Muscle hypertrophy (growth) – the most highly desired end-product of weight training is a multifaceted phenomenon. In this series of articles, I’ll be exploring some of the lesser-understood aspects that may influence your results. The greater understanding you have about the biological processes that influence muscle growth, the more efficient your quest will be. The less time you’ll waste with non-productive training programs. And believe me, this industry is littered with them.
In part-1 we looked at how strength improvements are the result of functional changes within the muscle that involve extensive remodeling of qualitative or intrinsic contractile properties such as Myosin Heavy Chain isoform expression. In turn, this ultimately influences muscle fiber type and distribution. In part-2 we’ll look at what hypertrophy is, the best type of training that initiates the hypertrophic response and the relationship between strength development and muscle growth.
What is Muscle Hypertrophy?
Muscle hypertrophy can be described as an increase in the cross-sectional area (CSA) of the muscle itself (determined by magnetic resonance imaging) or the individual fibres. In my research, I always correlated DEXA-scanned lean mass changes with muscle fibre hypertrophy which is assessed most accurately via needle biopsy and ATPase staining.
Muscle hypertrophy is essentially the result of a net increase in muscle protein. It is presumed that contractile (myofibrillar) protein volume increases in direct proportion with exercise-induced fibre hypertrophy. It is also presumed that sarcoplasmic or non-contractile proteins (such as alpha-actinin, myomesins, desmin, dystrophin, nebulin, titin, and vinculin) increase in proportion with the increased synthesis of myofibrillar protein. However, more recent studies have shown that consistent resistance training “refines” the acute stimulus so that it is preferentially directed towards contractile protein synthesis more so than non-contractile proteins.
The increase in myofibrillar protein that is associated with hypertrophy includes an increase in the number of myosin and actin filaments inside each sarcomere as well as the addition of new sarcomeres in a parallel force-producing arrangement.[5,6] As the contractile proteins make up at least 80% of the fibre space, minimal increases in contractile proteins may contribute significantly to increasing the size of the muscle fiber. Aside from athletic populations such as bodybuilders, significant muscle hypertrophy during resistance training has been documented in male and female adults of all age groups, including people over 90 years of age.[8,9] It appears as though we never lose the capacity to build muscle and virtually anyone of any age can induce significant changes. So let’s now look at the exact mechanical prerequisites – the best way to lift weights to trigger the hypertrophic response.
It’s interesting to note that under maximal activation, each type of muscle contraction (concentric, isometric and eccentric) is capable of stimulating hypertrophy. [10,11,12] However, conventional, high-overload resistance training with barbells and dumbbells, involves voluntary contraction of muscles as they undergo concentric, isometric and eccentric actions against a constant external load – the magnitude of which is limited by the individual’s concentric strength.
Maximum eccentric strength is 20-50% greater than concentric strength. So it’s interesting to note that, eccentric loading during typical bodybuilding training is always sub-maximal. However, it still results in significant myofibrillar disruption, including Z-band disruption and satellite cell activation. All three types of contraction appear necessary to induce the optimum stimuli for growth.
Conversely, when one type of muscle contraction is removed from the lift, such as in negative-only training, this optimum response for growth actually diminishes.[9,10,11] This finding has been confirmed in novices and trained individuals alike. While we’re on the subject, although high-overload resistance training does cause muscle damage, it’s important to point out that nowhere in the literature has it been shown that muscle soreness is a prerequisite for growth.
The take home message here is, from all the research on the mechanics of optimizing the hypertropic response, nothing seems to beat (or come-close) to conventional lifting with high-overload, (progressive) resistance using barbells and dumbbells.
A Question of Strength . . .
Maximal voluntary strength is typically measured by repetition maximum (RM) in the gym or isometric/dynamic torque in the lab.[15,16] The relationship between strength and hypertrophy is an intimate one – large alterations in one seldom occur without significant changes in the other. Exercise scientists have known for a long time that maximum voluntary strength is closely associated with muscle size. [20,30,31]. In fact, any textbook on muscle physiology will demonstrate that muscle fibres in general, show a linear relationship between their cross-sectional area (size) and the amount of force they can generate.
For example, force production is usually proportional to muscle fibre CSA. An increase in muscle fibre CSA is thought to underline most of the improvements in force production and strength that are achieved during training.  Hypertrophy actually contributes to improved force production by altering muscle architecture such as, an increase in the pennation angle in pennate muscles (such as quads, triceps, delts etc) and fascicle length of the muscle fibers themselves, such as increase in the number of sarcomeres in series.[18,19] These alterations basically improve the muscles’ position and shape which results in greater force production.
A bigger muscle is able to contract with much greater force – no big news there, but what about, neural adaptations? What role do they play in strength and hypertrophy development?
The capacity to generate force is essentially dependent on motor unit activation. In most skeletal muscles, motor units are composed of a single motor neuron (nerve) and the multiple muscle fibres that it innervates. Motor unit populations differ between muscles; in general, small muscles such as the external rectus of the eye have 2 or 3 muscle fibres per motor unit whereas larger muscles such as the gastrocnemius (calf muscles) or quads can have up to nearly 2,000 muscle fibres per motor unit.
To initiate movement, motor units are recruited according to their size; from small to large. Maximal force production requires the recruitment of all motor units. This includes the high-threshold motor units that are only recruited during high-overload, maximal efforts. The high-threshold motor units are the ones bodybuilders should target as they respond the most dramatically with an increase in size. Novices and people new to resistance training appear not to be able to voluntarily recruit their highest-threshold motor units very well. It’s a skill that has to be learned – it is perfected only by lifting heavy.
Overload – The Fundamental Principle
The capacity to recruit motor units muscle fibers and activate the mechanisms of muscle growth comes down to one aspect; overload.
For over 60 years we’ve known that the amount of resistance (overload) placed on muscle is the fundamental principle that underlines adaptations to exercise training.[35-37] For example, it is clear that the degree of overload placed on muscle determines the amount and type of motor units (muscle) that are recruited during movement.
If the amount of overload placed on muscle is such a key aspect of weight training for muscle hypertrophy then obviously, an individual’s strength determines the amount of overload applied.
A person’s strength becomes the limiting factor in their potential to build muscle and improve body shape. More particularly, improvements in strength will enable greater overload to be placed on muscles. A gain in strength enables greater overload which in turn provides a new stimulus for muscle growth! Previously, you may have thought you understood the importance of building strength, but now you know the science-based reason why!
It is true that many factors may influence the expression of strength (lever arms, muscle insertions, genetics etc). However, the universal rules do not change; size and strength gains are directly proportional to the magnitude of overload placed on muscle.[30-32]
An improvement in strength would enable greater overload to be placed on the targeted muscles and therefore, provide further potential for hypertrophy.[17,32-34] Therefore, with regard to program design, a clear focus on improving strength is a most effective way to optimize the muscle-building response.
Enhancing Neural Drive . . .
When a person starts resistance training, their muscles soon learn to contract with greater force. A large part of the initial strength improvements observed in people when they first start training is thought to be a result of an improved ability to recruit all motor units that initiate and control movement.[24-26]
Training consistently, using progressive overload techniques, will enhance neural drive – the recruitment and rate of firing of motor units. It also improves recruitment order efficiency and increases the synchronization of the motor units of various muscle groups. The more often you perform a lift, the more your muscles learn a particular neural pathway to create movement. That’s why correct technique in your key lifts is so important. If you don’t learn the correct technique from the get-go, you’re actually teaching your nervous system poor habits. Ingrained poor neural activation due to months or even years of training with poor technique is one of the biggest reasons for plateaus and lack of progress from consistent training.
Training can also alter the manner in which muscles are recruited by the central nervous system. This is associated with a change in the input–output properties of the corticospinal pathway, such that a greater degree of muscle activation is generated by the same amount of cortical input. However, there are other neural aspects that I teach my students which involve the deactivation of antagonist muscles along with the improved activation of agonist muscles  as well as decreased Golgi tendon organ inhibition.
It’s clear that neurological changes influence the expression of strength and the potential for growth. However, one question I often get asked is, at what time do these changes occur? What’s the time-line for strength and hypertrophy development, when does neural adaptations end and hypertrophy start?
We’ll look at this next month in Part-3.
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