Timing Resistance Training

There are over 600 skeletal muscles in the human body, and each one has its own internal molecular time clock, termed a muscle clock, that helps muscles learn to anticipate upcoming training sessions. Muscle clocks are one of many internal biological clocks, including the master clock in the brain, that have the body on a 24-hour, daily rhythm. Muscle clocks use specific cues to monitor time intervals and coordinate the molecular actions associated with resistance training outcomes to anticipate workouts.

The discovery of muscle clocks demonstrates how the timing of workouts is critical because it helps avoid interference or competition between modes of exercise. Resistance training and cardiovascular training are competing modes of exercise that initiate different muscle molecular actions and confuse muscles. When cardiovascular and resistance training are performed within a single session or even within the same day, muscle performance can be adversely affected. This is because muscles look for consistent cues to know which molecular actions to turn on. The molecular actions associated with different types of exercise outcomes are unique and seek different cues. When cues are dissimilar, such as jogging and a squat, and occur within an hour of each other, muscles don’t know what to do, so they shut down, and performance is negatively affected.

Molecular Interference and Muscle Confusion

Molecular Competition. Molecular competition occurs when competing modes of exercise are performed within a single session (i.e., concurrent training) or the same day. Molecular competition has been studied in the context of interference theory, which is the scientifically backed idea that the long-term physiological adaptations associated with muscle growth, strength, and power versus aerobic endurance outcomes compete during individual workout sessions and over time. Therefore, it can be counterproductive to train for cardiovascular endurance and muscle hypertrophy, strength, and power within the same training session or even the same day.

When training is chaotic or poorly designed, muscles get confused and don’t know which actions to activate. So, they shut down, and all outcomes are adversely affected. The mechanisms of aerobic endurance and strength improvements compete, diminishing the positive effects of training. Under chaotic, unscheduled training conditions, muscles cannot anticipate what will be required of them. If resistance training is done at 11 a.m. one day and randomly at 6 p.m. a couple of days later and then at 8 a.m. later that week, muscles cannot use their internal 24-hour clocks to anticipate what’s next. They don’t know when they should prepare the molecular actions associated with muscle hypertrophy, strength, and power versus aerobic endurance outcomes.

Interference Mechanisms

It is widely accepted that the long-term muscle adaptations associated with resistance versus cardiovascular endurance training compete during sessions. As science continues to evolve and examine the interference phenomenon, suggested mechanisms have emerged, including short-term chemical changes, long-term changes in muscle structure or morphology, and changes in metabolic or biochemical processes. These interference mechanisms include muscle contractility (metabolic), delayed-onset muscle soreness (structural and metabolic), testosterone levels (metabolic), and cortisol and blood lactate levels (metabolic).

Muscle Contractility. Prolonged cardiovascular exercise interferes with a muscle’s ability to contract, and decreased contractility diminishes the likelihood of positive muscle growth, strength, and power outcomes. Prolonged cardiovascular training, such as jogging, can adversely affect a muscle’s ability to contract efficiently, which supports the idea that when resistance training is done after cardiovascular training, strength is adversely affected.

Delayed-Onset Muscle Soreness (DOMS). DOMS causes a series of events, including microscopic damage to muscle fibers, that can have a negative effect on the muscle’s ability to contract optimally and yield strength improvements. There are also metabolic factors contributing to interference, such as substrate depletion and increased protein breakdown. Substrate depletion is the reduced availability of adenosine triphosphate (ATP), phosphocreatine (PCr), muscle glycogen, and blood glucose. The depletion of these substrates compromises muscle functions during training. Protein breakdown during prolonged muscle work also causes DOMS and protein is needed to build muscle. Therefore, when the substrates and protein are depleted, hypertrophy, strength, and power performance are adversely impacted.

Testosterone Levels. Testosterone levels are directly related to muscle growth, strength, and power outcomes. The greater the testosterone level, the greater the muscle growth, strength, and power improvements. Researchers measured testosterone concentrations during three different modes of training: strength training only, concurrent training (both cardiovascular and strength), or cardiovascular training only. The major finding was that testosterone levels increased in the strength only group. As expected, testosterone levels decreased in both the cardiovascular only and concurrent training groups.

Cortisol and Blood Lactate Levels. Blood lactate and cortisol levels are additional metabolic factors that determine the efficacy (or lack thereof) of concurrent training. Where it is desirable that testosterone levels be high to yield muscle hypertrophy, strength, and power improvements, the opposite is true of blood lactate and cortisol concentrations. These levels should be low, or they interfere with increases in muscle and strength. Cardiovascular training before strength training interferes with muscle hypertrophy, strength, and power by elevating blood lactate and cortisol levels, which interfere with molecular adaptations associated with the desired performance outcomes.

Avoiding Interference

It has been established that interference happens, and that cardiovascular training gets in the way of muscle hypertrophy, strength, and power improvements. The problem is that both cardiovascular and muscle endurance are required for conditioning. Therefore, the solution lies in the strategic use of a variety of factors that include type, frequency, and duration of cardiovascular exercise relative to resistance training.

Frequency. Resistance and cardiovascular training need to be scheduled differently to avoid interference. If a program includes strength and cardiovascular endurance goals, it is best to schedule each mode of exercise on alternate days. However, that does not just mean doing resistance and cardiovascular training on different days and the problem is solved. Alternating days is only the beginning of the programming puzzle. It is a good start and an easy programming suggestion, but, when the data are examined closely, the suggested frequency of each mode of training is more complex. When training more than once per 24-hour period, guidelines suggest a minimum of 3 hours rest after any workout before beginning another workout and at least 6 hours, up to 24 hours, rest between cardiovascular endurance training and resistance training.

The frequency of cardiovascular training is one of the main culprits for interference theory. Programming suggestions are to limit cardiovascular exercise frequency to less than 3 days per week to minimize the adverse effects on strength. Duration of cardiovascular training should not exceed 40 minutes. However, more conservative estimates suggest keeping cardiovascular training volume at no more than 20 minutes, with 30 minutes maximum. The type of cardiovascular exercise is critical to influencing the degree that cardiovascular training affects hypertrophy, strength, and power outcomes. Research shows that jogging or running has a greater negative effect on muscle contractility than cycling. Cycling does not include eccentric action, which is known to cause more microscopic damage to muscle fibers, hence contributing more to resistance training interference. Also, cycling loads muscles in a resistance training manner. The mechanical resistance from the parts of the bike during pedaling can act like an external weight (i.e., pushing a big gear is like doing a weighted lunge). Therefore, cycling is recommended for concurrent training programs instead of jogging or running.

Intensity. The intensity of cardiovascular endurance training has an impact on the extent of muscle force impairment. Specifically, moderate- to high-intensity cardiovascular training reduces the effectiveness of strength training. Therefore, the suggested intensity of cardiovascular sessions should be low (40% - 50% maximum heart rate) if hypertrophy, strength, and power are primary training goals.

Baseline Strength Recovery. General guidelines suggest that muscles need about 48 hours of rest after high-intensity resistance training (anything above 80% of 1-RM) to recover to baseline strength. However, too much rest is a bad thing. Rest periods should not exceed 96 hours, because after that the physiological processes of detraining begin.

Upper- Versus Lower-Body Training. Most of the data collected on the effects of concurrent training on muscle strength analyzed lower-body strength. In most cases, a 1-RM squat is used to measure lower-body strength. However, one study examined the effects of lower-body sprint interval training on upper-body hypertrophy and strength. Results showed that sprint interval training combined with resistance training adversely affected upper-body hypertrophy and strength. This finding is interesting because it shows that the effects of concurrent aerobic endurance and resistance training are not muscle-use specific. Lower-body sprint interval training had a negative effect on upper-body strength performance. The results showed that the effects of endurance training influence nonworking muscle. Hence, the data demonstrated that the mechanisms responsible for interference cannot be avoided by using different muscle groups for aerobic endurance and strength training.

Physiological Cues

Physiological cues are biological markers inside the body that internal clocks recognize as time cues. They are biochemical changes that reflect the time of day and changes in muscle activity, such as training and rest patterns, type of scheduled exercise, and naturally fluctuating hormone levels. Physiological cues include things like testosterone, human growth hormone (HGH), cortisol levels, and muscle pliability. Biochemical levels and natural patterns of highs and lows are influenced by both endogenous (inside the body) and exogenous (outside the body) factors. An example of an endogenous biological marker is the natural fluctuating testosterone levels throughout the day, while an exogenous factor is cortisol release caused by exercise.

Testosterone. Testosterone levels naturally fluctuate throughout the day. Testosterone levels are highest in the morning and begin to level off between 4 and 6 p.m., decreasing after that. Although testosterone levels naturally change throughout the day in a pattern consistent with most people, testosterone levels can be manipulated by factors such as resistance training. Resistance training influences testosterone levels during and after a workout session. Research has shown that testosterone levels remain elevated up to 30 minutes after resistance training.

Human Growth Hormone (HGH). HGH is a biochemical, like testosterone, that is critical to muscle growth and strength and power development. Just like testosterone, HGH levels fluctuate naturally. During sleep, 75% of HGH is released, with the majority released during the first hour of sleep. The fact that HGH is released during sleep emphasizes the critical role of rest to muscle recovery and performance. HGH release during sleep, specifically during the first hour of sleep, is an example of the body’s natural rhythms and how they are synchronized to activity-rest cycles during a 24-hour period. Just like testosterone, HGH is released into the bloodstream during exercise. Compound exercises such as squats and deadlifts, which use multiple joints and large muscle groups, are the most effective in releasing HGH. In addition, eccentric contractions cause more HGH release than concentric contractions.

Muscle Pliability. Tissue pliability is an indication of the natural elasticity of muscle, and it varies significantly from the time someone wakes to when he or she goes to bed. In accordance with body temperature changes throughout the day, muscle tissue is the least pliable first thing in the morning, but as the day goes on, the natural elasticity of muscle increases, peaking between 4 and 6 p.m., indicating that muscles will be most flexible during that time of day. Time of day that a muscle is most pliable is an important time cue for muscle clocks and a factor in muscle performance because muscles generate their greatest force just beyond resting length. A slightly stretched muscle generates the most strength and power.

Eating Habits. The timing of eating relative to any form of exercise is important to muscle performance and recovery. The nutrient makeup of meals and overall caloric intake are critical as well. Timed eating and muscle glucose uptake are cues that muscle clocks recognize, helping them determine the time of day and regularly occurring events.

Cortisol. Because exercise is a stressor (a positive one), it stimulates the release of cortisol, a stress hormone, into the bloodstream. Generally, cortisol is released in response to emotional stress (such as pain, anger, and fear) and physical work (such as scheduled resistance training). Like most of the body’s chemicals, cortisol also has its own natural daily rhythm. Natural cortisol levels are at their highest around 8 a.m. and lowest at 3-4 a.m. Like other biochemical markers in the body, such as testosterone and HGH, cortisol levels fluctuate throughout the day but also increase or decrease in response to their environment. For optimal muscle performance during resistance training, blood lactate and cortisol levels should be low, or they interfere with muscle growth, strength, and power improvements. Cardiovascular training before strength training also interferes with muscle hypertrophy, strength, and power by elevating blood lactate and cortisol levels, which interfere with the molecular adaptations associated with the desired performance outcomes.

Consistently high levels of cortisol create a dilemma because cortisol has been correlated to a decrease in skeletal muscle mass. Too much cortisol interferes with amino acid uptake that is critical to muscle growth. In addition to potentially decreasing muscle mass, cortisol increases free-floating glucose in the body, which ultimately leads to more fat mass. Too much fat combined with less muscle is highly undesirable to performance. Therefore, cortisol levels must be controlled, and scheduled exercise plays an important role in stabilizing those levels. Elevated levels of cortisol for an extended time cause a series of negative effects. However, resistance training can increase cortisol at scheduled times as a muscle clock entrainment cue and then decrease cortisol over time, leading to improved muscle function and possibly contributing to decreased fat mass, both of which are significant to improved performance.

Exercise Training and Programming

Successful resistance training programming relies on strategic timing. Muscle clocks use exercise training and programming cues to help regulate muscle performance and synchronize muscles with other body systems. The timing cues muscles look for are programming variables that all sport and fitness practitioners are familiar with, including mode, frequency, volume, duration, intensity, and rest periods. The only difference is that instead of being strictly programming variables, these same concepts are now viewed as muscle clock entrainment tools as well.

Forced Exercise and Clock Entrainment. The difference between forced versus voluntary exercise is that forced exercise is programmed or scheduled and voluntary exercise is recreational, by choice, spur-of-the-moment physical activity, such as a pickup basketball game. Scheduled exercise is powerful. It can override the natural light-dark cycle of biological clocks. Scheduled training is a cue that helps give muscles autonomy and flexibility in responding to the demands of their unique environment. It can teach muscles to turn on the associated actions of muscle performance at the desired time of day ahead of training, improving the effectiveness of training, reducing the likelihood of interference, and providing muscles with information about when to perform and when to recover. Regularly scheduled exercise at a set time of day over the course of weeks and months helps muscle clocks set an internal 24-hour rhythm and coordinate skeletal muscle tissue to anticipated resistance training sessions.

Training Frequency. Interference happens when cardiovascular and resistance training are performed too close together in time; however, interference can be avoided by allowing enough time to elapse between the two modes of exercise. Ideally, there should be 4 to 6 hours between cardiovascular and resistance training, and training sessions should never be within 30 minutes of one another. To provide consistent cues, cardiovascular training should be performed on alternate days from resistance training. Determining a day-to-day training schedule that alternates cardiovascular training with resistance training tells muscles when to be ready to turn on the molecular actions associated with cardiovascular versus resistance training outcomes.

Exercise Mode. The mode of exercise is the type of exercise performed. Interference occurs when cardiovascular exercise is performed less than 3 hours before resistance training. Research has clearly demonstrated that resistance training should be done before cardiovascular training to avoid interference when done in the same session or within 3 hours of one another. Additionally, when cardiovascular and resistance training are done within the same session, protein synthesis can be disrupted, leading to little or no muscle fiber size change and thus decreasing strength-related performance. Therefore, making sure that cardiovascular and resistance training are clearly distinguished and separated is important, because the mode of exercise is a cue for muscles to learn when to anticipate cardiovascular or resistance exercise and turn on the associated actions with either.

Biomechanical Similarity. Pairing exercises that use similar joint actions establishes biomechanical similarity, providing an important exercise and training cue for muscle clocks. Biomechanical similarity is a training method that pairs two exercises that are alike. Biomechanically similar exercises work the same or similar muscles, but they activate those muscles in different ways. Different movement patterns use different bundles of muscle fibers within the same muscle. For example, both a back squat and leg press train the muscles of the legs and hips. However, each exercise activates slightly different bundles of muscle fibers within the same muscles. The end programming result is a more comprehensive workout for the entire muscle group.

Intensity. The intensity of exercise is a critical factor for clock entrainment. The higher the intensity of exercise, the more testosterone, cortisol, and HGH are released into the bloodstream. All these biochemical events are timing cues that contribute to muscle clock entrainment; when released on a regular schedule within 24-hour periods, intensity-related biomechanical muscle changes can contribute to phase-shifting of muscle clocks.

Intensity-Rest Cycles. Another example of timing sessions is intensity-rest cycles. The amount of rest required for recovery from one set of an exercise to another varies by the intensity of the exercise, determined by the volume of muscle mass used, the percent of 1-RM, and the speed of muscle contraction. This is a key point because intensity is generally higher for lower-body exercises, and thus the length of rest required to recover is longer than what an intraset break allows. Muscles are monitoring all cues related to exercise mode, frequency, and timing; therefore, intensity-rest cycles give them another cue about what to anticipate and when.

The exercise programming suggestion for intermittent rest is the same as intermittent fasting: take 2 non-consecutive days of rest per week, while training the remaining 5 days per week. An intermittent rest schedule is contrary to the popular 5 days per week training with weekends off schedule. The rationale behind an intermittent-rest programming schedule is that the master clock in the brain synchronizes all clocks to a 24-hour schedule, and this is reset daily or every 24-hour cycle. Therefore, all clocks, including muscle clocks, are looking for 24-hour schedules and rhythms. They are monitoring external and internal events to set a schedule and coordinate physiological functions on a 24-hour cycle. Taking 72 hours off on the weekend or training on Friday morning and not again until Monday morning confuses 24-hour clocks.

Conclusion

Muscle clocks are internal timekeepers monitoring environmental and physiological signals that affect muscles. The muscle clocks learn over time what to expect, when, and how to respond by paying attention to a wide range of cues, including time of day from the master clock in the brain, regularly occurring physiological changes in local muscle tissue, and scheduled exercise programming and training cues. With the right time cues, muscle clocks are able to develop their own 24-hour daily rhythms that allow them to anticipate upcoming training sessions and activate in advance the molecular actions associated with muscle performance and recovery, improving resistance training outcomes.

Sources

[1] Ashmore, A. (2020). Timing resistance training: Programming The muscle clock for optimal performance. Human Kinetics.