Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemistry of Sports
Biochemical Characteristics of Recovery Processes during Muscular Activity
Sequence of Recovery of Energy Reserves after Muscular Work

The general patterns governing the Dynamics of Biochemical processes during the recovery period following muscular work are most clearly manifested in the restoration of energy substrates depleted during exercise—specifically, the replenishment of intramuscular phosphagen (ATP + PCr) and Glycogen reserves.

As previously noted (see Chapter 16), a linear relationship exists between exercise intensity and The rate of intramuscular phosphagen depletion. This same relationship links exercise intensity to the rate at which phosphagen reserves are replenished post-exercise. The maximum rate of phosphagen recovery is recorded immediately upon cessation of exercise, reaching approximately 20—25 mmol ⋅ L^-1⋅ min^-1. At the end of a workout, Muscle PCr reserves may drop by 70—90 % of their baseline levels. The rate at which they return to pre-exercise levels depends on the rate of aerobic ATP resynthesis, a process typically divided into two phases. The first, rapid phase has a "half-time" constant of about 22 seconds, covering the initial 3—4 minutes of recovery during which approximately 60 % of the PCr used during exercise is resynthesized (the "half-time" refers to the duration required for PCr reserves in working Muscles to increase by 50 % of their baseline levels). The second, slow phase of intramuscular PCr restoration has a half-time constant exceeding 3 minutes. Under normal conditions and with such kinetic characteristics, complete restoration of PCr reserves is achieved within 5—8 minutes post-exercise. Notably, the rate of phosphagen replenishment in muscles largely depends on the conditions under which recovery takes place. For instance, during occlusion (Blood flow cessation via tourniquet application), when the rate of aerobic ATP resynthesis is severely impaired, phosphagen levels in working muscles remain depressed throughout the entire occlusion period, with no pronounced supercompensation phase observed (Fig. 159).

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Fig. 159 Restoration of PCr reserves during the rest period following exhaustive exercise:

1 — fast recovery phase; 2 — slow recovery phase (black dots connected by a dashed line represent data from experiments with blood flow occlusion)

The rate of phosphagen recovery in muscles shows a close correlation with the payment rate of the alactacid (fast) component of the oxygen debt. This means that the greater the depletion of PCr reserves during exercise, the more oxygen must be delivered to working muscles during the post-exercise recovery period to ensure the restoration of creatine phosphate. As shown in Fig. 160, the bulk of the ATP required to fuel PCr restoration in working muscles is generated via the aerobic oxidative breakdown of CARBOHYDRATES and fats within the Krebs cycle and the mitochondrial Respiratory Chain. A fraction may also be derived from anaerobic Glycolysis, which continues to run parallel to oxidative processes in working muscles During the first few minutes of recovery.

Unlike the rapid replenishment of phosphagen reserves during post-exercise rest, the restoration of intramuscular glycogen reserves depleted during exercise takes many hours or even days. The recovery of intramuscular carbohydrate stores is significantly influenced by the type, intensity, and duration of the exercise, as well as The Nature and volume of carbohydrate intake during the post-exercise recovery period. Fig. 161 illustrates the time course of intramuscular carbohydrate recovery following a 20-minute bout of exhaustive exercise. The data show that during the rapid recovery phase (within the first hour post-exercise), the degree of carbohydrate replenishment is relatively minor, even under a high-carbohydrate diet. Achieving pronounced glycogen supercompensation in muscles requires at least 2 to 3 days. Restricting carbohydrates during recovery or imposing total fasting negatively impacts both the rate and absolute extent of carbohydrate restoration. Glycogen resynthesis in muscles post-exercise can utilize both internal substrates—specifically lactic acid and glucose formed from non-carbohydrate precursors—and additional carbohydrates ingested through diet.

Fig. 160 Energy Sources driving ATP resynthesis during the post-exercise recovery period

Fig. 161 Effect of dietary carbohydrate intake on the restoration of muscle glycogen reserves during post-exercise recovery: 1 — high-carbohydrate diet; 2 — high-protein, high-fat diet; 3 — fasting

Based on the linear relationship between intramuscular glycogen reserves and the time to onset of fatigue, specialized techniques have been developed and widely applied in athletic practice to boost the body's carbohydrate stores. The general pattern of changes in Liver glycogen reserves following heavy physical work, a multi-day carbohydrate-free diet, and a rapid "carbohydrate loading" phase the day before competition is illustrated in Fig. 162.

Fig. 162 Changes in liver glycogen content during carbohydrate deprivation and pre-competition carbohydrate loading