There are 3 main types of muscle contractions: (1) concentric, (2) eccentric, and (3) isometric. Concentric refers to a muscle shortening while it is contracted, think of curling a weight up. Eccentric refers to a muscle lengthening while it is contracted, think of a “negative” or slowly lowering a weight. Isometric refers to contracting a muscle without movement, think about holding a weight out to your side for a prolonged period of time.
Each type of muscle contraction has its own purpose and benefit during training. Ideally, you are using all three to accomplish your goals. If you find that all you do is concentric work, then you may want to build in some isometric and eccentric loading to help stave off injuries.
Eccentric overloading is a type of eccentric training that involves a weight/load/counterforce that exceeds the maximal effort of muscle contraction. This means that if you are pushing or pulling with as much strength as possible, the weight will still slowly win the battle and you end up controlling the weight as you lower it. You would be unable to concentrically contract to pull this weight up because the load outweighs your ability to do so. You can work on eccentric training without “overloading,” but the benefits of overloading will outweigh the benefits of performing negatives with a light load.
Benefits of Eccentric Training
The only real risk/side effect of eccentric training is that you will typically have more delayed onset muscle soreness (DOMS) if you dose/load it correctly.
When choosing a weight for eccentric training, you should select a heavy weight. This weight should be challenging or not possible to lift concentrically. You could use support straps, a lifting partner, or cheat reps to get into the loaded position before slowly lowering. One of my favorite ways to train eccentrically is to use both arms/legs to lift a weight concentrically, then slowly lower the weight with only one arm/leg. This way you can use good form in both directions. You can eccentrically train compound movements (squats, deadlifts, etc.) and isolated movements (bicep curls, front raises, etc.).
You should program heavy eccentric training into your routine consistently, but not too frequently. Start with once per week and then listen to your body. Eccentrics can be a great way to finish a workout session, especially if it was a lighter session.