In recent years, marked progress has been made in identifying the signaling substrates essential for muscle growth and development.
It has been reported that eccentric contractions can cause large increases in several genes involved in the infl ammatory responses. The resulting cytokines produced from inflammatory mediators are thought to play important roles in muscle degeneration/regeneration and growth.
Interestingly, concentric contractions don’t seem to activate this pathway, as concentric contractions don’t induce muscle damage as eccentric contractions do. Both animal and human studies point to the superiority of eccentric contractions in increasing muscle mass.
One study found dramatic differences in muscle size between subjects performing isokinetic concentric contractions as compared to isokinetic eccentric contractions. Type I muscle fiber percentages didn’t change significantly in either group, but Type II fiber area increased about 10 times more in the eccentric contraction group than in the concentric group.
It was concluded that adaptations to training with maximal eccentric contractions are specific to eccentric muscle actions that are associated with greater neural adaptation and muscle hypertrophy than concentric exercise.
In a new study, researchers wanted to examine IGF-1 (an essential growth factor) responses to eccentric training. Participants trained for eight weeks, loading the quadriceps in a shortened or lengthened position, followed by four weeks of detraining.
The resistance-training program was performed three times a week and involved a combination of free weights, machines, and body weight. Knee-extension exercises were performed for 3–4 sets of 8–10 reps at 80% (short-length group) or 55% (long-length group) of full range of motion 1RM. Quadriceps strength, vastus lateralis architecture, anatomical cross-sectional area, and serum IGF-I (insulinlike growth factor 1) were measured at Weeks 0, 8, 10, and 12.
Researchers concluded that greater increases in muscular strength and anatomical crosssectional area of the quadriceps resulted from resistance training at long muscle lengths in comparison with resistance training at short muscle lengths, despite the same mechanical loading and joint excursion in each case.
This should be a real wakeup call to bodybuilders to emphasize the stretch on the muscle when contracting for optimal muscle growth and strength. - FLEX