What Muscle Does That Work?

around 1.400 words, estimated reading time: 6-7 min.

 “What muscle does that work?” is a question frequently asked by casual gym-goers.

And yet, the question has in principle no relevance for sports, even those which use resistance training to build explosive strength (Olympic weightlifting), pure strength (powerlifting) or any type of strength-endurance (strong[wo]man and kettlebell lifting). Ask the question to any serious athlete about the competition movement of any of those sports, and they will most likely answer ‘all of them’.

If they answer at all.

In fact, the place of pride of this question among gym-goers is a testament to the influence of cosmetic bodybuilding in the mainstream health & fitness media.

And yet, in some contexts where strength, power, and endurance are the main concern, the question may actually have some merits in practice. In this post, we look at some of them.

The (mostly) wrong reasons

The question “What muscle does that work?” makes perfect sense relative to training routines of professional bodybuilders. These routines typically train body parts on dedicated days, with specific isolation exercises aimed at developing muscles for cosmetic purposes.

But isolation exercises are mostly useless for muscle-building in the absence of performance-enhancing drugs. The most popular performance-enhancing drugs among bodybuilders, anabolic steroids, contribute to building muscle tissue by elevating protein biosynthesis. This effect is systemic, occurs all over the body, and is largely independent of the type of exercise.

Steroids with and without exercise

A study published in 1996 found that supraphysiologic injections of testosterone without training stimulus were more efficient than a well-designed exercise program without injections for building muscle and nearly as effective for building strength — as shown in the right-hand figure (see also here for a popular presentation of the study’s results).

Bodybuilders can thus rely on steroids drugs to cause non-stop systemic muscle growth. Isolation exercises merely help their body prioritize the resources allocated to this growth. By contrast, ‘natural’ trainees can only stimulate short-lived local protein synthesis in response to exercise.

Muscle mass is furthermore metabolically costly to maintain. Without anabolic steroids to reduce maintenance costs, natural trainees quickly lose understimulated muscle tissue. Since stimulation through exercise lasts between 24 and 48 hrs, natural trainees cannot make long-term progress with routines that train body parts once per week and often regress when switching to them from whole-body routines. (This post on T-Nation gives a more detailed explanation.)

The best exercises for natural trainees are thus those for which the answer is as close as possible to ‘all of them‘. That being said, training for a strength sport will not build a ‘natural bodybuilder’ physique. Protocols that maximize performance in those sports do not typically promote muscle growth as efficiently as other types of mechanical work. But such work must be implemented with whole-body exercises, as exemplified by this program for natural trainees that only uses two whole-body exercises.

The right questions (and why they are asked)

Outside of the narrow context of drug-enhanced cosmetic muscle-building, the question “what muscle does that work?” can be interpreted in two different ways:

  • Normative/General: What muscles should contribute to a given movement under normal circumstances.
  • Descriptive/Case-based: What muscles do in fact contribute to a given movement under specific circumstances.

We can typically answer the first question deductively, that is by drawing logical consequences of our knowledge of physiology and biomechanics. Physiology looks at the origins and insertions of muscles in our skeleton and hypothesizes what movement should result from the contraction of said muscles. Biomechanics also looks at origins and insertions but factors in the forces that our structure (skeleton, tendons, etc.) as a whole must counteract in order to maintain its integrity while performing a given movement. Then it hypothesizes which muscle should contribute to maintaining said integrity.

We typically answer the second question empirically, that is through observation of actual subjects having developed abnormal movement habits or facing abnormally high demands. Common research topics are pathological motor patterns following injuries or cumulative trauma and sports movement that impose unusual demands on the body while requiring structural alignment (to prevent injury).

In practice, both questions require some empirical investigation. For instance, physiology and biomechanics agree most of the time,. But when they do not, it is necessary to carry out observations in order to determine which hypothesis is correct. This last point exceeds the scope of this post but we will return to it in the future. Let us now proceed with examples of the two topics for empirical research, namely pathological movements and extreme demands.

An example: glutes and lower back

Gluteal muscles. By Mikael Häggström, used with permission.

A common pathology is gluteal amnesia which happens when the gluteal muscles — gluteus maximus, medius, and minimus — fail to activate normally. This pathology is covered at some length in the ebook we prepared for the Philosophy Department of the University of Lund to which we will refer for additional details.

The inhibition of gluteal muscles is most likely a consequence of prolonged sitting, which entails both a lack of use of those muscles and cumulative trauma due to compression. Failure to activate them can happen during both normal locomotion or exercises that target the glutes either as principal movers (for instance glute bridges with or without weight) or as auxiliary (for instance squats, again with or without weight). In that case, other muscles that are not inhibited by sitting may take over, like the quadratus lumborum (QL). Since the QL also compensates for other muscles inhibited by sitting (like the erector spinae), it tends to be overworked as a result. Hyperactive QLs are thus a common cause of lower back pain.

Interestingly, the QL can also become involved when the gluteal muscles do work the way they should but need the assistance of other muscles due to the addition of an external load. An example of such circumstances is the ‘Yoke Walk’ event, as demonstrated below by Kristyn Whisman, multiple time winner of America’s Strongest Woman, carrying 290 kg (636 lbs) for just under 10 meters (30 ft).

In a study published in 2009, Dr. Stuart McGill and his collaborators found the demand imposed by a heavy yoke walk on the gluteal muscles could exceed their maximal power output. Still, athletes managed to complete successfully an ‘impossible’ task: the co-contraction of the trunk musculature required to maintain the structural integrity of the spine under such extreme loads also permitted to recruit safely the QL, which in turn developed the additional power needed to move the load.

Hence, the same motor pattern, here the recruitment of the QL to assist with gait, can result from pathological adaptations (inhibited gluteal muscles) or positive adaptations (fully functional gluteal muscle in need of assistance).

Conclusions: a glimpse at the longer story

Our central nervous system (CNS) keeps some muscle under tension even at rest. Tension at rest is called muscle tone and muscles that exhibit it are called tonic muscles. Other muscles alternate between phases of rhythmic contraction and phases of relaxation and are called phasic muscles. They are easily tired and our CNS tends to let them relax whenever possible. The difference between phasic and tonic muscles goes a long way to explain some pathological adaptations.

Gluteal muscles are phasic muscles that tend to be inhibited by sitting. The QLs, which are tonic, literally ‘pick up the slack’ of the glutes. Over time, they become hypertonic, inhibiting the glutes even further, and leading to irritation and lower back pain. Another common example is the hip flexors (tonic) taking over the rectus abdominis (phasic) following e.g. the performance of too many sit-ups and crunches, and causing lower back pain.

Descriptions of exercises specifying the targeted muscles can be misleading. They take physiology for granted and neglect:

  • biomechanical constraints that may impose that some muscle groups ‘off-load’ the work on others even under normal circumstances; and:
  • pathological adaptations to injuries or cumulative trauma leading hypertonic muscles to take over inhibited ones.

Strength athletes are usually more aware than regular gym-goers of biomechanical constraints. Unsurprisingly, the question “What muscle does that work?” has become an object of endless ridicule among them, to the point of becoming an Internet meme.

But since strength athletes also sit a lot and are particularly exposed to injury, they should also pay more attention to constraints that result from pathological adaptations, as they are not always weeded out by rehabilitation.

We will explore this topic in future posts.

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