Sliding Filament Theory of Muscle Contraction, Fiber Types, and Training Adaptations

It has been over fifty years since the sliding filament theory of muscle contraction was first proposed. Prior to the 1954 publication of two defining studies done independently by Hugh Huxley and Allan Huxley, the model of muscle contraction accepted by most academics was based on the protein, myosin, contracting in the presence of calcium ions (Anderson 2004; Cooke 2004). The role of the protein, actin, discovered by Straub in 1942, had yet to be explained (Szent-Gyorgyi 2004).

The findings of Hugh Huxley and Allan Huxley, published in Nature, May 1954, gave rise to the sliding filament model in virtually the form we see it today (Huxley 2004). The theory stated that skeletal muscle contracted when two types of filaments consisting of the proteins myosin and actin “slid” past each other without either filament’s length actually changing (Anderson 2004; Cooke 2004; McArdle, Katch, and Katch 2001). Termed the sarcomere, thousands of these filaments lay in a recurring pattern within each muscle fiber, and as the sarcomeres contracted, so did the muscle fiber.

As early as 1939, Engelhardt and Lyubimova reported that myosin showed ATPase activity. However, this was not widely accepted at the time. In 1957, Allan Huxley added to this report and proposed that the cross bridge structure of myosin, utilizing the energy released from the conversion of ATP to ADP, created the power stroke necessary to “slide” the filaments across each other by binding, rotating, and finally detaching from the actin (Szent-Gyorgyi 2004).

These cross bridge mechanisms by which the filaments “slide” past each other are referred to by McArdle et al. (2001) as the “molecular motors to drive fibre shortening.” A summary of the steps involved in muscle contraction is as follows:

Neural stimulation of a muscle fiber causes Ca2+ to be released from vesicles on the sarcoplasmic reticulum.
Ca2+ binds to troponin, a protein located on the actin filament, which along with tropomyosin, inhibits actin–myosin binding. This causes the myosin binding sites on the actin to be “switched on.”
Myosin cross bridges, fueled by ATP hydrolysis, bind to the actin and rotate, causing the power stroke that allows the actin to “slide” across the myosin molecule.
The myosin head detaches from the actin and recombines with an ATP molecule.
The above process happens continuously hundreds of times per second causing the length of the sarcomere, seen microscopically as the distance between the z-bands, to decrease. Thousands of sarcomeres are contained on each fiber, and as these shorten in length, so do the individual muscle fibers making up a muscle.
This contraction of the muscle continues until neural stimulation ceases and the Ca2+ is pumped back across the membrane of the sarcoplasmic reticulum. Myosin–actin binding is again inhibited, and the sarcomeres return to their usual length.
Two distinct types of muscle fibers exist in mammalian skeletal muscle, which can be differentiated on the basis of their relative reactivity with an altered pH solution of myosin ATPase. The first of these fiber types are classified as type 1 fibers, also referred to as slow twitch or red fibers. These fibers are smaller and contain less contractile proteins than their type 2 equivalents. They do not contract as fast or with as much force as type 2 fibers (McArdle et al.).

The majority of the time, type 1 fibers utilize the aerobic metabolism to produce ATP, hence their need for a higher level of capillarization than type 2 fibers. This, in combination with their highly developed mitochondrial reticulum which contains many iron-containing enzymes and has high myoglobin levels, gives these fibers their reddish appearance (Reaburn and Jenkins 1996). The aforementioned features also ensure that the type 1 fibers are resistant to fatigue. Hence, their role lies primarily in functions in which endurance is essential.

Other characteristic features of type 1 fibers noted by McArdle et al. (2001) and Reaburn and Jenkins (1996) include:

low myosin ATPase activity relative to type 2 fibers
slower Ca2+ processing ability
relatively poor glycolytic ability but a high capacity to utilize fat due to the development of the mitochondrial reticulum
Given the above, it should come as no surprise that type 1 fibers are found in the highest concentration in muscles which have a postural role. One example is the soleus muscle.

Type 2 fibers, commonly referred to as fast twitch fibers, tend to be larger than type 1 fibers. They contain less mitochondrial reticulum but have a larger amount of contractile proteins. McArdle et al. (2001) notes the following features of type 2 fibers:

high myosin ATPase activity in comparison to type 1 fibers
Ca2+ released and absorbed quickly by the sarcoplasmic reticulum
action potentials transmitted very quickly
Type 2 fibers contract more quickly and with more force than their type 1 counterpart. They can be further classified into three subgroups with varying properties. Type 2b fibers, the first subgroup, are often referred to as white fibers. They are the true fast twitch fibers, generating the greatest contractile forces of all skeletal muscle fibers, and are highly glycolytic, substituting mitochondrial reticulum development for increased amounts of contractile proteins. These fibers fatigue quickly and recover slowly, partially due to low levels of capillarization and the lactic acid produced by anaerobic glycolysis (Reaburn and Jenkins 1996).

The second subgroup, type 2a fibers or fast oxidative glycolytic fibers, have considerably greater mitochondrial reticulum than the type 2b fibers. While still considered fast twitch fibers, they have a greater capacity for aerobic metabolism, allowing them to be more fatigue resistant, and they have a decreased recovery time in comparison to the type 2b fibers (Reaburn and Jenkins 1996).

Type 2c fibers, the third subgroup, are said to be “rare and undifferentiated” and may be an intermediate in the conversion process between type 2a and 2b fibers (McArdle et al 2001).

The question of whether or not training has an effect on these fiber types inevitably raises the age-old nature versus nurture debate in regards to athletes. Are elite athletes born rather than trained? In some cases, it would seem so, especially when one considers the importance of type 1 fibers in endurance events. The proportion of type 1 fibers that an individual has is clearly determined by genetics, and no amount of training, either endurance or resistance based, can change this. That is, new type 1 fibers cannot be generated, and type 2 fibers cannot be converted into type 1 (Reaburn and Jenkins 2001; Pack 2005).

However, endurance training has been shown to increase muscle capillarization and thus the rate at which gases, nutrients, and metabolites can be exchanged. (McArdle, Katch, and Katch 2001; Harris 2005). This along with the increased efficiency of the cardiovascular system associated with endurance training can lead to increased levels of performance regardless of the lack of change in numbers or proportions of type 1 fibers (Harris 2005). Significant fiber size increases have also been observed in type 1 fibers after sprint training regimes (Ross and Leveritt 2001).

It has long been accepted that resistance training leads to increased muscle mass. Studies show that the majority of this increased mass is due to the increased cross-sectional area (hyperplasia) of the type 2 fibers (Benjamin and Hillen 2003; Phillips, Parise, Roy, Tipton and Tarnopolsky 2002; Bickel, Slade, Mahoney, Haddad, Dudley, and Adams 2005; Harris 2005). This increased fiber size appears to be linked to greater mRNA activity and increased protein synthesis in these fibers (Booth, Tseng, Fluck, and Carson 1998; Bickel, Slade, Mahoney, Haddad, Dudley, and Adams 2005).

Type 2 fibers appear to have a greater response to training, with the majority of the literature stating that while the number of type 2 fibers is genetically determined at birth, there seems to be a degree of conversion between the subgroups of type 2 fibers (Ross and Levritt 2001; Reaburn and Jenkins 1996; Pack 2005). High levels of endurance training have been shown to elicit increases in type 2a fibers while the number of type 2b fibers decreases (Reaburn and Jenkins 1996). This would seem to be beneficial in terms of performance in endurance activities given that type 2a fibers have a greater potential for aerobic metabolism and recover more quickly than type 2b fibers.

This conversion of type 2a fibers to type 2b fibers is well documented in resistance exercises as well as in training regimes requiring “explosive” bursts of activity such as sprints (Ross and Leveritt 2001). It could be assumed that this phenomenon as well as the hypertrophy of type 2 fibers associated with resistance training confers benefits in anaerobic performance as an athlete converts more fibers into a form better able to produce the high contractile forces required for events such as weightlifting and sprinting.

In summary, although numbers and proportions of type 1 and type 2 fibers are fixed at birth, athletes can benefit from specific training as they strive to increase their performance. However, based on the literature, it would seem that a select few are genetically predetermined to excel in either endurance or explosive events. These individuals, if trained correctly for the sport they are suited to, are likely to be the faces behind world record performances rather than well-motivated and trained athletes with “average” genetics.