Feature: training focus

[svenmeier]

Optionally alternate instructions each 10 strokes, to help focussing on different aspects of improved rowing:

- posture
  - pull your navel into your spine
  - lengthen your spine
- handle
  - use all of your back muscles to draw the handle in to the body
  - accelerate handle into body
- legs
  - initiating the Drive
  - push firmly away
- coordination
  - legs-pivot-arms sequence
  - stroke ratio
- breathe
  - ...

[svenmeier]

Not much interest so far, closing for now.

[svenmeier]

Another suggestion by @marcoemanuelecocco

- rhythm of rowing marked by a metronome, so you can keep the number of strokes per minute