Bruce Lee developed, exceptionally strong abdominal muscles, which were well defined and solid. Lee understood that every movement required generation of abdominal muscle force. He also understood that rock hard abdominal muscles help protect your rib cage and abdominal organs.

  
A Warrior’s Journey has also been published as The Game of Death

Bruce Lees exercise program involved the following:
CARDIO
Run (e.g., 2-6 miles, ranging 15 to 45 minutes, respectively)
Jump rope
Stationary bicycle

MUSCULAR ENDURANCE
Two-finger pushups (thumb and index finger, feet shoulder width apart)

Biceps Curls 80 lbs (8 reps)

‘If you’re talking about combat — as it is — well then, baby you’d better train every part of your body!’
— Bruce Lee

More training related quotes by Bruce Lee:
There must be a “being” instead of a “doing” in training. One must be free. Instead of complexity of form, there should be simplicity of expression.

In combat, spontaneity rules; rote performance of technique perishes.

Practice all movements slow and fast, soft and hard; the effectiveness of Jeet Kune-Do depends on split-second timing and reflexive action, which can be achieved only through repetitious practice.

Simplicity is the key to brilliance.

The first rule is to keep yourself well covered at all times and never leave yourself open while sparring around the bag. By all means use your footwork — side stepping, feinting, varying your kicks and blows to the bag. Do not shove or flick at it. Explode through it and remember that the power of the kick and punch comes from the correct contact at the right spot and at the right moment with the body in perfect position; not, as many people think, from the vigor with which the kicks or blows are delivered.

The highest technique is to have no technique. My technique is a result of your technique; my movement is a result of your movement.

The less effort, the faster and more powerful you will be.

The more relaxed the muscles are, the more energy can flow through the body. Using muscular tensions to try to “do” the punch, or attempting to use brute force to knock someone over, will only work to opposite effect.

The old-fashioned punching speed bag teaches you to hit straight and square; if you don’t hit it straight the bag will not return directly to you. Besides learning footwork, you can hit the bag upward too. Another important function is that after the delivery of the punch, the bag will return instantaneously and this will teach you to be alert and to recover quickly. The bag should not be hit in a rhythmic motion but instead in a broken rhythm. Actually fight the bag as if it is your opponent.

The techniques, though they play an important role in the early stage, should not be too restrictive, complex or mechanical. If we cling to them, we will become bound by their limitation. Remember, you are expressing the technique, and not doing Technique number two, Stance three, Section four?

Any technique, however worthy and desirable, becomes a disease when the mind is obsessed with it.

More Bruce Lee quotes on Wikiquote.org

Martial arts training objectives develop efficient strategy, technique, mental discipline, endurance and flexibility. Training with your own body weight used as resistance and light weights with medium to high repetitions are common. Exercises programs should develop strength, balance and flexibility.

Martial arts performance and training involves high forces and high velocity movements of the arms and legs. It is important to understand the importance of the core (abdominal muscles, and back muscles) in generating forces that move the arms and legs. This is especially important in grounding with the earth and using the ground to help generate forces that move the body and arms and legs.

Exercises that strengthen the stabilizer muscles of the arms and legs in their connection to the axial skeleton and core are very important. Rotator cuff muscles and hip adductors (inner (medial) thigh muscles) and abductors (muscles of the outer or lateral part of the hip), which help to support the back and hips are important for (1) transferring generated core forces to the arms and legs and (2) for absorbing forces from contact (especially of the feet and hands) with external objects.

Common injuries are sprains, strains and bruising (contusions). Strains, which occur in both muscles and tendons,  are caused by sudden and extreme forces that exceed the muscle or connective tissues area’s integrity.

Small and major bone fractures are also possible.

Much of martial arts training involves the careful understanding of using forces without damaging the body and helping the adaptation to attenuating greater forces during high velocity motion and contact.