TABLE OF CONTENTS The maximum force that a muscle fiber can generate depends on the velocity at which it is activated. Figure 3.51 shows the stress that can be generated as a function of strain rate for slow-twitch and fast-twitch muscles. We use slow-twitch muscles for long-range events (e.g., distance running) and fast-twitch muscles for explosive activities, such as sprinting or throwing a punch at our professor. Both muscles show a decreasing ability to generate stress as the strain rate is increased. However, the fast-twitch muscles show a lower decay.
The plot shown in Figure 3.51 is only schematic and represents the rat soleus (slow-twitch) and extensor digitorum longus (fast-twitch). The equation that describes the response in Figure 3.51 is called the Hill [29] equation. It has the form:
where ? 0 is the stress at zero velocity (equal to 200 kPa in Figure 3.51). The range of ? 0 is usually between 100 and 300 kPa; a and b are parameters and 
is the strain rate (obtained from the velocity).
A person is lifting a weight by contracting the biceps muscles. Assuming that each muscle fiber has the capacity to lift 300 ?g, and that each muscle fiber has a diameter of 5 ?m, what is the required cross section of biceps muscle needed to lift a mass of 20 kg?
Solution: The cross section...
