This exercise is from my book Classical Guitar Technique: Essential Exercises, Scales, and Arpeggios. The 122 page book includes: Practice Routines, Tips, 100 Open String Exercises, 120 Giuliani Arpeggios, Scales, Slur Exercises, Shifts, Finger Independence, Barre, Tremolo, Common Harmonics, and much more. YouTube Video Link (HD)
These exercises combine finger independence and stretches with a focus on keeping a good alignment in the left hand. Make sure you’ve covered the previous alignment exercises from the book and if you feel and pain or burning stop and rest!
Below is a closer look at the Horizontal Stretch Exercsie