Old men running: mechanical work and elastic bounce

Abstract

It is known that muscular force is reduced in old age. We investigate what are the effects of this phenomenon on the mechanics of running. We hypothesized that the deficit in force would result in a lower push, causing reduced amplitude of the vertical oscillation, with smaller elastic energy storage and increased step frequency. To test this hypothesis, we measured the mechanical energy of the centre of mass of the body during running in old and young subjects. The amplitude of the oscillation is indeed reduced in the old subjects, resulting in an approximately 20% smaller elastic recovery and a greater step frequency (3.7 versus 2.8 Hz, pZ1.9!10 K5 , at 15-17 km h K1 ). Interestingly, the greater step frequency is due to a lower aerial time, and not to a greater natural frequency of the system, which is similar in old and young subjects (3.6 versus 3.4 Hz, pZ0.2). Moreover, we find that in the old subjects, the step frequency is always similar to the natural frequency, even at the highest speeds. This is at variance with young subjects who adopt a step frequency lower than the natural frequency at high speeds, to contain the aerobic energy expenditure. Finally, the external work to maintain the motion of the centre of mass is reduced in the old subjects (0.9 versus 1.2 J kg K1 m K1 , pZ5.1!10 K6 ) due to the lower work done against gravity, but the higher step frequency involves a greater internal work to reset the limbs at each step. The net result is that the total work increases with speed more steeply in the old subjects than in young subjects.

The rebound of the body during uphill and downhill running at different speeds

“…The changes observed in uphill running are similar to those observed in old men running (Cavagna et al, 2008b). In older subjects, the average upward acceleration (a v;ce ) is lower than in younger ones, leading to a symmetric rebound.…”

Symmetry and Asymmetry in Bouncing Gaits

“…In other words, running with high, long leaps is a convenient strategy to adopt, provided that enough anaerobic power is at disposal to allow these leaps. It is interesting to note that old subjects never adopt the second strategy during running: the average upward acceleration never exceeds 1g and the bounce is symmetric at all running speeds (Figure 16) [57]. This allows development of a lower force during the push, but the increase in step frequency results in a greater internal power at high speeds.…”

“…Note that the landing-takeoff asymmetry is larger in the elderly (Figure 25) where the on-offground asymmetry is nil (Figure 16) due to a lower average acceleration upwards [57], whereas the contrary is true for the young subjects, where the landing-takeoff asymmetry is lower (Figure 25) and the on-off ground asymmetry is larger (Figure 16) due to a greater average acceleration upwards [57]. In both cases, age and average running speed, the landing-takeoff asymmetry, which suggests a lack of elastic storage and recovery in the rebound of the body, is associated with a lower force applied to the muscle-tendon units during their stretch-shorten cycle.…”

“…This allows development of a lower force during the push, but the increase in step frequency results in a greater internal power at high speeds. In contrast, the second strategy is adopted by young subjects, resulting in a lower step frequency, which limits the increase of the internal power at high speeds [57].…”

“…In order to maintain the step frequency, f = 1/(t ce + t ae ), tuned, as at low speeds, to the increasing resonant frequency of the bouncing system, f s = 1/(2t ce ), the effective aerial time t ae should decrease with speed similarly to t ce . Young and adult subjects however do not adopt this choice in human running: t ae is maintained about constant with the consequence that f becomes progressively less than f s [53,54,57]. In spite of this, the freely chosen step frequency f becomes greater than the frequency minimizing the step-average power.…”