In his book “Training Systems,” Charlie Francis defines recovery or regeneration to be the: continuous management of muscle tension/spasm, accelerated removal of the effects of fatigue, rapid restoration of body energy systems and substrates, and improved ability to renew physical activity without wasting unnecessarily the energy of the athletes. Low-intensity aerobic exercise, or sub-maximal exercise, for recovery purposes can be implemented in a variety of ways including running, biking, or swimming. This recovery strategy has the ability to attenuate some of the negative effects of fatigue via its circulatory effects and may aid in maintaining or improving performance in subsequent bouts of high-intensity exercise. Determinants of the type of exercise chosen, it’s volume, and intensity, will be contingent on the demands of the sport and the individual status of the athlete game-to-game.
Mechanism #1: Lactate Clearance
In order to discuss the mechanisms of action behind the recovery benefits of low-intensity aerobic exercise, we must first understand the acute effects of high-intensity exercise that may temporarily hinder performance. During anaerobic glycolysis, lactate accumulates as a by-product of ATP re-synthesis. This accumulation of lactate results in an increase in hydrogen ions and metabolic acidosis, which negatively affects the excitation-coupling process within a muscle cell thereby decreasing its ability to produce force. As a result, this accumulation of lactate must be cleared in order to maintain an athlete’s performance capabilities.
Lactate is removed via oxidation, which can occur at the muscle site where it is produced, or it can be transported via the blood to be oxidized in other muscle cells. Lactate can also be removed via the Cori cycle, where it is transported to the liver to be converted to glycogen. Therefore, the increased activity of the cardiovascular and pulmonary systems during low intensity exercise accelerate lactate clearance and recovery by delivering more oxygen and transporting lactate around the body’s cells for oxidation.
Mechanism #2: Reduction in Muscle Soreness
Another response to high-intensity exercise is delayed onset muscle soreness (DOMS). DOMS is caused by the eccentric component of exercise, which causes tearing and micro-trauma within a muscle cell, and a corresponding perceived level of pain within the athlete due to the resulting inflammation. This pain and microtrauma reduces both the range of motion and contractile strength of a muscle. As a result, the damaged muscle cells must be repaired in order to restore optimal function. Protein synthesis and skeletal muscle repair is a process that requires the delivery of both nutrients and hormones, which are delivered via the circulatory system. Therefore, the elevated heart rate and blood flow that occurs as a result of low-intensity exercise may aid the recovery process by increasing the rate of delivery of nutrients and anabolic hormones to the affected skeletal muscle.
Mechanism #3: Enhanced Circulatory System
Lastly, these circulatory effects are augmented by physiological adaptations that occur as a result of low-intensity exercise. Aerobic exercise has been shown to increase cardiac output and stroke volume resulting in an increase in overall circulation. Furthermore, long-term aerobic exercise increases the density of capillaries which act as the exchange sites for the diffusion of oxygen, nutrients, and waste. Consequently, these chronic cardiovascular adaptations cumulatively enhance the rate at which the aforementioned recovery processes take place.
How to implement low-intensity training
As with any training modality, recovery or not, each athlete has a set of individual circumstances that will dictate what exercise prescription will elicit the most benefit for them. For example, a swimmer who is not used to the eccentric forces that are faced while running may obtain superior benefit from a pool tempo session as opposed to a field-based one. Contrary to this, a large football player would likely swim inefficiently resulting in a higher perceived effort relative to if they had completed a pool-workout in shallow water or an on-field tempo workout. Therefore, when considering implementing the following modes of low-intensity aerobic exercise it is important to consider the physiological status of the athlete, their training history, injury status, skills, and training demands.
Extensive tempo running, if implemented correctly, can facilitate recovery. This exercise consists of athletes running intervals that are “performed strictly in the aerobic energy zone [which] promotes general fitness development and recovery via circulatory mechanisms,” (Hansen 2014). This training can often be completed on a field to allow for easy prescription and tracking of volume. However, if various circumstances such as weather, size of athlete or their preferences, or injury may preclude the use of a field, the same concepts can be applied to completing a workout on a stationary bike or treadmill and using time as the measurement of work:rest as opposed to distance.
As an alternative to land-based workouts, athletes and coaches can opt to facilitate recovery in a pool. This option has that added benefit of hydrostatic pressure which “may be beneficial in reducing the symptoms of muscle damage and general fatigue,” (Joyce, 2014). Furthermore, as water is weight bearing, impact on the lower extremities is reduced relative to running which may be beneficial for larger athletes or injured athletes. A typical pool recovery session may incorporate calisthenics and dynamic motions such as water jogging, knee lifts, side-shuffles, and lunges in the shallow end of the pool in addition to swimming lengths at an easy pace.
As alluded to earlier, intensity must be manipulated in a way that is conducive for recovery. A sprint coach, of runners or swimmers, may use a percentage of best time for a given distance (Francis, 2008 & Lomax, 2012). Specifically, Hansen suggests that “…coaches and athletes shoot for a 60-65% effort to be on the safe side, particularly for newcomers to the technique who have yet to find their tempo ‘groove’,” (Hansen, 2014). This is congruent with findings by Lomax who found that 20 minutes of swimming intervals at 65% of maximum velocity resulted in an approximate 82% reduction in blood lactate levels (2012). This method is only valuable to athletes in sports where time is the standard measurement of performance.
The efficacy of this method of recovery, like any training, is partially contingent on if it is practical to implement and how it is implemented. Although this type of recovery modality can be extremely useful, one must make sure to keep the goal of recovery in mind and not let excessive volume or intensity creep into the sessions: “excessive use of intensive tempo workouts can result in excessive fatigue, overtraining, blunted recovery and central nervous system disruptions if the volume is too high or sessions are too frequent,” (Hansen, 2014). Furthermore, sometimes athletes do not have access to the required facility or they simply lack the energy to complete even a light workout. In these instances, alternative recovery strategies should be sought out.
High-intensity training sessions, where an athlete is producing maximum forces and velocities, are necessary for positive physiological adaptations. However, these high-intensity sessions elicit acute effects that temporarily impair performance. These negative effects include central and peripheral fatigue, muscle damage, the accumulation of lactate, and the depletion of energy stores. As a result, it is imperative for an athlete to facilitate the recovery process in order to attenuate these adverse effects prior to subsequent training sessions or competition. Low-intensity exercise provides a viable active recovery measure that augments the natural regenerative processes in the body. Namely, low-intensity aerobic exercise increases both circulation and capillary density, which aids in the delivery of nutrients and elimination of waste from skeletal muscle cells. This type of recovery can be implemented in multiple ways and the chosen mode should be representative of the needs of the athlete as it relates to familiarity, physiological status post-game, and availability of facilities. It seems that approximately 65% of best time or effort is an appropriate level of intensity with volume being contingent on the training status of the athlete and demands of the sport. Furthermore, athletes and coaches alike must keep the goal of recovery in mind and resist increasing intensity or they risk perpetuating fatigue as opposed to attenuating it.
Special thanks to Kit Wong, MKin, CSCS for his contribution to this article. Kit was a colleague of mine at SFU, and a fellow graduate student at UBC. He is currently the IST Lead and Head Strength Coach for Canada Cycling and the National Men’s Field Hockey Team.