The development of the SR is more pronounced in fast muscle compared to slow muscle. This is due to higher levels of RYR, SERCA, lumenal Ca, and a higher DHPR/RYR ratio. These factors contribute to the creation of a larger and faster intracellular Ca transient in fast muscle, which is crucial for quick and forceful contractions. Aside from the aforementioned differences between fast and slow fibers, other muscle proteins are also expressed in a fiber type-specific manner. These proteins include the three troponin subunits, tropomyosin, and C protein. The variation in troponin and tropomyosin isoforms affects the dependence of contraction on Ca++. Slow fibers initiate tension at lower concentrations of Ca++ compared to fast fibers. This difference in Ca++ sensitivity is partly attributed to the presence of a single low-affinity Ca++-binding site in the troponin C isoform of slow fibers, whereas fast fibers have two low-affinity binding sites. However, changes in the dependence of contraction on Ca++ are not solely limited to troponin C isoforms. Distinctions in troponin T and tropomyosin isoforms are also observed. Consequently, the regulation of contraction dependence on Ca++ is complex and involves multiple proteins on the thin filament. Phosphorylation of the regulatory light chain of myosin by Ca++/calmodulin-dependent myosin light chain kinase can enhance the Ca++ sensitivity of contraction, particularly in fast muscle fibers due to the higher activity of myosin light chain kinase in these fibers.

Increasing the frequency of electrical stimulation of skeletal muscle results in a greater force of contraction. This is attributed to the prolongation of the intracellular Ca++ transient and is known as tetany. Incomplete tetany occurs when another intracellular Ca++ transient is initiated before the muscle has fully relaxed, resulting in the summation of twitch forces. To increase the force of contraction, a muscle can recruit more muscle fibers. Recruitment of additional motor units is achieved by activating more muscle fibers within a motor unit. Slow-twitch motor units, which are innervated by easily excited α motor neurons, tend to be small and are recruited first. Fast-twitch motor units, on the other hand, tend to be larger and are innervated by α motor neurons that are more difficult to excite. Therefore, as the force requirement increases, fast-twitch motor units are recruited. This recruitment strategy ensures that the initial muscle fibers recruited are highly resistant to fatigue. Additionally, the small size of slow-twitch motor units allows for precise motor control at lower force levels. This process of increasing the force of contraction by recruiting additional motor units is referred to as spatial summation, as the forces from muscle fibers are combined within a larger area of the muscle. This is distinct from temporal summation, which will be discussed later. Action potentials in skeletal muscles are uniform and lead to the release of a consistent pulse of Ca++ from the SR.
The SR is more highly developed in fast muscle than in slow muscle, with higher levels of RYR, SERCA, lumenal Ca, and a higher DHPR/RYR ratio, all of which promote the development of a larger, faster intracellular Ca transient in fast muscle, which is important for quick, forceful contraction. In addition to the differences between fast and slow fibers just noted, other muscle proteins are also expressed in a fiber type–specific manner. Such proteins include the three troponin subunits, tropomyosin, and C protein. The differential expression of troponin and tropomyosin isoforms influences the dependency of contraction on Ca++ . Slow fibers begin to develop tension at lower [Ca++] than fast fibers do. This difference in sensitivity to Ca++ is related in part to the fact that the troponin C isoform in slow fibers has only a single low-affinity Ca++-binding site, whereas the troponin C of fast fibers has two low-affinity binding sites. Changes in the dependence of contraction on Ca++ , however, are not restricted to differences in the troponin C isoforms. Differences in troponin T and tropomyosin isoforms are also found. Thus regulation of the dependence of contraction on Ca++ is complex and involves contributions from multiple proteins on the thin filament. Phosphorylation of the regulatory light chain of myosin by Ca++/ calmodulin-dependent myosin light chain kinase, however, can increase Ca++ sensitivity of contraction, particularly in fast muscle fibers (partly because of the reported higher activity of myosin light chain kinase in fast muscle fibers).
•Fig. 12.16. Increasing the Frequency of Electrical Stimulation of Skeletal Muscle Results in an Increase in the Force of Contraction. ThisisattributabletoprolongationoftheintracellularCa++transientandistermedtetany. IncompletetetanyresultsfrominitiationofanotherintracellularCa++transientbeforethemusclehascompletelyrelaxed.Thusthereisasummationoftwitchforces.Seetextfordetails.Action potential Time 1 sec Myoplasmic Ca++ Incomplete tetanus Twitch force Tetanus force
Modulation of the Force of Contraction
A simple means of increasing the force of contraction of a muscle is to recruit more muscle fibers. Because all the muscle fibers within a motor unit are activated simultaneously, a muscle recruits more muscle fibers by recruiting more motor units. As already noted, muscle fibers can be classified as fast-twitch or slow-twitch. The type of fiber is determined by its innervation. Because all fibers in a motor unit are innervated by a single α motor neuron, all fibers within a motor unit are of the same type. Slow-twitch motor units tend to be small (100 to 500 muscle fibers) and are innervated by an α motor neuron that is easily excited (see
Table 12.2 ). Fast-twitch motor units, in contrast, tend to be large (containing 1000 to 2000 muscle fibers) and are innervated by α motor neurons that are more difficult to excite. Thus slow-twitch motor units tend to be recruited first. As more and more force is needed, fast-twitch motor units are recruited. The advantage of such a recruitment strategy is that the first muscle fibers recruited are those that have high resistance to fatigue. Moreover, the small size of slow-twitch motor units allows fine motor control at low levels of force. The process of increasing the force of contraction by recruiting additional motor units is termed spatial summation because forces from muscle fibers are being “summed” within a larger area of the muscle. This is in contrast to temporal summation, which is discussed later.
Action potentials in skeletal muscles are quite uniform and lead to the release of a reproducible pulse of Ca++ from the SR (