The simplified explanation of this common misconception is that glycogen is often discussed as a car’s gasoline tank – we can fill it up to 100% capacity, and expect the car to function with a high level of consistency until the gas becomes fully depleted. In other words, you don’t expect your car to function differently when your gas tank is 75% full versus 25% full; if it has gas, it works, and when the gas runs out, it doesn’t. So, what’s the likelihood that your resistance training workout will fully deplete your muscle glycogen?

Extremely low. As noted previously, studies often find that whole-muscle glycogen stores are only depleted by around 24-40% in a typical resistance training workout (2, 3, 4, 5, 6). These observations, combined with the gas tank metaphor, have fueled a common perspective that glycogen storage (and, by extension, carbohydrate intake) is largely irrelevant for the typical lifter.

The main problem with the gas tank metaphor is that it doesn’t work. In human muscle, glycogen is stored in three major compartments (7): intramyofibrillar glycogen (located within the myofibrils, mostly near the z-line), intermyofibrillar glycogen (located between the myofibrils), and subsarcolemmal glycogen (located just beneath the sarcolemma). These storage compartments are separate, functionally distinct, and become depleted at different rates. While a standard resistance training session might only deplete whole-muscle glycogen by around 24-40%, the intramyofibrillar storage depots will deplete most rapidly (2). Notably, this glycogen storage depot is most directly related to muscular fatigue development via impairment of calcium release from sarcoplasmic reticula (7). In addition, different individual muscle fibers deplete at different rates, and muscle function is impaired long before a muscle fiber reaches full intramyofibrillar glycogen depletion.

With these factors in mind, it’s clear that many fitness enthusiasts have been asking the wrong question when it comes to the relationships between carbohydrate intake, glycogen storage, and lifting performance. Rather than aiming to avoid full depletion of whole-muscle glycogen levels, lifters should be aiming to avoid scenarios in which enough intramyofibrillar glycogen depletion occurs in enough individual muscle fibers to collectively have a negative impact on the ability to maintain adequate force production during exercise. While the former can be achieved with just about any dietary approach, the latter can (in some cases) require a more nuanced assessment of the physiological demands of an individual’s training program and their carbohydrate intake and distribution throughout the day or week (8).

The needs of a lifter will be extremely context-dependent. For example, do you typically train with relatively high rep ranges? Does your program typically fall on the higher end of the spectrum for total training volume? Do you typically complete multiple training sessions for a given muscle group with fewer than 24 hours between sessions? Does your program include concurrent endurance training or sport-specific training in addition to your lifting volume? These are not all of the pertinent questions to consider – for example, many people compete in sports with weight-related constraints that significantly impact their diet, which adds an additional layer of complication to the mix. However, if you find yourself answering “yes” to several of these questions, you’re more likely to be someone who should err on the side of higher carbs, and err on the side of prudent glycogen replenishment strategies in the hours following a training session.