How can we select exercises to fit different training frequencies?

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When writing a bodybuilding training program, we need to consider individual training variables (including rep range, proximity to failure, volume, frequency, rest period duration, exercise selection and others). However, we also need to consider how each of these variables interacts with the others. This is easiest to appreciate in the context of volume and frequency (higher workout volumes require lower workout frequencies and vice versa). Yet, another variable that also affects frequency is exercise selection.

What determines training frequency?

Training frequency is largely determined by the amount of long-lasting fatigue caused by each workout. Long-lasting fatigue is made up of several individual types of fatigue, including central nervous system (CNS) fatigue and multiple types of peripheral fatigue (mainly excitation-contraction coupling failure and myofibrillar damage).

#1. CNS fatigue

CNS fatigue is particularly important for determining training frequency, because it involves the temporary inability to recruit high-threshold motor units. When high-threshold motor units cannot be recruited, then their muscle fibers cannot be trained. Thus, when CNS fatigue is still present from the previous workout, the stimulus gained from a workout will be much smaller than it would otherwise be. This has a bigger impact than most people realize, because (1) high-threshold motor units control many times more muscle fibers than low-threshold motor units, and (2) high-threshold motor units control the muscle fibers that are most responsive to the stimulus of a strength training workout.

Although some strength coaches have suggested that CNS fatigue dissipates very quickly after a workout, this is not true. In fact, it has frequently been measured lasting several days after strength training, and can even last for over a week, in some more susceptible muscles.

A couple of hypotheses have been put forward to explain what causes CNS fatigue after a workout, but the most likely explanation is that it arises due to the inflammatory signaling that is triggered by myofibrillar damage. This would explain why workouts that cause more long-lasting fatigue also usually cause greater CNS fatigue.

#2. Peripheral fatigue

In the days after a workout, peripheral fatigue mechanisms (predominantly excitation-contraction coupling failure and myofibrillar damage) reduce the force that can be produced by some of the fast twitch fibers of high-threshold motor units. Unlike CNS fatigue, which completely switches off whole motor units (controlling thousands of muscle fibers each), peripheral fatigue mechanisms reduce the force that individual muscle fibers can exert, either by reducing the amount of calcium ions that are released into the cytoplasm in response to a signal from the CNS, or by reducing the force that can be produced by myofibrils or transmitted by the cytoskeleton.

Importantly, however, peripheral fatigue does not prevent many muscle fibers (and certainly any undamaged sections of muscle fibers) from contributing to muscle force, and thereby experiencing the mechanical tension that triggers hypertrophy. Whether it reduces mechanical tension of whole, affected fibers sufficiently to cause a smaller stimulus is not clear. Even so, training a muscle fiber while it is still experiencing myofibrillar damage may have a different detrimental effect, insofar as it may interrupt the repair processes that are ongoing, and lead to an incomplete or less-than-optimal repair. This may not have immediate implications, but could lead to long-term overuse injury (albeit there is no strong evidence to indicate that this does happen).

What types of exercise cause more long-lasting fatigue?

Certain training variables cause more long-lasting fatigue than others. For example, training with a higher workout volume causes more long-lasting fatigue than training with a lower workout volume. Yet, factors relating to exercise selection are also very relevant.

For example, exercises that involve larger ranges of motion (or which involve the peak force at longer muscle lengths) cause greater and more long-lasting fatigue than exercises that involve shorter ranges of motion (or which involve peak force at shorter muscle lengths).

Also exercises that involve a smaller amount of CNS fatigue *during* a strength training workout probably involve a larger amount of long-lasting fatigue after the workout. While this seems counter-intuitive, there is actually a very clear biological rationale. When an exercise involves a larger amount of CNS fatigue *during* a strength training set, then the fast twitch fibers controlled by high-threshold motor units will not be activated for as long a duration, and so they will not be as fatigued. Fatigue of fast twitch muscle fibers during exercise is a major factor that determines their damage (and therefore the amount of long-lasting fatigue) after exercise, since much of the damage results from biochemical processes and not from mechanical tension.

The greater the amount of muscle mass involved in an exercise (and the greater the cardiovascular demand), the greater the amount of CNS fatigue that occurs *during* a strength training set. Thus, multi-joint exercises cause more CNS fatigue *during* a strength training set than single-joint exercises, and two-limb exercises cause more CNS fatigue *during* a strength training set than single-limb exercises. As a result, multi-joint and two-limb exercises will likely cause less long-lasting fatigue after a workout than single-joint and single-limb exercises, respectively.

How does this relate to hypertrophy?

#1. Stretch-mediated hypertrophy

While exercises that involve larger ranges of motion (or which involve the peak force at longer muscle lengths) cause greater hypertrophy in some muscles (such as the quadriceps), they do not cause greater hypertrophy in other muscles (such as the triceps). This is likely because sarcomere stretch (not muscle stretch) is what causes the stretch-mediated hypertrophy effect.

Thus, for some muscles (such as the quadriceps), there is a trade-off between using full range of motion exercises to cause more hypertrophy in a workout but also simultaneously causing more long-lasting fatigue. In contrast, for other muscles (such as the triceps), there is no such trade-off. Using full range of motion exercises (or exercises that involve the peak force at longer muscle lengths) does not cause any more hypertrophy than partial range of motion exercises (or exercises that involve the peak force at shorter muscle lengths), but it does cause more long-lasting fatigue. This indicates that some muscles are always best trained with exercises that involve the peak force at shorter muscle lengths, if training frequency is to be optimal.

#2. High-threshold motor unit recruitment

Exercises that involve a smaller amount of CNS fatigue *during* a strength training workout (such as single-limb, single-joint exercises), likely allow the fast twitch fibers controlled by high-threshold motor units of the trained muscle to be activated for a longer duration of time. Consequently, they may provide a slightly larger hypertrophic stimulus on each set. Indeed, there is a small amount of evidence suggesting that single-joint exercises can cause more muscle growth for the targeted muscle than multi-joint exercises.

How does this work in practice?

To see how this works in practice, let’s look at an example.

The elbow flexors (which include the biceps brachii) are easily-damaged muscles that almost certainly do not experience additional stretch-mediated hypertrophy from full range of motion exercises (or exercises that involve peak force at long muscle lengths).

When training the elbow flexors (including the biceps), an exercise that would be expected to cause a substantial amount of long-lasting fatigue might be a single-arm incline bench dumbbell curl (single-limb, single-joint, peak force at long muscle length). In contrast, an exercise that would be expected to cause much less long-lasting fatigue is a narrow grip row (two-limb, multi-joint, peak force at short muscle length).

It makes sense never to use include exercises that involve peak forces at long muscle lengths for the elbow flexors, since they cause more long-lasting fatigue but no extra hypertrophy, so incline bench curls and preacher curls are actually not great choices, despite their popularity. Yet, we might program single-arm dumbbell curls (single-limb, single-joint, peak force at moderate muscle length) if we were training the muscle group once a week, standing barbell curls (two-limb, single-joint, peak force at moderate muscle length) if we were training the muscle group twice a week, and narrow grip rows (two-limb, multi-joint, peak force at short muscle length) if we were training the muscle group three times a week.

What is the takeaway?

When designing a training program with a higher muscle group training frequency (or when writing a training program for a muscle or an individual who has a tendency to experience a large amount of long-lasting fatigue), it is logical to select exercises that cause the least amount of long-lasting fatigue. In contrast, when writing a training program with a lower muscle group training frequency (or when writing a training program for a muscle or an individual who has a tendency to experience little long-lasting fatigue), it is possible to include exercises that cause more long-lasting fatigue, but it is only worth doing this if they also stimulate more muscle growth.

If you enjoyed this article, you will like my second book (see on Amazon).