There are four important parameters that are usually measured to indicate the performance of a skeletal muscle fibre: maximum force (kN/m2), maximum shortening velocity (Vmax, fibre lengths/s), power (in kN/m2 ⋅ FL/s) and the sensitivity of the contractile apparatus to calcium.
Like a car engine, the measurements and outputs provide us with vital information that can be related to the performance of the individual muscle fibre, and the overall muscle (e.g. the biceps). Combining these two components can provide us with the performance capacity of the athlete. (Our lab focusses on the contractile properties of single muscle fibres).
What is important to note about the data of the graphs below is that all the single fibre experiments were conducted at 12˚C. Other laboratories perform their experiments at 15˚C or even 20˚C, but seldom at 37˚C (body temperature). The primary reason is because the integrity of the contractile apparatus of the fibre (i.e. the myosin and the actin) seems to be better preserved so that more than one contraction or experiment can be performed using one muscle fibre. All graphs discussed below were adapted from Bottinelli et al. (1999) using human muscle fibres.
a. Maximum force
The figure no the right shows the maximum force generating capacity of each fibre type. In other words, how much force can a fibre produce when it is maximally activated. As force is directly related to the size of the fibre, force must always be expressed relative to the size of the fibre. It is clear that the amount of force each fibre type can produce, is different.
The type I fibres produce less force than the type IIA fibres, irrespective of the size of the fibre. This is an important characteristic between the MHC isoforms. The force production of the hybrid fibres are usually between that of two pure fibre types (i.e. I vs. IIA vs. IIX).
b. Maximum shortening velocity (Vmax)
Another crucial property of the different muscle fibre types is the difference in how fast a muscle fibre can contract (hence the term slow twitch and fast twitch). It seems that the speed of contraction is directly related to the speed of how fast the MHC can breakdown ATP. The faster ATP is broken down, the faster the contraction speed. From the figure on the right, type I fibres (black bar) are slow in contraction speed relative to type IIA fibres. The latter is slower than type IIX fibres (light blue bar).
c. Maximum power
The formula for power is calculated by multiplying force with shortening velocity (power = force x velocity). Power can also be seen as the amount of work that is performed and may also be expressed as watts. However, in skeletal muscle it is generally expressed as the product of the units from specific force and velocity. The figure on the right shows the maximum power output of fibres from different types.
It is therefore not surprising that type I fibres produces the least amount of power, and type IIX fibres the greatest. The power of the type IIA and the respective hybrid fibres produce power that are in between these two extremes.
d. Calcium sensitivity
The on the right shows the amount of force a fibre can produce at a specific calcium concentration (pCa) in an experiment. (Clarification: pCa is the log scale of the calcium concentration: the lower the pCa value, the higher the concentration of calcium in the solution.)
The principle of sensitivity is based on the fibre’s ability to respond to calcium. For the same force output, the lower the concentration of the Ca (the higher the pCa), the more sensitive the fibre is to calcium. (The same principle applies for insulin resistance: the less insulin is required to reduce blood glucose, the more insulin sensitive the body is.) From the figure it is clear that type I fibres are less sensitive to calcium than type IIA and IIX fibres.
Why is sensitivity important? Well, it’s exact role is still not really known. All that is known is that muscle weakness can be caused by a decreased calcium sensitivity. Exercise training, on the other hand, improves the sensitivity that can translate to improved efficiency of the fibre.
e. The effect of temperature
The temperature of the solutions at which these experiments are performed has a major effect on the overall contractile properties and is primarily determined by the researcher. But does it affect the contractile properties? Indeed! Temperature affects force, shortening velocity and calcium sensitivity. To illustrate, an increase in temperature from 12˚C to 17˚C results in the force output to double and shortening velocity to triple.
From 17˚C to 22˚C the effect is minimal on force, but shortening velocity increases a further two-fold (thus 6x that achieved at 12˚C). Why is temprature important? The primary answer is that when properties from various studies are to be compared, it is imperative that temperature be considered prior to drawing conclusions. That is also why the inclusion of a control group is so important in all experiments.
The next section focusses on the fuel and metabolism of a whole muscle in order for contraction to occur.