Effects of the transition time between muscle-tendon stretch and shortening on mechanical efficiency

The net mechanical efficiency of positive work (η_pos) has been shown to increase if it is immediately preceded by negative work. This phenomenon is explained by the storage of elastic energy during the negative phase and its release during the subsequent positive phase. If a transition time (T) takes place, the elastic energy is dissipated into heat. The aim of the present study was to investigate the relationship between η_pos and T, and to determine the minimal T required so that η_pos reached its minimal value. Seven healthy male subjects were tested during four series of lowering–raising of the body mass. In the first series (S ₀), the negative and positive phases were executed without any transition time. In the three other series, T was varied by a timer (0.12, 0.24 and 0.56 s for series S ₁, S ₂ and S ₃, respectively). These exercises were performed on a force platform sensitive to vertical forces to measure the mechanical work and a gas analyser was used to determine the energy expenditure. The results indicated that η_pos was the highest (31.1%) for the series without any transition time (S ₀). The efficiencies observed with transition times (S ₁, S ₂ and S ₃) were 27.7, 26.0 and 23.8%, respectively, demonstrating that T plays an important role for mechanical efficiency. The investigation of the relationship between η_pos and T revealed that the minimal T required so that η_pos reached its minimal value is 0.59 s.