Strength

Muscles have seven characteristics related to physical ability. Each characteristic is associated with a different aspect of physical ability. Taekwondo demands certain requirements of muscles depending in the circumstances. Sometimes you need maximum strength to release from a grip. Sometimes you need quickness for strikes. Sometimes you need speed to run away. Of these seven characteristics, the first five are the most important to develop for martial artists.   

• Speed. How fast the muscle fibers contract. The most important muscle trait for a martial artist. For example, if Zorro (small man with small, fast sword) fought Conan (huge man with large, bulky sword) who would win? Zorro would probably cut Conan fifty times before he could swing his broad sword.
• Strength. How many muscle fibers contract at once. For the greatest strength, many muscles must contract together or in sequence.
• Stretch. How far muscles stretch (flexibility). To kick high and quickly while preventing injuries, you must have flexibility that allows a wide range of motion.
• Stamina. Your endurance or VO2 uptake capacity. Most fights finish in a few seconds. So why do you need Stamina? Good endurance allows you to repeat techniques many times in training with proper form. You must repeat a technique thousands of times before it becomes instinctive. For that type of training, you need endurance.
• Smarts. Muscle memory or learned neural pathways. Muscles have memory. Once you train a technique thousands of times, it become automatic and you do not have to think about the action. 
• Shape. Amount of fat around muscle. Some people call this muscle tone or definition. If you have a lot of body fat your muscles, will not show and you will be round and soft looking. If you remove that fat, your muscles will look cut and firm. It is the same muscle, but the amount of fat around it makes the shape change. Fat slows you down by adding extra weight you have to move, but it also provides padding against impacts and insulation against body heat loss.
• Size. Amount of muscle mass. The most powerful weight lifters are not as big as most body builders yet they have more strength.

Shape and size are not of great importance to martial artists, but they do seek to increase one or more of the other five characteristics.