Muscle contracts by lifting, holding, and lowering versus a weight. Each type of contraction serves a purpose.
You have three levels of strength. These levels are positive, static, and negative. When lifting, each gives a higher level of strength successively for a given weight.
Phases
The concentric or lifting phase is positive. Your strength overcomes the weight, which shortens the muscle length. With free weights, the weight moves upward against gravity. This phase feels harder than the others since it is the weakest phase for a load.
Positive failure occurs when you can no longer lift a weight. This is the most common way to train to failure.
Positive work is needed to achieve the best cardio. It activates more fibers and demands more energy. Moving with lots of muscle will raise your heart rate the most.
The isometric phase is static and has no movement. The force equals the weight and no change in muscle length occurs. This occurs most often during co-contractions, when opposing muscle groups contract together to stabilize a body part.
This phase would seem to allow for the ideal point on the length-tension curve by keeping the exercise at the right muscle length. You would just hold the weight at the midpoint of a good exercise and stop when you could no longer keep it there. This idea carries some flaws though. Trainees may put forth less effort due to the lack of clear feedback that occurs when moving. Moving seems to involve the nervous system more deeply too, which boosts effort and performance. This would also ignore the benefits of negative work described later.
Static failure occurs when you can no longer hold a position. This can occur throughout any part of the range of motion. With fatigue being equal, you could hold a weight for longer in positions of better leverage, even though these fail to stimulate growth as well.
Work, as defined by physics, equals the force times the distance moved. Muscles can perform contractions without performing work in physics terms. Viewing intensity in this sense then may mislead you. You do not need much mechanical work to stimulate growth. Some argue that performing reps with as great of a range of motion as possible increases the total work performed and therefore adds to the growth stimulus. This is not true. Too much range of motion actually reduces tension, the main stimulus for more strength and size, in addition to risking your safety.
The eccentric or lowering phase is negative. This lengthens the muscle since it fails to overcome the weight either by your choice or due to fatigue. You then resist it from moving faster downward.
This phase is between 20-40% stronger than the positive. It requires fewer fibers to activate for the same given load yet demands more tension per fiber than the other phases. This makes it the most important phase for building strength and size.
Negative failure occurs when you can no longer lower the weight safely. If you feel the weight dropping instead of lowering under your control then you have reached this form of failure.
This is the final level of strength. It can be dangerous to approach without safety features in place, such as a power rack, since the weight will fall freely when you hit it.
Negative work is a double-edged sword. Research shows that it is essential to strength training. Nonetheless, it also inflicts great damage.
Keep in mind that negative strength serves as your last reservoir. It gives you a brake for positive work. If you kick with your leg and then stop before your knee locks out, you rely on negative strength to limit positive strength. With this function disabled, you would have had serious repercussions in the past, perhaps offering yourself as easy prey. While this thought experiment really gives us no certainty on how this truth affects training, it should remind you to approach negative work carefully.
You must strike a balance between stimulation and stress. You do need to perform some negative work for the best results but not too much.
Conclusions
Exercise to stimulate, not to annihilate.
– Lee Haney
Lifting seems to auto-regulate the amount of lowering you need during a set.
We only need to reach the threshold of improvement possible from a session. There is a fine line between stimulation and overtraining. Too much tension for too long could harm your results. You may have already turned on that light switch with less work so to speak.
I suggest including all three phases during a set by training to positive failure.
Static contractions still matter to some degree. A large range of motion away from the ideal length-tension point that exists in the middle of a good exercise will reduce results. Keeping this range close to the midpoint will make you more likely to reach the static form of failure. It also allows you to ensure you have truly reached positive failure.
For both safety and results, reaching positive failure seems enough, perhaps more than enough. You still have to work hard. Consider continuing to push for a few seconds afterward too. You may reach static failure this way. Make sure you cannot lift the weight any higher and then end the set. With enough effort, positive failure closely approaches negative failure anyway. This maximizes motor unit recruitment to hit the fast-twitch fibers, but just applied with a reasonable cutoff.
Some positive work during lifting may also stimulate the secondary growth factors. This could lead to hormonal changes and build up some lactic acid and the pump that may spur recovery.
You need positive work to perform effective cardio that raises the heart rate. This explains partially why sprinting, stairclimbing, and jumping work better versus high rep squatting for example. These movements involve mostly positive work.
Use all types of muscle contraction. Realize though that negative work is necessary for building strength and size best. Positive work also best stimulates cardio.