One thing that seems to frustrate people new to the training game is how strength increases occur rapidly early on with no detectable change in muscle size, which is usually their priority. While it certainly puzzles many a new trainee, the phenomenon has been studied for decades now, and while we still don’t have a precise answer, combining a little research with some practical experience and common sense provides us with a pretty simple explanation to this question.

Neural adaptations dominate early training adaptations

Early strength training studies were quick to note many instances where strength increased following training with no muscle hypertrophy. While there is a rough correlation between muscle strength and size (1), where large muscles are often capable of producing more force, it was initially difficult to determine what the source of the increased strength was. This lead to a division in the adaptations to strength training, where initial gains in strength were attributed to nervous system adaptations, and that as the training program continued, alterations in nervous system function contributed less, while changes in muscle structure and size caught up (2). The graph below shows how this maps out, and is adapted from Moritanti and Devries (2), whose work was one of the first to document this phenomenon.

So while we’ve hypothesized about what these nervous system adaptations are, and we have decades of observational research to suggest that resistance training induces neural adaptations, we still don’t know exactly what they are. To simplify things, there are two main ways we can rapidly increase strength with no change in muscle size:

1. Increase muscle activation
2. Decrease activation of opposing muscles (antagonist co-activation)

Increased muscle activation seems like a no-brainer, use more muscle, increase force production, but a quick look at the literature suggests that it isn’t that simple. The first problem is that, while we like to think that experienced trainees recruit more muscle mass, our untrained counterparts can achieve high, often near maximal levels of activation (3,4), and training doesn’t consistently improve activation (5-9). So if trained and untrained people can use similar amounts of muscle mass, there is hardly room for adaptation here. Unfortunately it isn’t that cut and dry, because many studies have shown that there is an increase in the electrical activity within the muscle (demonstrated through electromyography), that could suggest an increase in muscle activation with training (2,10-14). This adaptation is not consistently observed (5-9), and when it is, it’s often disproportionate to the gain in strength, suggesting any gain in muscle activation can’t entirely explain the increased strength. Couple that with the fact that increased EMG signals may not actually represent increased muscle activation and it’s hard to conclude that increased activation occurs following training (15).

So if increased muscle activation can at best only partially explain the effect, that leaves us with a decrease in activation of opposing muscles. The specific term for this is co-activaiton of antagonistic muscles, and while it sounds like a bad thing, it is actually essential to the integrity of the musculoskeletal system. Any movement we perform consists of a dynamic balance between the prime mover muscles (Agonist), which act to produce the movements, and antagonist muscles, which often perform the opposite function of the agonist muscles. While this may put a damper on the force produced by the prime mover, it is an essential strategy used to control movement to prevent injury, provide joint stability, and produce deceleration during movements. To demonstrate how this acts as an adaptation to strength training, we need to think back to when we were learning or watching someone learn an exercise for the first time.

Remember your first time…

Watch anyone the first time they have a barbell on their back descend into a squat and they will look tense and uncoordinated, with movements that are choppy and cautious. Many muscles are active, usually the wrong ones at the wrong times, making most exercises look nothing like the intended target. To contrast this, take a moment to watch the clip below of a skilled Olympic weight-lifter (Hossein Rezazadeh) front squatting. Watch how he descends rapidly into the hole, no hesitation, the movements are quick, fluid, automatic; look how rapidly he reverses the movement to fire the weight back to the top. Much less awkward than your first time, I’m sure.

So how does this relate to the role of antagonistic co-contraction in the adaptations to strength training? The problem is not one of activating enough muscle, it’s the exact opposite. During those awkward, cautious movements, the level of co-contraction is very high, essentially limiting the strength of the prime-mover (agonist) muscles for the movement. While often times the right muscles are active, the wrong ones are too, and usually at the wrong time. In the scientific literature, it’s been shown that co-contraction often decreases as skill increases (16,17), and this effect is rapid and can occur in around a week (15). So one of the first neural adaptations to strength training is not necessarily to recruit more muscle mass (although this may play a role), it’s actually to recruit LESS of the other muscles, obviously dependent on the specific movement (8,15,18).

As we train and perform the exercise more, we refine our motor program, getting the timings down and building confidence, which is ultimately the foundation for strength (18,19). This also highlights the great importance of an experienced coach, as they know the proper cues to optimize your motor program more rapidly than if you were left to your own devices.

There’s rarely one answer

While human nature makes us crave a single, unified answer to every problem, seems these days that rarely exists. This problem is no different, as there are a host of factors that could produce increased strength without hypertrophy but using a combination of existing literature, practical experience, and common sense I’m willing to put my money on what we’ve discussed here as one of the prominent early (one to two weeks) neural adaptations to training.