Preheat the oven to 375F.
In a large bowl, stir together the flour, brown sugar, baking soda and salt. In a smaller bowl, whisk together the yogurt, oil and egg. Add to the dry ingredients and stir until almost combined; add the rhubarb and stir just until blended.
Divide between 8-10 paper-lined muffin tins. In a small bowl, stir together the pecans, brown sugar, butter and cinnamon and sprinkle a bit over each muffin.
Bake for 20-25 minutes, until golden and springy to the touch. Makes 8-10 muffins.