Most experienced strength coaches will promote the use of "negatives." One of the most popular exercises that individuals use eccentric training for is pull-ups. For those of you who do not know what a "negative" is, it is when you would use an aid to get your chin over the bar - in the case of improving at pull-ups, and then lower your body down from the bar slowly and under your own power.
Our muscles are always strongest eccentrically. Eccentric muscle contractions occur when the muscle lengthens. One could consider the stronger eccentric contraction as a safety mechanism built in to our muscles to not only decrease the risk of strains and pulls, but to also allow us to perform actions that we wouldn't be able to perform otherwise if we had equal amounts of strength concentrically (muscle shortening) and eccentrically (muscle lengthening). The analogy I usually use is if you were to pick up a heavy object and carry it a certain distance. The object may feel extremely heavy, almost to the point of not being able to pick it up (muscles shortening - concentric contraction) but once you have the object up, you are able to hold on to it for much longer because the muscles are acting isometrically which is very similar to eccentric contractions. Although the muscle isn't necessarily lengthening, the external load is certainly trying to lengthen it, and your muscles are able to hold on for longer.
So why do individuals train pull-ups eccentrically (negatives) but not generally other exercises? Well, a recent study published in the Frontiers in Physiology looked to study the advantages of eccentric training.
As training experience increases it becomes more challenging to induce further neuromuscular adaptation. Consequently, strength trainers seek alternative training methods in order to further increase strength and muscle mass. One method is to utilize accentuated eccentric loading, which applies a greater external load during the eccentric phase of the lift as compared to the concentric phase. Based upon this practice, the purpose of this study was to determine the effects of 10 weeks of accentuated eccentric loading vs. traditional isoinertial resistance training in strength-trained men. Young (22 ± 3 years, 177 ± 6 cm, 76 ± 10 kg, n = 28) strength-trained men (2.6 ± 2.2 years experience) were allocated to concentric-eccentric resistance training in the form of accentuated eccentric load (eccentric load = concentric load + 40%) or traditional resistance training, while the control group continued their normal unsupervised training program. Both intervention groups performed three sets of 6-RM (session 1) and three sets of 10-RM (session 2) bilateral leg press and unilateral knee extension exercises per week. Maximum force production was measured by unilateral isometric (110° knee angle) and isokinetic (concentric and eccentric 30°.s−1) knee extension tests, and work capacity was measured by a knee extension repetition-to-failure test. Muscle mass was assessed using panoramic ultrasonography and dual-energy x-ray absorptiometry. Surface electromyogram amplitude normalized to maximum M-wave and the twitch interpolation technique were used to examine maximal muscle activation. After training, maximum isometric torque increased significantly more in the accentuated eccentric load group than control (18 ± 10 vs. 1 ± 5%, p < 0.01), which was accompanied by an increase in voluntary activation (3.5 ± 5%, p < 0.05). Isokinetic eccentric torque increased significantly after accentuated eccentric load training only (10 ± 9%, p < 0.05), whereas concentric torque increased equally in both the accentuated eccentric load (10 ± 9%, p < 0.01) and traditional (9 ± 6%, p < 0.01) resistance training groups; however, the increase in the accentuated eccentric load group was significantly greater (p < 0.05) than control (1 ± 7%). Knee extension repetition-to-failure improved in the accentuated eccentric load group only (28%, p < 0.05). Similar increases in muscle mass occurred in both intervention groups. In summary, accentuated eccentric load training led to greater increases in maximum force production, work capacity and muscle activation, but not muscle hypertrophy, in strength-trained individuals.
Despite the admitted disadvantages in the study, I think the findings are pretty telling.
It should be noted that the final sample size in the present study (n = 10+10+8) was likely statistically underpowered to detect differences, particularly between the intervention groups. Results of our priori suggested that sample sizes of 11–12 per group would be sufficient for a power of 0.8. Consequently, due to difficulties in recruiting trained subjects to perform the study in addition to the number of drop-outs and magnitude of strength improvements during the intervention (+10–30% rather than our expected +5–15%) probably increased the likelihood of type II errors.
So how do you implement eccentric training to continue to elicit strength gains? Well, eccentric training is, by its nature, very dangerous. Your primary goal is to overload the muscles with an external load that is greater than the force you can generate concentrically. In other words, you're using more weight than you can lift.
For example, if you are doing bench press eccentric training, you would load a bar up with more weight than your 1-rep max, liftoff bringing the bar directly over your chest, lower the bar to your chest under your own control, and then stop there. You will want to use either a couple of spotters to then help you unload the weight from your chest, or use safety pins/rails to accept the load rather than having it stuck on your chest.
Since our muscles are able to generate and accept more force eccentrically compared to concentrically, using loads slightly above your concentric 1-rep max should be considered safe and effective training, just make sure to practice safe training techniques (spotters and/or safety pins/rails) to reduce your risk of injury!