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Why is everybody making a fuss?
To put it simply, the question “What muscle does that work?” is virtually irrelevant for strength sports or any serious physical activity that involves resistance training.
Stimulation through exercise lasts between 24 and 48 hrs, non-enhanced trainees cannot make long-term progress with body part splits and often regress when switching to them from whole-body routines. (Christian Thibaudeau has recently addressed this topic in T-Nation a lot beginning with this post.) The body-part-split-plus-cardio regimen that most non-enhanced trainees stick too in the hope of looking like bodybuilders who train like that is just what prevent them to look like they lift. But their VO2max is probably higher than average, which is not that bad.
In most circumstances, the best exercises for non-enhanced trainees are those for which the answer is as close as possible to ‘all of them‘. That’s clearly the case when training for strength, and especially for strength sports, but it also holds true for hypertrophy. As for strength, given the principle of specificity, training sports movements or close variations of those movements is the best way to train sport-specific strength. And since sports movements involve “all the muscles” the close variations are going to involve them as well.
What everybody should really be making a fuss about
Let’s backtrack a bit and re-consider the question itself, as it is in fact not entirely unambiguous. Applying a common distinction from analytic philosophy, we can interpret it as either a normative or a descriptive question. Hand-waving the details of the normative vs. descriptive distinction, it boils down to this in our present case:
- Normative/General: What muscles should contribute to that movement under normal circumstances, when a fully-functional human being performs it?
- Descriptive/Case-based: What muscles do in fact contribute to that movement under the specific circumstances of me or you performing that movement?
We need to attend to some details about the normative vs. descriptive distinction for the special case under consideration, so let’s get to it.
Let’s just take the special case of pathological adaptations to sitting. The muscle map below shows the tonic muscles and the phasic muscles that are respectively shortened (contracted) and lengthened (relaxed) when we sit. Eventually, imbalances and compensations in the neck, lower back, shoulders and around elbows and knees will happen, turning into chronic discomfort or pain.
The relation between muscle imbalances and chronic pain is the basis of the Janda approach. And as McGill later showed, chronic pain results in non-normal muscle activation during motion. This turns the question “what muscle does that work?” into a tricky one as soon as some of the compensation has to get factored in.
Of course, this only applies if to people who sit a lot. But if that’s you (chances are that it is) most of the standard description of exercise libraries are inaccurate as far as you are concerned. They take physiology for granted and assume a ‘normal’ case while neglection 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.
 This is a theoretical maximal under the assumption that each additional word carries marginal information. The actual additional information content of the full post may, in fact, be lower.