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

Biochemistry of Sports
Biochemical foundations of athletes' speed and strength qualities and methods for their development
Biochemical characteristics of speed and strength qualities

The most important speed-strength qualities of an athlete are strength, speed, and the power of the muscular effort generated. Their manifestation depends on a range of psychological, physiological, and Biochemical characteristics of the body.

Maximum values of speed-strength qualities are achieved through an extremely high concentration of volitional effort. This ensures optimal excitation in motor centers and maintains the maximum frequency of impulses in motor nerves, thereby recruiting the largest possible number of motor units. The manifestation of speed-strength qualities largely depends on The ratio of fast- and slow-twitch fibers within the Muscle and the specifics of its internal biochemical composition—in particular, the orientation of tendon tracts and the arrangement of muscle fibers relative to them (which determines the magnitude of the total force developed at the attachment points of the tendon terminals to the skeletal levers). It also depends on movement coordination (combining the efforts developed by synergistic Muscles, counteracting antagonistic muscles, the sequential temporal activation of individual muscle groups, etc.).

At the level of individual motor units, the manifestation of speed-strength qualities is determined by the frequency of impulses reaching the synaptic structures on the outer membrane of the muscle fiber, the speed of electrical excitation transmission from the outer membrane to myofibrils, the flow rate of Ca²⁺ ions released from the internal cisternae of the sarcoplasmic reticulum into the intracellular space, The rate of activation development in the myofibrils, as well as the total quantity, enzymatic properties, and Structural Features of myofibrillar contractile Proteins, and so forth.

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Fig. 171 Dependence of maximum muscle strength on sarcomere length and Actin content in muscle fibers

The primary biochemical factors limiting the manifestation of speed-strength qualities can be identified using "fundamental relationships" for Skeletal Muscle. The first of these relationships describes the conditions for generating maximum muscle strength (Fig. 171). Experimental studies on various Human and Animal muscles show that the magnitude of maximum muscular effort is directly proportional to the sarcomere length or the length of thick Myosin filaments—that is, the degree of myosin polymerization—and the total content of the contractile protein actin in the muscle. As noted previously, the force developed during the interaction of Actin and myosin filaments in myofibrils is proportional to the number of formed cross-bridges: the greater the area of overlap between thin Actin filaments and thick myosin filaments within each sarcomere, the greater the maximum force developed by the muscle. The maximum possible contact area between filaments is determined by the length of the thick myosin filaments or an individual sarcomere. The longest sarcomeres are found in mollusk catch muscles; these muscles are capable of generating forces 3 to 4 times greater than the maximum human muscle strength. The shortest sarcomeres are located in the flight muscles of insects and hummingbirds: the maximum strength of these muscles is approximately 3 times less than that of humans. In human skeletal muscles, the average sarcomere length is 8 µm, and the length of myosin filaments is about 1 µm. In terms of maximum strength, human muscles occupy an intermediate position between mollusk muscles and insect flight muscles.

Sarcomere length, or the degree of myosin polymerization in the thick filaments of myofibrils, is a genetically determined factor; therefore, it remains unchanged during ontogenesis and training. Sarcomere length varies across different fiber types within various muscles. At the same time, the actin protein content in muscles changes significantly during individual development and under METABOLISM/18.html">The Influence of training. This indicator reflects pronounced differences in muscle fibers of various types and in muscles with different functional purposes.

The actin content in muscle myofibrils is linearly dependent on the total amount of creatine (Fig. 172). Both indicators—actin content and total creatine concentration in muscles—can be used to monitor The Development of muscle strength and predict athletic performance levels in speed-strength exercises.

The second fundamental relationship describes The connection between the maximum muscle shortening velocity, sarcomere length, and relative myosin ATPase activity. The highest shortening velocity is observed in the flight muscles of insects and hummingbirds, which feature the shortest sarcomeres, while the lowest is found in mollusk catch muscles, which contain the longest sarcomeres (Fig. 173). The maximum shortening velocity varies among different muscle fiber types: in fast-twitch white fibers, it is approximately 4 times higher than in slow-twitch red fibers.

In human voluntary movements, what matters is not the isolated manifestation of strength or shortening velocity, but their combined effect, evaluated by the power of the generated effort. Since power is the product of force and velocity, based on the already established relationships for force and shortening velocity, it is easy to derive a third relationship describing changes in power During Muscle contraction. The power developed by a muscle depends on the total ATPase activity, i.e., the overall rate of ATP Hydrolysis (Fig. 174).

Fig. 172 Dependence of actin protein content on the total amount of creatine in skeletal muscles

Fig. 173 Dependence of maximum muscle shortening velocity on sarcomere length and myofibrillar ATPase activity

Fig. 174 Dependence of maximum muscle power on total myofibrillar ATPase activity

The values of maximum power, much like maximum shortening velocity, vary significantly across different muscle fiber types and change noticeably during adaptation to specific types of motor activity. In fast-twitch fibers, maximum power is about 155 W ⋅ kg^-1 of muscle mass, whereas in slow-twitch fibers, it is 40 W ⋅ kg^-1.

Total ATPase activity is higher in fast-twitch fibers. Accordingly, the maximum contraction power of a muscle is closely related to the percentage of these specific fiber types in the working muscles (Fig. 175). Sprint runners, in whose gastrocnemius muscle the proportion of fast-twitch fibers reaches 60%, significantly outperform long-distance runners in maximum power values (120 W ⋅ kg^-1 versus 85 W ⋅ kg^-1), in whom fast-twitch fibers account for only 35%.

Fundamental muscle relationships also include the so-called Hill's characteristic equation, which defines the relationship between the manifested force and the shortening velocity. As seen in Fig. 176, the maximum force is manifested under isometric conditions at a shortening velocity equal to zero, while the maximum shortening velocity is developed at a relative force of approximately 0.2 of the individual maximum isometric effort. The characteristic curve is equally applicable to both fast- and slow-twitch muscle fibers (Fig. 177).

In human skeletal muscles, the isometric maximum contraction force ranges between (15–30)×10⁴ N ⋅ m^-2, and this value does not differ substantially between fast- and slow-twitch fibers. At the same time, the maximum shortening velocity of white fibers is 4 times greater than that of red fibers. Since in most skeletal muscles red and white fibers are present in specific proportions, the contractile properties of these muscles will fall within the region of the characteristic curve bounded by the extreme values for red and white fibers.

Fig. 175 Dependence of maximum muscle power on the percentage of fast-twitch (FT) fibers: 1 — hypokinesia; 2 — control; 3 — long-distance runners; 4 — sprinters; 5 — runners

Fig. 176 Dependence of relative force on skeletal muscle shortening velocity

Fig. 177. Relationship between relative force and maximum shortening velocity in fast- and slow-twitch muscle fibers

Based on the described relationship between muscle force and contraction velocity, it is possible to establish the core requirements for exercises aimed at developing speed-strength qualities. Specifically, when developing strength capabilities (improving maximum muscle strength), the resistance to be overcome should be 70–100% of the individual's isometric maximum for the given muscle group; when developing contraction velocity, it should be 20–40%; and when improving the combined manifestation of force and contraction velocity—that is, power—it should be 40–70%. A fundamental requirement for speed-strength exercises is their maximum Specificity to The Structure of the primary exercise and the creation of conditions allowing execution with maximal effort.