Strength is the ability to exert force against a resistance. All muscle movements require strength, and everybody can benefit from increasing it. Whether you are an athlete, a mother, a father, a gardener, a golfer, a weekend warrior, or a grandparent, muscular strength is as important to the success of your daily activities as breathing is to life.
There are approximately half a thousand voluntary muscles in the body, under the direct control of the will. About 50 of these are in each arm and leg. This accounts for 200, leaving 300 in the central part of the body, the neck, trunk and head.
Some are as small as a pinhead and others weigh several pounds. They have different shapes—long, short, flat, round, etc. They all have their purposes. Some are more useful than others.
The abdominal wall
You may have the strongest legs in the world, but if your abdominal muscles are weak, you cannot be a good runner or jumper, or good in any sport requiring strength or endurance. If you have a hard, solid, elastic, muscular body, your legs will a hard, solid base to work from. If your abdomen is soft and flabby, any effort makes it bulge weakly, the arms and legs have no reliable substance behind their action and there is no power. The most important voluntary muscles of the body are the abdominal muscles. They are far more important than the biceps in spite of the fact that we always put up the arm and try to make it bulge when we want to show off.
The abdominal muscles consist of more muscles than just the rectus abdominis: the six-pack muscles. External and internal oblique muscles as well as the infinitely important transversus abdominis complement the rectus abdominis muscle to provide trunk stability and the foundation of strength.
One of the first secrets of strength is, spend your time where you get the best results. Work on the most important muscles, develop a foundation, or you get nowhere. Every jump, kick, throw, hit, dodge and stride you make is three-quarters trunk muscles’ work. The [person] who has a strong trunk will outswim, outrun, outclimb the [person] who develops only his [or her] arms and legs. This is one of the great secrets of success in all lines of athletics. The great coaches of athletic teams and track athletics know this and they have their several ways of putting it into practice.
In every athletic effort the abdominal muscles take an essential part. The high jump, the hurdle, every step of the sprint or distance runs, every movement in baseball, football, tennis, swimming, boxing, use the abdominal muscles as the foundation element. It’s always there in the middle of things. [As of 1936,] very few athletic trainers fully realize this and the public knows it not at all. Now you know it.
Strong arms, strong legs are futile unless there is a strong abdomen to work from. A strong abdomen is one of the great secrets of strength.
In addition to the abdominal wall muscles, the muscles of the lower back are just as important for maintaining posture and facilitating power generation. The quadratus lumborum and the erector spinae are especially important.
For exercises and stretches specific to each of these muscles I've mentioned, I refer you to ExRx.net, a very useful site. If you would like more hands-on instruction, consult with a qualified, certified personal trainer. NASM, NSCA, or ACSM certifications are awarded by quality organizations.