Muscle Fiber Type, Achilles Tendon Length, Potentiation, and Running Economy

The purpose of this investigation was to develop a potential model for how muscle fiber type, Achilles tendon length, stretch-shortening cycle potentiation (SSCP), and leg strength interact with running economy. Twenty trained male distance runners 24–40 years of age served as subjects. Running economy (net oxygen uptake) was measured while running on a treadmill. Leg press SSCPforce and SSCPvelocity were determined by measuring the difference in velocity between a static leg press throw and a countermovement leg press throw. Vertical jump SSCP was determined by measuring the difference in jump height between a static jump and a drop jump from a 20.3-cm bench. Tendon length was measured by magnetic resonance imaging, and muscle fiber type was made from a vastus lateralis muscle biopsy. Type IIx muscle fiber percent (r = 0.70, p < 0.001) and leg strength (r = 0.95, p < 0.001) were positively and independently related to late eccentric force development. Achilles tendon length (r = 0.42, p <= 0.05) and late eccentric force during stretch-shortening cycle (r = 0.76, p < 0.001) were independently related to SSCPforce. SSCPforce was related to SSCPvelocity, which in turn was related to running economy (r = 0.61, p < 0.01). These results suggest that longer Achilles tendon length, type II fiber, and muscular leg strength may enhance the potential for SSCP, running economy, and physiological effort while running.