Maintaining continuous tension in the muscle throughout the repetitions is a good way to increase difficulty without increasing weight. This requires that you not relax your muscles at any time during the exercise.
As you know, most weight training exercises have a top portion and a bottom portion, during which the muscles have a moment to rest. At the top of an incline press, for instance, when the arms are fully extended, the skeleton supports the load, relieving the tension on the muscles for a brief moment before the weight is lowered.
To maintain continuous tension, you must avoid the completely extended portion of the exercise by keeping your arms or legs slightly bent (contracted) at all times. This causes an intense muscle burn because of intracellular asphyxiation that occurs as a result of blocked circulation. Without oxygen, the muscles produce a lot of waste (lactic acid) while synthesizing energy.
Here is how to apply the principle to various exercises:
• During back, biceps, and hamstring movements, do not straighten your arms or your legs completely in the stretched position.
• During chest, shoulders, triceps, and quadriceps exercises, do not straighten your arms or your legs completely in the contracted position.
Note: Except for the deadlift, there is no loss of tension at the top of the contraction phase in back and hamstring movements. This differs from most of the basic chest and quadriceps exercises, in which tension is often lost at the top of the contracting phase.