This paper reviews three of our recent studies on human muscle architecture in vivo. 1. Hypertrophic changes: From B-mode ultrasonograms, pennation angles and thickness of triceps brachii were determined for normal subjects and highly-trained bodybuilders. There was a significant correlation between muscle thickness and pennation angles. It was confirmed that hypertrophy was accompanied by an increase in pennation angles. 2. Variation of fascicle architecture: Fascicle lengths and pennation angles were obtained from different positions in the gastrocnemius muscle while the subjects relaxed and performed isometric plantar flexion. The fascicle length was uniform throughout the muscle and shortened by contraction (30-34% at 50% of the maximal force). On the other hand, pennation angles differed among positions and increased by contraction. The muscle thickness did not change by contraction. Pen-nation angles were significantly correlated with muscle thickness within muscle. 3. Joint position-fascicle length relationships: Ultrasonic images of the gastrocnemius and soleus muscles were obtained while the subject performed maximal isometric plantarflexion at various joint positions, from which fascicle lengths and angles were determined. The length-force relationship of each muscle was estimated. It was suggested that human muscle architecture has an ability to make substantial changes to adapt to environmental conditions.