The relationship between load (P) and work can be understood by comparing a contracted muscle to a spring that, when loaded with a 10-g weight, has increased in length by 10 cm (Fig. 3-2). If the increase in length is linearly proportional to increasing load (Hooke’s law), length will
have increased 1 cm for each gram of added load; conversely, if an initial 10-g load is decreased in 1-g steps, the spring will shorten 1 cm for each gram removed from the load. These relationships are shown in Table 3-1, where column a describes the stepwise decrease in load when a series of lighter weights is hung on the spring after it had been stretched by the 10-g weight. Column b describes the extent of shortening at each lighter weight, while column c lists the work that would have been done if the stretched spring had lifted each of the lighter weights, calculated by multiplying each new load (column a) by the distance that the spring would have shortened at each new load (10-column b). Plotting the work in column c as a function of load demonstrates that no work is done when the load is the same as the maximal load (P = 10 g), because the spring cannot shorten at this load, nor when the spring is completely unloaded (P = 0) because in spite of the large extent of shortening, no force is generated. Instead, work is maximal at the intermediate loads (Fig. 3-3).

Work-load relationships like that for a spring also describe the influence of load on the work performed by an activated muscle. This similarity led early physiologists to postulate that the transition from rest to activity occurred when a resting muscle formed new spring-like bonds. Although now known to be incorrect, this new elastic body theory is described at this point because recognition of the fundamental error provided the foundation for our modern understanding of muscle energetics.

Work-load curves, such as that shown in Figure 3-3, were initially explained by postulating that the transition from rest to activity in a muscle causes new elastic bonds to be formed within the contractile machinery (Fig. 3-4). Contraction was therefore thought to be initiated when chemical energy is used to form spring-like bonds that increase the ability of the muscle to shorten and generate tension. This theory explained why stretching a resting muscle (Fig. 3-4, left) generates only a small amount of tension (resting tension) and the greater stiffness of active muscle (Fig. 3-4, right). According to this theory, activation adds a fixed amount of energy that causes the muscle to become a stiffer spring; that is, a “new elastic body.”

A key assumption of the new elastic body theory was that excitation adds a fixed amount of energy to the muscle to establish the new bonds; for this reason, the total energy available for release by the active muscle (work + heat) should be independent of load. This theory therefore predicted that curves relating load to total energy release would resemble Figure 3-5. At maximum load, all of the energy added during the transition from rest to activity would appear as heat because no work is done. In an unloaded contraction, all of the energy added to establish the new cross-links in the muscle would also appear as heat, again because no work is done. At intermediate loads,
where the release of energy as work is maximal (see Table 3-1 and Fig. 3-3), heat liberation should be minimal because the new elastic body theory predicts that total energy release is independent of load (Fig. 3-5).

The new elastic body theory was widely accepted until Fenn (1923) showed that the total energy liberated by a contracting muscle is not constant, but instead depends on load.

Fenn’s decisive contribution to our understanding of muscle contraction was that the total energy released as work and heat increases when more work is performed (Fig. 3-6). This finding, called the “Fenn effect,” proved that the energy available for release by a contracting muscle is not
determined at the time of activation, but instead depends on load because when muscle does more work, more energy is liberated. This finding indicated that a muscle resembles a gasoline or electric motor, where fuel consumption increases when the motor is more heavily loaded. The Fenn effect was confirmed directly in the early 1960s when high-energy phosphate utilization, like energy release as work plus heat, was shown to be maximal at intermediate loads, where the highest levels of work are performed (Fig. 3-7).

It is a historical curiosity that the Fenn effect had been documented in cardiac muscle almost a decade before Fenn’s report. In a paper that had been overlooked by most muscle physiologists of the time, but which Fenn cited in his 1923 article, Evans and Matsuoka (1914–1915) reported that cardiac oxygen consumption increases when the heart does more work. This observation led Starling, in his Linacre Lecture describing the “Law of the Heart” (1918), to equate the extra oxygen consumption caused by an increase in cardiac work to the added fuel consumption by a motorcycle when it is ridden up a hill.

The force-velocity relationship, which plots the influence of load on the velocity of muscle shortening, provides additional evidence that contracting muscle does not behave like a stretched spring. The new elastic body theory, which viewed muscle as made up of parallel elastic and viscous elements (Fig. 3-8), predicted that shortening velocity would increase in a linear manner when load is decreased (curve A, Fig. 3-9). In 1935, however, Fenn and Marsh found this relationship to be hyperbolic (curve B, Fig. 3-9). Although this hyperbolic relationship could be explained in the context of the new elastic body theory by assuming special characteristics for muscle elasticity and viscosity, A. V. Hill demonstrated in 1938 that the hyperbolic shape is also obtained from measurements of work and heat, and so reflects the fundamental energetic properties of the contractile machinery.

Muscle liberates three types of heat (Fig. 3-10, Table 3-2). Maintenance (resting) heat, the slow liberation of heat by resting muscle, is not related to contraction and so is not considered further. The other two types of heat, which together represent the activity-related heat, are generated when the muscle contracts; these are initial heat and recovery heat. Initial heat appears during contraction, while recovery heat is liberated after the contraction has reached its peak.