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Whether you are a student who wants to be fitter, a netballer who wants a faster more powerful throw, a sprinter who wants to win that race or a weightlifter who wants to lift heavier weights, you are trying to make your muscles work better.
There are three major factors that affect how well your muscles perform – strength, power and endurance.
Strength
Muscle strength is also a result of the combination of three factors:
- Physiological strength, which depends on factors such as muscle size, the cross-sectional area of the muscle and responses to training.
- Neurological strength, which looks at how weak or how strong the signal is that tells the muscle to contract.
- Mechanical strength, which refers to a muscle’s pulling force and the way those forces can be changed using bones and joints as levers.
When we talk about the strength or muscles, we are describing the maximum force a muscle can exert. Muscle strength is directly dependant upon the size of the cross-sectional area of muscle, so if after a period of training, you increase your muscle size by 50%, you will also increase the force the muscle can develop by 50%.
For every 1 square centimetre of cross sectional area, muscle fibres can exert a maximum force of approximately 30–40 newtons (the weight of a 3–4 kg mass).
Example: Emily can lift 21 kg (210 newtons force) using muscles that have a cross-sectional area of 6 cm2. Use this formula to work out how many newtons per square centimetre her muscles can pull with:
Emily’s friend Alisha has larger muscles that have a cross-sectional area of 8 cm2. Use this formula to work out what weight Alisha should be able to lift if her muscle tissue is similar to Emily’s:
Power
When muscles contract or stretch in moving a load they do work, and energy is transferred from one form to another. The power of muscles refers to how quickly the muscles can do this work and transfer the energy.
Example: A weightlifter lifts 100 kg up a distance of 1.5 m. 100 kg has a weight force of 1000 newtons. Use this formula to calculate the work done (energy transferred) by the weightlifter:
If the weightlifter lifts the 100 kg explosively and takes only 0.5 seconds to make the lift, use this formula to calculate the power their muscles produce:
Where does the energy come from and where does it go?
The energy for muscle contraction comes from glucose transported by the blood and deposited in muscle tissues. In the weightlifter example, the energy has been transformed to gravitational potential energy. Also, heat energy will be generated in the muscle tissues themselves. This means that the muscles will have transferred even more energy than the amount calculated above.
Putting the relationships together
There are three different equations that can be simplified to make an even more useful equation:
Because
the formula can be rewritten: power = force × velocity
Sports scientists use this formula to measure the power profiles of particular sets of muscles by measuring both the force of the muscles and the speed with which they are contracting or lengthening. They have found that the greatest power is produced when the load is much less than the maximum load on the muscles.
Endurance
Muscle endurance refers to how well the muscles can exert and hold maximum force over and over and over again.
Increasing or decreasing the number of motor units active at any one time changes the amount of force produced by a muscle. In the 1960s, Elwood Henneman and his colleagues at Harvard Medical School found that steady increases in muscle tension could be produced by progressively increasing the activity of axons that provide input to the relevant pool of lower motor neurons. This gradual increase in tension results from the recruitment of motor units in a fixed order according to their size. By stimulating in an experimental animal either sensory nerves or upper motor pathways that project to a lower motor neuron pool while measuring the tension changes in the muscle, Henneman found that the smallest motor neurons in the pool are the only units activated by weak synaptic stimulation. When synaptic input increases, progressively larger motor neurons are recruited: As the synaptic activity driving a motor neuron pool increases, low threshold S units are recruited first, then FR units, and finally, at the highest levels of activity, the FF units. Since these original experiments, evidence for the orderly recruitment of motor units has been found in a variety of voluntary and reflexive movements. As a result, this systematic relationship has come to be known as the size principle.
An illustration of how the size principle operates for the motor units of the medial gastrocnemius muscle in the cat is shown in Figure 16.6. When the animal is standing quietly, the force measured directly from the muscle tendon is only a small fraction (about 5%) of the total force that the muscle can generate. The force is provided by the S motor units, which make up about 25% of the motor units in this muscle. When the cat begins to walk, larger forces are necessary: locomotor activities that range from slow walking to fast running require up to 25% of the muscle's total force capacity. This additional need is met by the recruitment of FR units. Only movements such as galloping and jumping, which are performed infrequently and for short periods, require the full power of the muscle; such demands are met by the recruitment of the FF units. Thus, the size principle provides a simple solution to the problem of grading muscle force: The combination of motor units activated by such orderly recruitment optimally matches the physiological properties of different motor unit types with the range of forces required to perform different motor tasks.
The frequency of the action potentials generated by motor neurons also contributes to the regulation of muscle tension. The increase in force that occurs with increased firing rate reflects the summation of successive muscle contractions: The muscle fibers are activated by the next action potential before they have had time to completely relax, and the forces generated by the temporally overlapping contractions are summed (Figure 16.7). The lowest firing rates during a voluntary movement are on the order of 8 per second (Figure 16.8). As the firing rate of individual units rises to a maximum of about 20–25 per second in the muscle being studied here, the amount of force produced increases. At the highest firing rates, individual muscle fibers are in a state of “fused tetanus”—that is, the tension produced in individual motor units no longer has peaks and troughs that correspond to the individual twitches evoked by the motor neuron's action potentials. Under normal conditions, the maximum firing rate of motor neurons is less than that required for fused tetanus (see Figure 16.8). However, the asynchronous firing of different lower motor neurons provides a steady level of input to the muscle that causes the contraction of a relatively constant number of motor units and averages out the changes in tension due to contractions and relaxations of individual motor units. All this allows the resulting movements to be executed smoothly.