 7.6 kg, stature 1.80 ± 0.06 m) were tested for 1-RM of the three chest-press exercises in counterbalanced order with 3–5 days of rest between the exercises. One-repetition maximum and electromyographic activity of the pectoralis major, deltoid anterior, biceps, and triceps brachii were recorded in the exercises. The dumbbell load was 14% less than that for the Smith machine (P ? 0.001, effect size [ES] = 1.05) and 17% less than that for the barbell (P ? 0.001, ES = 1.11). The barbell load was ?3% higher than that for the Smith machine (P = 0.016, ES = 0.18). Electrical activity in the pectoralis major and anterior deltoid did not differ during the lifts. Electrical activity in the biceps brachii increased with stability requirements (i.e. Smith machine