Skeletal muscle structure and function
2.4 Muscles in action
Table 2.2 Typical muscle fibre composition in elite male athletes and non-athletes. Data taken from Costillet al. (1976)
Per cent slow Per cent fast (Type I) (Type IIa/x) Distance runners 65–75 25–35
Sprinters 20–30 70–80
Non-athletes 45–55 45–55
Exercise Intensity
Fibre Recruitment
I II A
II X
Figure 2.11 Muscle fibre recruitment as a function of exercise intensity
the bioenergetics of this type of activity in detail in Chapter 10.
0 10 20 30
Time in milliseconds (msec)
40 50
Relaxation period Latent
period
Force of contraction
Contraction period
Figure 2.12 A muscle twitch contraction. (From Tor- tora and Derrickson,Principles of Anatomy and Physiol- ogy, Twelfth Edition, 2009, reproduced by permission of John Wiley & Sons Inc.)
formation. During the contraction phase, the sarcomere changes length to a point where peak tension occurs. Muscle tension then decreases in the relaxation phase (which usually occurs at a slower rate the contraction phase), when the sarcomere returns to its original length. In this phase, Ca2+ are pumped back into the SR so that they are available for contraction upon the delivery of the next action potential.
2.4.3 The length-tension relationship
The actual tension developed by a muscle fibre during contraction is a function of the sarcomere length at the onset of contraction, and thus the number of crossbridges formed between the thick and thin filaments. This relationship is known as the length-tension relationship and is shown in Figure 2.13. Here you can see that if the contrac- tion were to begin at the initial resting sarcom- ere length shown in the green zone, then maxi- mal tension would develop. This initial position is known as the optimal resting sarcomere length. If the contraction was to begin when there is very little overlap at the long sarcomere length, then not much force would develop at the beginning of contraction. Similarly, if the thick and thin fil- aments have too much overlap initially, where the thick filaments can only pull the thin filaments a short distance, then little force will be generated.
40
Tension developed (percent of maximum) 20
40 60 80
100 1.8 µm
2.2 µm
3.8 µm Thick Filaments Thin
60 80 100 120
Resting sarcomere length (percentage of optimum)
Optimal length
Overstretched Understretched
140 160
Figure 2.13 The length-tension relationship which dictates that the tension produced during contraction is dependent on the initial sarcomere length. (From Tortora and Derrickson,Principles of Anatomy and Phys- iology, Twelfth Edition, 2009, reproduced by permission of John Wiley & Sons Inc.)
Furthermore, if there are no more binding sites available for myosin to form new crossbridges then no force will be generated.
2.4.4 Tetanus contractions
It is important to appreciate that the type of muscle contractions that occur in sport- and exercise-related activities are not the result of a single twitch contraction generated by the delivery of only one action potential. Rather, these types of contractions are due to multiple action potentials stimulating the muscle fibre within a short period of time. As a result of this increasedfrequencyof action potentials, the muscle fibre does not have time to relax in between each twitch contraction and the actual force produced is additive, to create a much greater force than a single twitch. This process is known assummation.
If the frequency of stimulation (i.e. the number of action potentials per second) is high enough (usually around 60–100 Hz), relaxation between twitches is diminished until the fibre achieves a
state of maximal contraction, known as atetanus.
In this situation, the contraction is said to be a fused tetanus, as the individual twitches have
‘fused’ to produce maximal tension. They remain there (providing there is continual stimulation) until muscle fatigue occurs. Alternatively, if the frequency of stimulation is lower (i.e. <30 Hz) and the fibre can partially relax between stimuli, the result is a wavering type contraction known as anunfused tetanus, as complete fusion has not taken place.
In physiological situations such as weak or strong voluntary muscle contractions, the fre- quency of motor unit firing is typically around 5 Hz and 70 Hz, respectively. It is important to note that the force produced during contractions is not only affected by frequency of stimulation but also by recruiting an increasing amount of motor units, as would be the case when we increase exercise intensity. An example myogram illustrating differences in tension produced during single, unfused and fused contractions is shown in Figure 2.14.
2.4.5 Force-velocity relationship
In most sporting situations, it is important to appre- ciate that the tension produced by a muscle fibre not only depends on fibre type, but also on the relationship between velocity of shortening and the load placed on the muscle. This relationship can be shown graphically by the force-velocity curve outlined in Figure 2.15. This relationship dictates that the absolute force produced during contrac- tion is greatest at slow, concentric speeds. In other words, the speed of contraction (i.e. movement) is greatest at low workloads – a relationship which is true for both slow and fast twitch fibres. Indeed, it is easy to flex your biceps muscle quickly when no dumbbell is in your hand, compared to when you are holding a 20 kg dumbbell!
Given that most sporting events require the development of power (a product of force × velocity), it follows that there is a trade-off between training with the optimal load and velocity in order to develop the power-generating capacity of the muscle.
(a) Single twitch Action potential
Myograms
Force of contraction
(b) Wave summation (c) Unfused tetanus (d) Fused tetanus Time (msec)
Figure 2.14 Differences in tension produced during single and multiple twitches. (From Tortora and Derrickson, Principles of Anatomy and Physiology, Twelfth Edition, 2009, reproduced by permission of John Wiley & Sons Inc.)
Load on Muscle
Velocity of Shortening
Figure 2.15 Force-velocity relationship during skeletal muscle contraction
2.4.6 Muscle fatigue
One of the most highly research areas within the sport and exercise sciences are the mechanisms by which muscles become fatigued during exercise.
Fatigue can be most simply defined asan inability to maintain a given force or power output during repeated muscle contractions.
Though relatively straightforward in definition, actually undertaking research to ascertain the underlying physiology and biochemistry under- pinning exercise-induced fatigue has proved a major challenge for exercise scientists (see Allen et al., 2008 for an excellent review). Furthermore, much of this research has also been conducted in animal models using isolated single fibres.
While such approaches have proved insightful in terms of elucidating potential detailed biochemical mechanisms of fatigue, it is likely that the findings produced are not entirely relevant to the exercising human performing whole-body exercise.
Despite the uncertainty surrounding this area, two general theories of fatigue have emerged.
The first describes central fatigue, meaning that the processes underpinning the decline in force
are due to disturbances at the level above the NMJ. Alternatively, the cause of fatigue could be peripheral in location (i.e. peripheral fatigue), due to metabolic disturbances below the NMJ and in the muscle itself (refer back to Figure 2.7 for an overview of the chain of events involved in force production).
Given that the focus of this text is on skeletal muscle, we are concerned with those events that are peripheral in nature which could lead to inabil- ity to produce force. Examination of Figure 2.8 suggests this could be due to a failure of propaga- tion of the action potential along the sarcolemma, in EC coupling, in crossbridge formation and also reduced energy availability. Furthermore, the pre- cise location(s) and cause(s) of fatigue within this chain of events is also likely to be highly depen- dent on the type of contraction, the predominant muscle fibre type involved, exercise mode, dura- tion, intensity and environment, as well as the nutritional and training status of the individual, etc.
For this reason, potential causes of muscle fatigue are discussed in Part 3 of this book, where we will examine specific exercise scenarios of high- intensity, endurance and intermittent exercise.