The truth of the matter on muscle hypertrophy and growth factors is that no one knows with certainty how muscle grows. No one. Not trainers, not any scientist at Yale University or Harvard, or any bodybuilding expert or guru out there. What is known is that the IGF-1s (protein with a similar structure to insulin) appear to be players in the process of muscle growth. To learn more about muscle growth factors, read How to Harness Your Body's Growth Factors.

Once you understand growth factors, it’s time to discuss training strategies to help you increase their levels so that you can get on with the business of getting huge. Training can elicit a boost in both circulating IGF-1 levels and muscle expression of IGF-1s. But not just any ole workout will send your growth factor levels through the roof. These four strategies are backed by science to get your growth factors surging.


HOW IT WORKS Muscle damage is important to activating dormant muscle satellite cells. And that instigates an increase in the production of MGF in the muscle. This will get you more muscle nuclei in your muscle fibers, and that equates to more growth that you keep for the long haul. In fact, U.K. researchers reported that men performing an eccentric (negative rep) leg workout experienced significant increases in muscle expression of MGF. Another study, from the University of Alabama-Birmingham, reported that subjects performing 8 sets of 8 eccentric reps using 110% of their 1-rep-max weight increased muscle expression of IGF-1 by more than 60%. Yet when they performed only positive reps, the increase in expression was only 40%.

DO THIS: Every four to six weeks, do a negative- rep training workout for each muscle group. Do 3 sets of 5 to 8 negative reps on 2 to 3 exercises. Use a weight that’s about 110-120% heavier than your one-rep max on that exercise. Have a spotter assist you through the positive part of the rep and then lower the weight on your own through the negative parte of the rep. Each negative rep should last for three to five seconds. Follow the 3 sets of negatives with 2 to 3 sets of regular for 6 to 8 reps. 

Train solo? You can still do negatives on a Smith machine. Use both arms to move the weight through the positive portion of the rep and then resist the weight on the negative part of the rep and then resist the weight on the negative part of the rep with just one arm or leg. Switch limbs each rep until you have completed 5 to 8 negative reps on each side. When you come back to the next negative rep-training workout, try to choose different exercises than the previous week’s to ensure that you are damaging muscle fibers in different areas of the target muscle.


HOW IT WORKS Training with a weight that allows you to complete about 10-12 reps is best for muscle growth. It’s what decades of experienced bodybuilders have found in gyms, and it’s what decades of experienced bodybuilders have found in gyms, and it’s what strength scientists have confirmed in the lab. This rep range had also been shown in the lab to better boost expression of local IGFs from the muscle, as well as boost circulating GH and IGF-1 levels. Researchers from Finland reported robust increases in both IGF-1Ea and MGF following a leg workout that consisted of 5 sets of leg presses for 10 reps per set and 4 sets of Smith machine squats for 10 reps per set.

DO THIS: Although you should never focus on just one rep range all the time, be sure to frequently hit the 10-12 rep range in your workouts to boost both locally produced IGF-1s in the muscle and circulating IGF-1 levels. This will help to increase both muscle nuclei number and muscle protein synthesis in the target muscles. 



HOW IT WORKS If you look around your gym you’ll notice that most guys do a set and rest a solid two to three minutes before doing the next set. This is a common weightlifting prescription that works well for increasing strength. After all, the better recovered the muscles are, the more weight they can lift and the more reps they can complete on the next set. But when it comes to growth, fatigue appears to be important. Fatigue sets off a cascade of chemical events in the muscle that encourage growth. In fact, a recent study reported that subjects reducing the rest between sets by 15 seconds each week until they were resting just 30 seconds between sets in the final week gained more muscle than those resting a steady two minutes between sets. Although they didn’t measure IGF-1 levels in this study, it may be one of the reasons why the subjects decreasing their rest time were able to build more muscle. 

Researchers reported at the 6th European Sports Medicine Congress that only when subjects rested one minute between sets of presses did they experience a boost in circulating IGF-1 levels as compared with when they rested 90 seconds or two minutes between sets. One-minute rest periods have also been shown to better boost GH levels, which may be involved in stimulating the expression of the IGF-1s in muscle fibers.

DO THIS: Keep rest periods to a maximum of one minute between reps, or consider using a six-week periodization program that decreases rest by 15 seconds each week. In Week 1, start at two minutes of rest, and end in Week 6 at 30 seconds of rest. 


HOW IT WORKS To really boost growth factors, you need to train with balls-to-the-walls intensity. That means taking the last set of each exercise to failure…and beyond. A great way to do this is with the Weider Training Principle known as forced reps–research from Finland shows that they boost GH levels higher than when you end a set at failure.

Although the did not measure IGF-1 levels, the researchers reported that when subjects performed a workout using forced reps, their GH levels were three times higher than when they did a normal workout where each set ended upon reaching failure (higher GH levels typically mean higher IGF-levels in the circulation and/or in the muscles). In a follow-up study, the same research team found that trained men using forced reps recruited more fast-twitch muscle fibers, as well as more total muscle fibers during a workout as compared with when they used normal sets taken just to failure.

The forced reps also led to higher fatigue of both their muscle fibers and their nervous systems. The researchers suggested that this would likely result in greater muscle growth over time due to the increased mechanical stress placed on the muscle fibers and the higher release of anabolic hormones and growth factors induced by greater fatigue. 

DO THIS: The key is how much help you get from your spotter. Too little assistance and you will not be able to complete an adequate amount of forced reps to stimulate muscle growth. Too much help will not adequately overload the muscle. The spotter must help just enough to get you past the sticking point but allow you to do the majority of the work.

You can still used forced-rep training even if you’re solo. Do single-arm or single leg exercises, and after reaching failure, assist yourself using your other arm or leg. Tempted to take every set to failure and do forced reps to boost IGF-1 levels higher? Don’t be. Spanish researchers found that men training for 16 weeks and taking every set to failure had lower circulating IGF-1 than those not taking sets to failure. Keep it to one set per exercise.