The higher the incline, from the flat bench position, the more the delt's come into play. The lower the decline, from the flat bench position, the more the pec's come into play. Though, in either case, the delts and pec's still are involved in the exercise load, perhaps to a lower degree depending of the bone length, leverage and muscle inserts for each lifter.
With inclines, the delt-pec tie-in becomes more pronounced. The risk of shoulder joint injury tends to be reduced a bit also. The declines are more akin to dips, reason the pec's receives a good deal of involvement. (my personal choice would be dips..anytime, over declines)
If choosing the flat bench version, a three grip position in a chest workout has worked well with many a trainee, as a change of pace from the regular chest programs.
Example when working the flat bench presses. A warmup would be advised. Might suggest a spotter also.
1...extra wide grip, elbows out, lowering the bar to the upper chest/neck area. Not going to use an extra heavy weight, more in the moderate range, and going for a rep range of 12-15. Favorite of many 70's BB'ers. If having longer arms, like me, than a collar to collar grip is ideal.
2...Middle grip (little wider than shoulder width), lowering to the center of the chest. This is pure BB'ing, so touching the chest each rep is advised, with a slight lockout at the top position.. You want control of the bar it's self. 8-10 reps
3...close grip ..hand about 8 to 12 inches apart. Lower the bar to the lower rib box/upper abs. Try keeping the elbows closer to the body at all times. Don't want them any wider out, which can put too much stress on the wrist and elbows. 8-10 reps.
Including an option exercise, your choice. Using a very close grip...4-6 inches and just do rapid lockouts from about 12 inches. Tends to strengthen the ligaments and tendons around the elbows/triceps. A couple of sets should do.
Would suggest 2 or 3 sets each benching grip position. That would be 9 sets of max total reps. Try not doing any other direct chest movements when doing this. This protocol also works well with chinning and BB curls. Even squats, with a three position foot spacing. ..extra wide, middle and close. Doesn't seem to work that well with steeper inclines, for me anyway. If for anyone else, go for it.
Good Luck..
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