Provide anatomy and physiology advice to clients Muscle action part 2

Muscle action In our previous session we looked at how muscles actually contract and we saw that contraction was generated by a nerve impulse, which in turn liberated acetyl choline. That had the effect of liberating calcium throughout the muscle cell, which in turn caused a change in the conformation of proteins that made up the filaments of muscle. Finally the muscle contracted when energy from ATP was supplied. This slide will remind you about energy sources that supply ATP for muscle contraction.

Muscular contraction Each muscle has at least one nerve attached to it and where the nerve enters the muscle it branches into a number of axon terminals, which all join to a single fibre. We saw this in our last session. A motor neuron and all the muscle fibres that it innervates is called a motor unit. When a motor neuron fires, all the fibres that innervates respond by contracting. Large weight bearing muscles have large motor units. This means that there may be several hundred muscle fibres per motor unit. Muscles that exert fine control, such as moving the fingers have small motor units with only a few fibres. The muscle fibres in a single motor unit are spread throughout the muscle, so that a single motor unit stimulation causes a weak contraction of the entire muscle. You can see a motor unit in this slide.

The muscle twitch The response to a brief neural stimulus is called a muscle twitch. The muscle contracts briefly and then relaxes. Of course, the twitch can be weak or strong, and this depends on the number of motor units that are activated. Our muscles can contract with different frequencies and by changing the strength of each stimulus. Successive contractions in a muscle can add to the contraction size by reducing the relaxation time between twitches. So this adds to the strength of the contraction. But this cannot continue indefinitely and muscle fatigue soon sets in. In this case glycolysis stops because NADH builds up. So, pyruvate is converted to lactate and NAD is restored. Lactate then diffuses out of the cells and goes back to the liver, where it is converted back to glucose for more contractions. Muscle contractions are also made stronger by the involvement of several motor units in contraction.

Muscle fibres Muscle fibres can have hundreds of nuclei and during foetal growth progenitor cells form myoblasts that become myofibrils. Genes control the production of isoforms of these fibres which give rise to subtypes of fibres such as slow twitch and fast twitch. It’s actually variations in the myosin protein that gives muscle its different contraction times. Fast twitch muscles are sub divided into fast twitch oxidative, which means that their primary energy source is aerobic and fast twitch glycolytic. Here their primary energy source is anaerobic glycolysis. Slow twitch fibres primarily use aerobic energy sources. This table gives information about different isoforms of muscle fibres. Some people believe that different types of training can stimulate changes in muscle types but that has yet to be proven.

Slow (Type I Fibres) Slow twitch fibres have rich vascularisation (very good blood supply) and abundant myoglobin. They use aerobic energy for ATP production. You can see in this slide the dark spots that represent NADH. The energy from oxidation of glucose now resides on NADH prior to it being shunted into the oxidative phosphorylation chain. These muscle fibres are slow to contract and slow to fatigue. We use them in endurance exercise and in simply maintaining our upright position.

Fast Fibres (Type II) This slide shows abundance of glycogen which is associated with anaerobic or fast twitch. The red fibres are Type I, others are Type IIA and Type IIB. Fast twitch fibres Type IIA are quick to contract and have reasonable fatigue resistance. They also use aerobic energy supplies. We use these in walking and sprinting. On the other hand fast twitch Type IIB muscles use stored glycogen for their energy supplies. They are fast to twitch and fast to fatigue. They are the muscles that we use in power movements such as lifting weights.

Energy used in exercise We use all our muscles fibres in exercise or sports. Generally, the stored ATP and CP energy lasts about 15 seconds and is used in power. It can be rebuilt in about 60 seconds. The other anaerobic system system can be used in power for about 60 seconds and then it needs about 60 minutes to be replenished. We can actually train our muscles to store more glycogen for both power and aerobic energy. This is done by eating/drinking carbohydrate such as a sports drink for up to 30 minutes after exercise. This trains our muscles to store more glycogen. Good marathon runners have about 2 hours worth of glycogen stored.

Muscle Adaptation Any regular exercise changes our muscles. We build up a greater capillary network within the muscle and the number of mitochondria increase in number. Remember that mitochondria provide us with aerobic ATP. These changes are more dramatic in slow twitch fibres. When we engage in high intensity resistance exercise, such as weight lifting, then muscles increase in bulk and generally the Type IIB increase their fibre diameter. It is thought that the fibres add more glycogen, water and connective tissue when high intensity resistance training is used. There also seems to be more connective tissue laid down between the fibres. Note that the number of fibres probably do not increase, only their diameter.

Muscle Strength We all know that some people are stronger than others. What gives them their strength is a combination of muscle force, contraction velocity and duration of contraction. The force of contraction is related to the diameter of the muscle fibres and the time it takes to make the muscle contract. The velocity of contraction is also related to the contraction time and the type of fibre present. We’ve already seen the difference between fast twitch and slow twitch muscles. And we’ve looked at the duration of contractions.

Muscle fatigue Muscles have glycogen stores that allow them to contract for up to several hours and after that they can call on fat supplies to provide ATP. Now during anaerobic contraction oxygen is not required, but an oxygen debt results because oxygen still must be used to deal with the by products of anaerobic metabolism. So if you run 100 metres very quickly then you will need oxygen when you’ve finished. The high levels of lactic acid in your blood trigger the respiratory centre of your brain to make you breathe very deeply and gulp air. As you train and become fitter your oxygen carrying capacity improves and you therefore “puff” less after anaerobic exercise.

Activity Prepare a presentation that describes to clients the physiology behind fast and slow twitch muscle fibres. As a group discuss answers to the following questions: Can a person retain the strength that they had as a youth? Why is it said that you should never stretch before you are warmed up? Why is it that some people are stronger than others yet seem the same size and weight?