Energy Cost of Force Production After a Stretch-Shortening Cycle in Skinned Muscle Fibers: Does Muscle Efficiency Increase?

Abstract

Muscle force is enhanced during shortening when shortening is preceded by an active stretch. This phenomenon is known as the stretch-shortening cycle (SSC) effect. For some stretch-shortening conditions this increase in force during shortening is maintained following SSCs when compared to the force following a pure shortening contraction. It has been suggested that the residual force enhancement property of muscles, which comes into play during the stretch phase of SSCs may contribute to the force increase after SSCs. Knowing that residual force enhancement is associated with a substantial reduction in metabolic energy per unit of force, it seems reasonable to assume that the metabolic energy cost per unit of force is also reduced following a SSC. The purpose of this study was to determine the energy cost per unit of force at steady-state following SSCs and compare it to the corresponding energy cost following pure shortening contractions of identical speed and magnitude. We hypothesized that the energy cost per unit of muscle force is reduced following SSCs compared to the pure shortening contractions. For the SSC tests, rabbit psoas fibers (n = 12) were set at an average sarcomere length (SL) of 2.4 μm, activated, actively stretched to a SL of 3.2 μm, and shortened to a SL of 2.6 or 3.0 μm. For the pure shortening contractions, the same fibers were activated at a SL of 3.2 μm and actively shortened to a SL of 2.6 or 3.0 μm. The amount of ATP consumed was measured over a 40 s steady-state total isometric force following either the SSCs or the pure active shortening contractions. Fiber stiffness was determined in an additional set of 12 fibers, at steady-state for both experimental conditions. Total force, ATP consumption, and stiffness were greater following SSCs compared to the pure shortening contractions, but ATP consumption per unit of force was the same between conditions. These results suggest that the increase in total force observed following SSCs was achieved with an increase in the proportion of attached cross-bridges and titin stiffness. We conclude that muscle efficiency is not enhanced at steady-state following SSCs.

Association and Agreement between Reactive Strength Index and Reactive Strength Index-Modified Scores

“…The primary role of the SSC is to optimize mechanical loading of the muscle-tendon complex, which may lead to a metabolically efficient and forceful muscle contraction [ 2 ]. Neurophysiological mechanisms responsible for SSC enhancement of muscle performance are debated [ 2 ], yet may include storage and recapture of elastic potential energy [ 2 , 3 , 4 , 5 , 6 ], skeletal muscle pre-activation [ 3 , 4 , 5 , 6 ], increased active state [ 2 ], residual force enhancement [ 3 , 4 , 5 , 6 ], pre-synaptic facilitation of alpha motor neurons from supraspinal drive [ 7 ], and involuntary spinal reflex pathways [ 3 , 4 , 5 ]. Given the role of the SSC in optimizing muscle efficiency and force, it is essential to identify measures that effectively differentiate SSC utilization during functional movement tasks.…”