Active Muscle Work, Not Springs, Determine Maximal Effort Hopping Performance in Humans

Abstract

Biological systems typically require movement patterns that involve either efficiency or maximal power. Mammalian propulsion systems consist of the active and passive work elements of muscle actuators and a muscle and tendon springs. These components can be used in an array of motor-driven or spring-driven models to optimize performance and task accomplishment. The purpose of this investigation was to examine muscle-tendon unit kinematics and kinetics in human subjects asked to perform a locomotor task (hopping) for maximal performance with variational preceding milieu. Based on previous evidence it was hypothesized that a motor-driven model would be utilized. Twenty-four subjects were allocated post-data collection into those subjects with an average hop height lower than 0.1 m (LH, n = 11, age = 22.3 ± 2.7 yrs, height = 1.68 ± 0.09 m, body mass = 64.8 ± 13.7 kg, avg hop hght = 0.08 ± 0.01 m) or higher than 0.1 m (HH, n = 13, age = 21.5 ± 2.1 yrs, height = 1.79 ± 0.05 m, body mass = 83.1 ± 15.2 kg, avg hop hght = 0.13 ± 0.01 m). Subjects were placed on a carriage attached to rails on a customized sled at a 20 degree angle while standing on a force plate. Subjects only used their dominant ankle for all testing and their corresponding knee was completely immobilized and thus all movement was isolated to that single ankle joint and corresponding propulsive unit (triceps surae muscle complex). Subjects were asked to hop as high as possible during a single dynamic countermovement hop (CMH) and drop hops from 10 cm (DH10) and 50 cm (DH50). Drop hops involved dropping the carriage from the afore mentioned heights and asking the subject to hop upwards after impact with the force plate. Three-dimensional motion analysis was performed by utilizing an infrared 9-camera VICON motion analysis system and a corresponding force plate. In addition, an ultrasound probe was placed at the mid-muscle belly of the triceps surae muscle complex of the subject for muscle fascicle imaging. HH hopped significantly higher in all hopping tasks in comparison to LL. The HH group concentric ankle work (CMH = 2.55 ± 0.93 J•kg-1, DH10 = 2.72 ± 0.75 J•kg-1, DH50 = 4.48 ± 1.56 J•kg-1) was significantly higher in comparison to LL (CMH = 1.49 ± 0.56 J•kg-1, DH10 = 1.98 ± 0.59 J•kg-1, DH50 = 3.14 ± 1.43 J•kg-1) during all of the hopping tasks as well (Figure 1). In addition, active muscle work was significantly higher in HH (CMH = 3.26 ± 1.18 J•kg-1, DH10 = 3.74 ± 0.90 J•kg-1, DH50 = 4.98 ± 3.22 J•kg-1) in comparison to LH (CMH = 1.01 ± 0.79 J•kg-1, DH10 = 1.85 ± 1.34 J•kg-1, DH50 = 2.66 ± 2.0 J•kg-1). However, passive tendon and passive muscle work were not significantly different between the groups. Across both groups and hopping tasks, active muscle work was significantly correlated with hopping height (r = 0.97) and contributed to approximately 49% of the total work. The data indicates that humans primarily use a motor-driven system. Thus, muscle actuators and not springs maximize performance in hopping locomotor tasks in humans.