There are three general attributes of skeletal muscle that athletes try to develop: strength, power, and endurance. Although these attributes are not really separable, they reflect the different physical capacities of muscles.
Strength
Strength is simply the maximum contractile force that a muscle can exert. Functionally, this is defined as your one repetition maximum, or 1RM. For example, if the absolute maximum amount of weight you can curl one time is 40 lbs., that gives you an estimate of the strength of your biceps brachii. Physiologically, strength is determined predominantly by cross-sectional muscle area. Basically, the bigger the muscle is, the stronger it is. (Although other variables are involved, such as the communication between nerves and muscles and the efficiency of energy metabolism, these have a much smaller overall effect on strength.) To understand the link between cross-sectional muscle area and strength, think of muscle as being a little bit like rope; the more strands there are in the rope, the greater the load it can lift without snapping.
Power
Power is the amount of work you can do per unit of time. Unlike strength, which is simply a measurement of contractile force, power is the application of that force to do work (e.g., to move something or move yourself) as a function of time. Consider two examples, based on the one above.
If you can curl 40 lbs. in 1 second, that is twice as powerful as curling 40 lbs. in 2 seconds.
If you can curl 80 lbs. in 1 second, that is twice as powerful as curling 40 lbs. in 1 second.
This is why power is often used interchangeably with concepts like "explosiveness"; it is not just the force your muscles exert but the way that force is applied.
Although cross-sectional muscle area is a component of power, other variables, such as nerve-muscle communication, muscle fiber recruitment, and muscle fiber composition, play a much larger role.
Endurance
Endurance is the ability of a muscle to sustain repeated contractions for an extended period of time. Functionally, let's return to our previous example. If your 1RM is the maximum amount of weight you can move one time, endurance is the number of times you can move a certain amount of weight. Since we know your 1RM is 40 lbs. for the curl, to test the endurance of your biceps brachii, you may instead determine how many times you can curl, say, 15 lbs. before you reach muscle failure (the inability to initiate a contraction). That gives you an estimate of muscle endurance.
Endurance is determined mostly by muscle fiber composition, cardio-pulmonary efficiency, and energy metabolism.
Now, as mentioned above, you cannot completely separate these three capacities of skeletal muscle in reality. All workouts will contribute to all three attributes, but you can emphasize one over the others to some extent. There are many many ways to design workouts that focus on one or two of these components (many of which will be the subjects of future posts), but here is a general rule: as you move from power to strenth to endurance, increase the number of repetitions (by decreasing the resistance) and decrease the rest between sets:
Power: 3-5 sets of 1-5 repetitions, 2-5 minutes of rest between sets
Strength: 3-5 sets of 8-12 repetitions, 1-2 minutes of rest between sets
Endurance: 3-5 sets of >15 repetitions, <1 minute of rest between sets
Future posts will explore more sophisticated ways to train these attributes.
