This is an excerpt from M.A.X. Muscle Plan 2.0-2nd Edition, The by Brad Schoenfeld.
The metabolic phase is a preparatory phase that conditions your body for hypertrophy training. The goal here is to optimize training efficiency by packing more exercise into less time, i.e., increasing training density. This is accomplished by training with a combination of high repetitions (15-30 reps per set) and short rest intervals (approximately 30 seconds or less). Rest intervals progressively decrease over the course of the cycle to bring about the desired metabolic adaptations.
Although the hypertrophic benefits of metabolic training may not be readily apparent, it indeed can have positive effects on muscle development. First and foremost, metabolic training increases your lactate threshold, the point at which lactic acid begins to rapidly accumulate in working muscles. From a muscle-building standpoint, lactic acid is a double-edged sword. On one hand, there is evidence that it serves to stimulate hypertrophy, at least in animal models (5-7). Although the mechanisms are not entirely clear, lactate conceivably may function as a signaling agent that turns on intracellular anabolic pathways. On the other hand, excessive buildup of lactic acid (specifically the hydrogen ions) can interfere with muscle contraction, thus reducing the number of reps you can perform in a set. Here’s where metabolic training comes into play. Adaptations associated with metabolic training include an increase in the number of capillaries (tiny blood vessels that facilitate the exchange of nutrients and metabolic waste) and an improved muscle-buffering capacity, both of which help delay lactic buildup. The upshot is that you’re able to maintain greater time under tension at a given workload without compromising the proposed hypertrophy-related benefits of lactate accumulation. In addition, you develop a greater tolerance for higher volumes of work—an important component of maximizing hypertrophy.
Metabolic training also improves glycogen storage capacity. Glycogen is the term for stored carbohydrate. The majority of glycogen is stored in muscle tissue, with the balance deposited in liver cells. Here’s the kicker: Each gram of stored glycogen attracts 3 grams of water into the muscle. Increase muscle glycogen stores and you increase overall muscle size, a phenomenon called sarcoplasmic hypertrophy. Although sarcoplasmic hypertrophy does not meaningfully contribute to strength capacity, it does enhance muscular aesthetics, improving the overall shape of your physique. If you aspire to maximize muscular gains, sarcoplasmic hypertrophy is your ally.
In addition, metabolic training boosts your recovery ability. As mentioned, training in a metabolic fashion increases the network of capillaries that deliver nutrients and other substances (such as oxygen, hormones, and so on) to body tissues. A greater capillary density allows for greater nutrient transfer to your muscles. This facilitates better recovery after an intense workout in that it supplies damaged muscles with the necessary materials for remodeling.
Finally, metabolic training may help to fully stimulate the growth of slow-twitch (i.e., type I) muscle fibers (4). Research remains inconclusive on the topic, but there is some evidence that these fibers might respond better to higher rep ranges given their endurance-oriented nature. Although slow-twitch fibers are often dismissed as inconsequential from a muscle-building standpoint, don’t discount their importance to overall muscle development. Superior slow-twitch fiber hypertrophy is one of the hypothesized reasons bodybuilders display greater muscularity compared with powerlifters (3). To maximize muscle size, it is necessary to maximally stimulate the full spectrum of fibers, including slow-twitch fibers.
Before you conclude that including extended cycles of metabolic conditioning in a hypertrophy program is beneficial, remember that this type of training is intended to set the stage for muscle development, not to maximize hypertrophy. In fact, metabolic training potentially can negatively affect strength gains made in the previous phase if carried out over a prolonged time frame. Thus, limit metabolic cycles to relatively short time periods (i.e., four weeks) to avoid any detrimental impact on force-producing capacity.
Moreover, the weights used in this phase are comparatively light, but that doesn’t mean the workouts will be a walk in the park. Quite the contrary. Metabolic training can be even more physically and mentally demanding than heavy weight training. Pushing past the intense burn that continues to build up during a high-rep set requires a high tolerance for discomfort and lots of resolve. It’s definitely not for the weak of mind!