S5.2-1 Sunday, Jan. 5 10:00 Is the spring in our step the spring in our leg? LEE, David V*; MCGOWAN, Craig P; ISAACS, Michael R; University of Nevada Las Vegas; University of Idaho Moscow; University of Nevada Las Vegas firstname.lastname@example.org
Virtual leg spring stiffness k leg has long been used to characterize bouncing gaits such as running, hopping, and trotting. Although k leg is a spring constant, it is divorced from the physical leg because virtual spring deflections are determined solely from vertical and fore-aft displacements of the center of mass (CoM) rather than measured deflections of the physical leg. The virtual leg assumption that time-varying force and deflection are symmetrical sinusoids is the exception rather than the rule for the physical leg. Here we use a serial actuator-spring model, acting as a radial pogo-stick from the hip to the ground, to determine a spring constant k rad for the physical leg based upon actual time-varying force and length changes. Radial leg spring constants krad of bipedal runners and hoppers increase significantly with speed - a phenomenon not observed in kleg. The effect of speed on krad indicates that an increasingly stiffer leg spring would minimize the work required of the in-series actuator. While the radial leg spring constant krad is similar to kleg during bipedal hopping, it is three times greater than kleg during human bipedal running. Hence, dynamics of the radial leg of wallabies more closely match the virtual leg model. The three-fold greater krad compared with kleg found during human running is also observed during quadrupedal trotting. Consistent with the observation that the bipedal hopping leg more nearly approaches an idealized virtual leg spring, we find that the in-series actuator does one-third of the radial leg work during hopping but more than half of the leg work during human bipedal running. Our results support the concept that legs utilize stiffer built-in springs than the virtual leg spring model would predict and suggest that large bipedal hoppers may represent an exception to this rule.