If you concentrate on what your legs are actually doing during a stride,there are really only two fundamental actions: (1) limbs must exert force against the ground to support and propel the body and (2) limbs must swing through the air to be repositioned for the next support/propulsion phase. These two actions occur in discrete phases of a stride cycle called stance and swing, respectively. Although it is clear that the metabolic cost of legged locomotion derives mostly from the recruitment and actions of limb muscles during the stride, what has remained unclear is the relative cost of stance-versus swing-related muscle activity. Does the cost of swinging one's limbs contribute significantly to the overall cost of locomotion?
One school of thought posits that the energetic cost of swinging the limbs is minimal. Early evidence for this idea came from load-carrying experiments by Dick Taylor and colleagues. Their results suggested that forces exerted by the limb against the ground during stance largely determined the energetic cost of locomotion, rendering swing-related muscle actions energetically irrelevant. However, recent work by Rich Marsh and colleagues at Northeastern University suggests that we should not yet relegate the swing phase to obscurity in the context of locomotor energetics.
So how does one go about trying to partition the costs of stance versus swing during a stride? The Northeastern group reasoned that,during aerobic exercise, measuring blood flow to active muscles might be the best means of estimating their energy use. To measure muscle blood flow,colored, microscopic (15 μm) polystyrene spheres were injected into the left ventricle of champion runners: guinea fowl. The animals were then exercised on a treadmill over a 5-fold range of speeds. During these locomotor trials, spheres travelled through the bird's circulatory system until they eventually became lodged in capillary beds due to their size. Assuming the spheres were well mixed within the blood, the number found lodged in a muscle's capillary beds should be proportional to the blood flow to that muscle and, presumably, the amount of energy used by that muscle while the bird was running. After the exercise trials, the bird's limb muscles were dissected out and digested to get at the trapped spheres. Taking advantage of the dye carried within the microspheres, Marsh's group used spectrophotometry to assay how many microspheres had lodged in each limb muscle.
Looking at muscles that were electrically active during the stride, the team found three-quarters of the spheres lodged in muscles active mainly during stance. Thus, a full quarter of the blood flowing into the limb musculature during running goes to muscles that are active during the swing,suggesting that the swing phase accounts for approximately 25% of the energy used in a stride. Their experiments also showed that this fraction, which is higher than many of us might have guessed a priori, remains constant regardless of speed.
As those of us interested in locomotor mechanics and energetics know, it is often the seemingly simple questions that are the hardest to answer. Fortunately, innovative approaches such as those used by Marsh and colleagues are giving us new insights into the locomotor energetics of an individual stride and, importantly, have shown that swinging the limbs is indeed more costly than we thought.