The lateral rectus of the eye contracts rapidly upon receiving an action potential, reaching peak tension within 8 msec, and then relaxes quickly, resulting in a brief contraction. On the other hand, the soleus muscle of the leg takes 90 msec to reach peak tension in response to an action potential and relaxes slowly. The gastrocnemius muscle falls in between, taking 40 msec to reach peak tension due to the presence of both fast-twitch and slow-twitch muscle fibers.

The difference in contraction speed between fast-twitch and slow-twitch muscles is correlated with the activity of myosin ATPase, which in turn reflects the type of myosin present in the muscle fiber. Fast-twitch muscle fibers contain myosin isoforms that hydrolyze ATP quickly, while slow-twitch muscle fibers contain myosin isoforms that hydrolyze ATP slowly.

The type of myosin present in a muscle fiber is associated with its MHC expression. Type I MHC expression is related to antigravity and sustained weight-bearing functions, while type IIa and IIx MHC expression is associated with burst weight-bearing function. Figure 12.15 provides a comparison of the three basic motor unit phenotypes in skeletal muscles of the extremities and trunk.

Converting a slow-twitch muscle fiber into a fast-twitch fiber is challenging but can be achieved through cross-innervation, where two motor neurons are surgically interconnected. When the soleus muscle and extensor digitorum longus muscle underwent cross-innervation, the soleus muscle exhibited increased contraction speed and myosin ATPase activity, while the extensor digitorum longus muscle experienced a decrease in shortening velocity and myosin ATPase activity. The motor innervation of the muscle fiber plays a crucial role in determining the expression of myosin isoforms.

Further research has shown that the intracellular calcium concentration in the muscle is a determinant of whether the muscle fiber expresses the slow or fast myosin isoform. The myosin isoforms in skeletal muscle can be distinguished based on the composition of the myosin heavy chain. (Refer to Figure 12.15 for more details)
Fig. 12.14A , the lateral rectus of the eye contracts very quickly in response to an action potential, reaching peak tension within 8 msec, and then relaxes quickly, which results in a short duration of contraction. The soleus muscle of the leg, in contrast, requires 90 msec to reach peak tension in response to an action potential, and then it relaxes slowly. The gastrocnemius muscle requires an intermediate time to reach peak tension (40 msec) because of the presence of both fast-twitch and slow-twitch muscle fibers in this muscle.
The difference in speed of contraction between fast-twitch and slow-twitch muscles is correlated with myosin ATPase activity (see
Fig. 12.14B ), which in turn reflects the type of myosin present in the muscle fiber (Fig. 12.15
Table 12.1 ). Thus fast-twitch muscle fibers contain myosin isoforms that hydrolyze ATP quickly, whereas slow-twitch muscle fibers contain myosin isoforms that hydrolyze ATP
MHC expression: Type I MHC expression: MHC expression: Function: antigravity, IIa, IIx IIb, IIx weight bearing, and Function: sustained Function: burst •Fig. 12.15. Comparisonofthreebasicmotorunitphenotypesinskeletalmuscleofextremitiesandtrunk.MHC,myosinheavychain.(RedrawnfromBaldwinK,HaddadF,PandorfC,etal.Alterationsinmusclemassandcontractilephenotypeinresponsetounloadingmodels:roleoftranscriptional/pretranslationalmechanisms.Front Physiol. 2013;4:284.) *Human“fastglycolytic”skeletalmusclefiberstypicallyexpressthetypeIIxmyosinisoenzymeinsteadofthetypeIIbmyosinisoenzyme.Incontrast,rodentsexpressonetypeofslow-twitchmusclefiber(typeI)andthreetypesoffast-twitchmusclefibers(expressingtypeIIa,typeIIx,ortypeIIbmyosinisoenzymes);thetypeIIxmusclefibersexpressmetabolicpropertiesintermediatebetweenthoseoftypeIIaandtypeIIbmusclefibers.ThetypeIIxmyosinisoenzymemaybecoexpressedwithtypeIIamyosinisoenzymein“fastoxidative”musclefibers.Intext,thesimpledesignationtype II fiber referstofastglycolyticmusclefibers.
slowly. These two types of myosin isoforms have the same basic structure described previously, with two heavy chains and two pairs of light chains, although they differ in amino acid composition.
It is very difficult to convert a slow-twitch muscle fiber into a fast-twitch fiber, although it can be accomplished by cross-innervation, which involves surgically interconnecting two motor neurons. As shown in
Fig. 12.14B , when the soleus muscle and extensor digitorum longus muscle underwent cross-innervation, so that contraction of the soleus muscle was controlled by the extensor digitorum motor neuron (and vice versa), the speed of the contraction and the myosin ATPase activity of the soleus muscle increased (labeled X-SOL in
Fig. 12.14 ), whereas the extensor digitorum longus exhibited a decrease in shortening velocity and myosin ATPase activity (labeled X-EDL). Thus the motor innervation of the muscle fiber plays an important role in determining which type of myosin isoform is expressed in the muscle fiber. Further study showed that the intracellular Ca concentration in the muscle (secondary to differences in the activity pattern of the motor neuron) was an important determinant of whether the muscle fiber expressed the slow myosin isoform or the fast myosin isoform (see the section
The myosin isoforms expressed in skeletal muscle can be distinguished on the basis of myosin heavy chain composition (see
Fig. 12.15