PURPOSE: The purpose of this study was to examine muscle morphological and neural activation adaptations resulting from the interaction between concurrent strength and endurance training. METHODS: Thirty sedentary healthy male subjects were randomly assigned to one of three training groups that performed 10 wk of 3 d/w high-intensity strength training (S), cycle endurance training (E), or concurrent strength and endurance training (CC). Strength, quadriceps-muscle biopsies, computed tomography scans at mid-thigh, and surface electromyogram (EMG) assessments were made before and after training. RESULTS: S and CC groups demonstrated similar increases (p < 0.0001) in both thigh extensor (12 and 14%) and flexor/adductor (7 and 6%) muscle areas. Type II myofiber areas similarly increased (p < 0.002) in both S (24%) and CC (28%) groups, whereas the increase (p < 0.004) in type I area with S training (19%) was also similar to the nonsignificant (p = 0.041) increase with CC training (13%). Significant increases (p < 0.005) in maximal isometric knee-extension torque were accompanied by nonsignificant (p <= 0.07) increases in root mean squared EMG amplitude of the quadriceps musculature for both S and C groups. No changes (p > 0.38) in the EMG/torque relation across 20 to 100% maximal voluntary contractions occurred in any group, A small 3% increase (p < 0.01) in thigh extensor area was the only change in any of the above variables with E training. CONCLUSIONS: Findings indicate 3 d/wk concurrent performance of both strength and endurance training does not impair adaptations in strength, muscle hypertrophy, and neural activation induced by strength training alone. Results provide a physiological basis to support several performance studies that consistently indicate 3 d/wk concurrent training does not impair strength development over the short term.

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