In every strength training program, there is a period of time between workouts in which we “recover” from the workout.
Some workouts do not cause any detrimental effects that we need to “recover” from, and some in fact produce a temporary “potentiating” effect whereby performance is improved for a short time afterwards. Yet, other workouts (especially those involving high volumes of eccentric contractions in unaccustomed exercises), do cause a sudden and prolonged reduction in our ability to produce force.
The time it takes for our ability to produce force to return to pre-workout levels is called “strength recovery” and is often taken as the best available measure of whether we have “recovered” from a workout or not.
What causes losses in strength after a workout?
Immediately after a workout, our strength is reduced because of the fatigue that we experience during the repeated muscular contractions, in addition to the muscle damage that is caused.
The fatigue can arise from changes inside the central nervous system (CNS) or from inside the muscle. This is why the expression “neuromuscular fatigue” is sometimes used. This means that there are three key factors: (1) peripheral fatigue, (2) central fatigue, and (3) muscle damage.
#1. Peripheral fatigue
Peripheral fatigue can include a transitory accumulation of metabolites, altered calcium and sodium-potassium pump functions, and a decrease in intra-muscular glycogen.
Exercise intensity and duration affect which factors are most important. Short, intense exercise produces a larger transitory accumulation of metabolites, while long, sustained or intermittent (but still fatiguing) exercise produces changes in calcium ion pump function, and glycogen depletion.
Strength training clearly falls into the category of short, intense exercise that involves an elevation in metabolites. As you might expect, these metabolites are cleared from the system fairly quickly, and do not impact strength recovery from one day to the next.
#2. Central fatigue
Central fatigue is defined as an impairment in the ability to activate a muscle voluntarily. We can measure this using the interpolated twitch method, which involves comparing the ability of a muscle to produce force in voluntary and involuntary (electrically-stimulated) contractions.
New research has now shown that central fatigue is quite transitory after strength training, except when it occurs because of muscle damage.
In fact, the nature of central fatigue is such that it increases with increasing exercise duration, and decreases with exercise intensity. Thus, long cycle rides actually cause more central fatigue than short, intense ones.
This suggests that if muscle damage from a workout is minimal, then central fatigue after strength training will not be observed.
#3. Muscle damage
Some strength training workouts can cause muscle damage, especially when they are different from the type of workouts that an individual has performed in the recent past.
The damage can occur to the extracellular matrix, causing it to pull away from around the muscle fiber, as well as to the muscle fibers themselves, most commonly to the Z disks that link one sarcomere to the next, and to the sarcolemma that surrounds the fiber.
When muscle fibers incur mild damage, they are repaired. However, after severe muscle damage, muscle fibers becomes necrotic and die. This then requires muscle regeneration (making a new fiber), which occurs inside the cell membrane of the necrotic fiber, and takes quite a while to happen.
Mild muscle damage is common, albeit not universal, after strength training. And while severe muscle damage leading to necrosis is not commonly observed after strength training workouts involving voluntary contractions (in contrast to workouts involving electrically-stimulated contractions), necrotic muscle fibers have been observed in high-level powerlifters (but whether this happens after individual workouts, or is a cumulative effect from repeated training is not known).
This suggests that a very varied amount of muscle damage can occur after strength training, and the amount of damage is likely what determines how long it takes strength levels to return to normal.
What causes muscle damage?
Muscle damage is most commonly observed after workouts involving eccentric contractions, although isometrics at long muscle lengths are also very effective at producing damage, and there is some research out there showing that concentrics *can* cause damage too.
Muscle damage is increased by higher volumes, a closer proximity to failure, longer duration (isometric) contractions, heavier loads, larger ranges of motion, a more elongated muscle, and by using a constant load rather than accommodating resistance.
Most of these features suggest that muscle damage is primarily caused by high muscle forces, as these cause more fascicle strain (elongation), and the greater stretch on the muscle fiber causes more damage.
Indeed, muscle forces are higher when using heavier loads, as well as when the passive elements contribute more to force production, which is what occurs during eccentric contractions, when working through larger ranges of motion and at longer muscle lengths, and when using a constant load rather than accommodating resistance.
Equally, higher volumes, longer duration (isometric) contractions, and closer proximity to failure also increase muscle damage. This suggests that time under tension and the accumulation of metabolites also have damaging effects. Indeed, studies using animal models indicate that longer time under tension for each contraction might cause more muscle damage because of calcium overload, which is further increased by fatigue and hypoxia.
Finally, it is important to remember that muscle damage is always higher when the exercise is unfamiliar. This may be because fascicle strains often vary quite a bit throughout a muscle, and different exercises probably cause more fascicle strain in one region than in another.
How much of a problem is muscle damage?
To accelerate recovery from a strength training workout, bodybuilders, strength athletes, and team sports athletes must minimize muscle damage. Clearly, this requires managing the factors that cause muscle damage within the workout, which are: (1) muscle force, (2) fatigue, (3) time under tension, and (4) exercise familiarity.
Unfortunately, many of the adaptations that are sought by bodybuilders, strength athletes, and team sports athletes require workouts to incorporate one or all of these factors.
Bodybuilders use strength training solely to produce muscle growth.
Hypertrophy is produced by the signaling cascades that result from detecting mechanical loading at the membrane of muscle fibers. Like muscle damage, hypertrophy is enhanced by greater muscle forces, longer time under tension, and greater fatigue (insofar as greater fatigue increases the forces experienced by single muscle fibers).
Additionally, to maximize hypertrophy, a variety of exercises are often used, because this helps produce muscle growth in all regions of a muscle. This can increase the need to use exercises that are not performed regularly, thereby exposing the individual to unfamiliarity.
#2. Strength athletes
Strength athletes like powerlifters use strength training to increase maximum (concentric) strength.
The adaptations that produce these gains in maximum strength include increases in muscle size, voluntary activation, tendon stiffness, load-specific coordination, and lateral force transmission, and they all arise in response to high muscle forces.
#3. Team sports athletes
Team sports athletes use strength training to improve sprinting performance, change of direction ability, and jump height.
We can analyze these movements biomechanically to determine the types of strength that they require, and while eccentric strength is valuable for the hamstrings in sprinting, and for the quadriceps in changing direction, all of the movements benefit from increasing high-velocity strength.
Importantly, high-velocity strength is best improved using light loads and fast bar speeds, because it is achieved through increases in early phase neural drive (large bursts of rate coding), reduced coactivation, and changes to muscle fiber contractile properties. And since this type of training involves low muscle forces, little fatigue, and minimal time under tension, it produces little muscle damage.
How can we minimize muscle damage?
Clearly, bodybuilders have the greatest challenge ahead of them for reducing muscle damage, as the exact same factors that influence the extent of muscle damage also affect the amount of muscle growth that is stimulated. The optimal approach will likely involve a careful balance between the factors, and the best recipe may actually be quite individual.
In contrast, if team sports athletes focus the majority of their efforts on high-velocity strength training (often misleadingly called “power” training), they can avoid muscle damage altogether. While such athletes still require eccentrics for both injury prevention and certain aspects of performance, these can be allocated over the week in order to avoid adverse effects.
Strength athletes fall somewhere between the two extremes. They can avoid fatigue, long time under tension, and exercise unfamiliarity by focusing their efforts on a smaller number of exercises performed with lower volumes of very heavy loads, but with long rests and avoiding failure. This means that they will only incur muscle damage from high muscle forces.
What does this mean?
The time it takes for our strength to return to pre-workout levels is called “strength recovery” and is the standard way of assessing whether we are fully recovered from the workout, and ready to perform the next one. Strength recovery is affected by three factors (peripheral fatigue, central fatigue, and muscle damage), but the amount of muscle damage is the key element.
Muscle damage is increased by higher muscle forces, greater fatigue, longer time under tension, and exercise unfamiliarity. Since all of these factors also positively influence muscle growth, balancing the stimuli that produce hypertrophy with the stimuli that cause muscle damage is challenging.
Conversely, only high muscle forces are required to produce gains in maximum strength, so strength athletes can compromise on the other factors that produce muscle damage in order to improve the rate of strength recovery. Team sports athletes can focus on developing high-velocity strength using light loads and fast bar speeds, which causes no muscle damage at all.
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