Why are higher intensities better for strength? PART 3: Neurological Adaptations

Brian Minor, MS, CSCS In Part 1 of this article series, I began by introducing a wide lens, a binary model commonly put forth when discussing maximal strength expression: greater muscular size increases one’s potential for strength, and increases in nervous system ‘drive’ allow one to further express that strength potential. While a bit reductionist,…

Brian Minor, MS, CSCS

In Part 1 of this article series, I began by introducing a wide lens, a binary model commonly put forth when discussing maximal strength expression: greater muscular size increases one’s potential for strength, and increases in nervous system ‘drive’ allow one to further express that strength potential. While a bit reductionist, this model allows most coaches and athletes to begin to understand the utility and theoretical underpinnings of topics such as periodization, and areas of relative emphasis based on training experience. However, the value lies within some of the more specific details here. Part 1 closed by introducing the more comprehensive categorization of adaptations as either morphological or neurological in nature, along with some of the primary adaptations for strength within each that would be examined in more detail later in the series.

While a comprehensive overview of every specific adaptation within each “category” extends beyond the utility of this series, Part 2 focused on two of the primary morphological adaptations: increases in muscle size, and changes that occur within connective tissue. The roles of each on increasing strength was explored, as well as the influence (or lack of) that load had on these adaptations. A big take away from Part 2 was that while hypertrophy does not appear to be intensity specific, heavier loading does seem to have a greater impact on connective tissue adaptations, namely increases in tendon stiffness and lateral force transmission. To no surprise though, the benefits of high intensity loading on strength extend well beyond these connective tissue adaptations. In Part 3 now, we will focus on some of the primary neurological adaptations, and how high intensity may influence them.

Anyone who has been training for a while has likely noticed that the rate of strength gain is disproportionately greater than the rate of muscle gain. While morphological adaptations such as tendon stiffness and lateral force transmission can increase applied force independent of hypertrophy, much of this disparity is attributed to neurological adaptations. Take for example a novice lifter. It is very common for a novice who has never learned or performed the squat to be able to add kilo’s to the bar each week despite not building any appreciable amount of muscle on their quads. How can this be? Well, the nervous system adapts at a much faster rate than many other systems. As a novice lifter acquires the skill of a movement and enhances their proficiency, they can progress in strength at a rapid rate.

Our nervous system plays a vital role in movement. In the absence of the signal to contract, our muscles are essentially just “dumb pieces of meat” (credit to Jacob Schepis). Fortunately, our nervous system allows for the voluntary activation of our skeletal muscle, and for motor learning and coordination to take place, resulting in an increase in movement efficiency.

There are a couple of areas in particular where these neurological adaptations can have a profound impact on our ability to set new PRs: Agonist activation and coordination.

Agonist Activation

Agonists are simply muscles working in a concentric manner during a lift. The somatic nervous system contains motor neurons which control and innervate a given amount of skeletal muscle fibers, which are recruited on an as-need basis.

How do changes in agonist activation impact strength?

Let’s start by explaining what all we are referring to when it comes to agonist activation. It’s common to think of it simply as the ability to recruit a motor unit, when it’s really referring to the whole pipeline of agonist activity:

The ability to recruit motor units (voluntary activation)
The rate at which we recruit motor units (rate coding)
The speed at which we initiate muscle contraction upon receiving an action potential from the motor neuron (muscle fiber conduction velocity)

The amount of force a MU can produce is based on a couple of variables. First, the size of the motor unit, since larger motor units contain more muscle fibers and in turn, contractile tissue. The other is the rate at which they can send the twitchs/repeated action potentials to the muscle fibers they control. This is referred to as rate coding. In the untrained, increases in agonist drive appear to be the primary factor for strength expression early in training (1). This is thought to be one of the primary reasons for the rapid increase in strength that is observed at the onset of training. Since voluntary activation is usually pretty high even in the untrained (5) increases in rate coding are likely to be steering much of that increase.

Now once a fiber receives the message to contract, additional signalling (an action potential) occurs within the fiber itself, initiating the contractile process. The conduction velocity refers to the speed at which this signal travels within the muscle fiber. This has implications for how fast we can reach max MU recruitment. The rate at which we can reach maximal MU recruitment influences our rate of force production (RFD).

Rate of force development (RFD) is the rate at which peak force can be generated. It has been shown that a greater fiber conduction velocity in trained individuals is primarily responsible for the larger rate of torque development (RFD) in trained vs untrained individuals, likely resulting in an increased rate of maximal MU recruitment (2). This also provided evidence that fiber conduction velocity and rate of motor unit recruitment can be improved in well-trained individuals.

In a sport like powerlifting, with no apparent time requirement, one may wonder why this matters much.

To understand the importance of RFD, think about the most basic reason why people fail a max attempt: they are unable to meet the force demand presented by the load (and where one is within the range of motion). As obvious as that sounds on the surface, there are a few potential paths to that outcome:

Their capacity for peak force production simply wasn’t sufficient to overcome the most demanding portion of the lift.
They were able to reach their peak force, but unable to sustain it long enough due to muscular fatigue.
They weren’t able to reach their peak force by the time they hit the most demanding portion of the lift.

The third scenario is when rate of force development may be the bottleneck in one’s ability to complete a lift. There is a point in each lift where the muscular demands are going to be greatest (as a result of the external demands being the greatest). A narrow window of time exists to reach the force required to get past that point in the lift.

If peak force, RFD, and ability to sustain adequate output are all-sufficient to overcome the most demanding portion of the lift, then the athlete will be able to make it through that point which (unless downstream fatigue within the rep presents an issue) ultimately means their max would be greater than had they not been able to get through that point. As a result of the heavier load, bar speed will have been lower. Not surprisingly then, it’s been shown experienced athletes can perform 1RMs at slower velocities than novices (7), which within powerlifting is often referred to as one’s “ability to grind” at max effort. This is likely largely explained by RFD increasing with training experience (2).

Does absolute intensity influence agonist activation?

As mentioned in Part 2, hypertrophy is not dependent on intensity, but on placing sufficient mechanical tension on high threshold motor units (HTMU). Being that active tension (muscle contraction/cross-bridging) is dependent on motor unit activation, one may expect MU activation to be similar between both high and low intensity conditions when hypertrophy is equivalent. In other words, don’t we have to “activate” a muscle fiber to grow to begin with? So what does it mean exactly when stating high intensities increase agonist activation?

In an acute sense, training at 80% loads resulted in greater peak EMG readings when compared to 30% loads when performing leg extensions to failure (3). This may come as a surprise given the relationship between heavy and light loads with hypertrophy. However, surface EMG looks at activity at a specific moment in time. It’s possible that by the time HTMU are recruited with low intensities, that lower threshold fibers have fatigued to the point where their decreased activity is reflected through a lower EMG reading late in the set.

What about the impact of intensity on agonist activation following training? It also appears training with higher intensities can increase agonist activation over time. A 2017 study by Jenkins, et al (3) examined the effects of training at 80% or 30% 1 RM loads (using leg extension) on markers for strength, hypertrophy, and muscle activation after 6 weeks. It was demonstrated that 80% loads were superior to 30% loads for increasing both 1 RM and maximum voluntary isometric contraction (MVIC), however, there were no differences in hypertrophy (adding to a wealth of data there). In support of the notion that the strength difference was influenced largely by neurological adaptations, the 80% group also had greater levels of voluntary activation (VA) and EMG amplitude when performing MVIC after 6 weeks. The authors of the study also found lower VA and EMG amplitude in the 80% group at submaximal intensities. On the surface, this may seem contradictory, but what it’s saying is there was an increase in efficiency and likely less MUs were required to be activated to meet the force demand at sub max intensities despite the capacity for peak VA and EMG amplitude increasing. In other words, low(er) threshold MUs may have been doing a better job in meeting the force demand at submax loads.

Coordination

Coordination, in the context of strength expression, is primarily referring to the relative roles of different muscle groups (agonists, antagonists, and synergists), based on the specific profile of what’s being performed. The more skilled the movement, the more specificity in terms intensity seems to matter for 1 RM strength.

How do changes in coordination impact strength?

The most efficient way to become stronger is to become a more efficient lifter. Pushing for refining or maintaining proficiency allows us to optimize the production of force via optimizing coordinated efforts between muscle groups and joints. This requires sufficient practice performing the specific task we want to improve.

Does absolute intensity influence coordination?

The more skilled the movement, the more specificity of load seems to matter for strength. This was demonstrated in a 2017 meta-analysis (6) that compared the effects of low (<60%) and high (>60% 1 RM) intensities on hypertrophy, multi-joint 1 RM strength, as well as isometric strength. There were no significant differences between groups for hypertrophy outcomes, but the high load group had greater increases in multi-joint 1 RM strength. However, there were also minimal differences between groups in maximal isometric strength (exerting as much force as possible against an immovable object), providing evidence that specificity of load is more important with multi-joint movements. Why may this be the case?

As intensities go up, the relative contribution of prime movers can change, and subsequently, impact the way the lift is performed. This can be observed by watching a powerlifting meet and seeing lifters work up to their third attempts on meet day. If technique visually changes across those attempts then the relative force contributions of the involved muscle groups have also changed. For example, we know that as intensity increases on back squats, the relative contribution of glutes increases, with the relative contribution of quads decreasing. On the bench press, activation of the anterior delts and the long head of triceps increases with heavier loads (4). This appears to largely occur in concert to a shift to a more efficient bar path.

With multi-joint movements like the 3 power lifts, sufficient exposure to high intensities within one’s program plays a large role in becoming coordinated and proficient with the movement and the relative contribution of different muscle groups at high intensities. No real surprise there.

Neurological Summary

Both agonist activation and aspects related to coordination play important roles in the maximal expression of strength. While neurological adaptations are considered primarily responsible for early increases in strength, they still play important roles across an entire training career. The final Part 4 of the series is next and is all about practical application and some additional food for thought related to programming theory for strength. Stay tuned!