Some thoughts about Stroke Rate and Stroke Power.

A possibly unique feature of rowing, and even more clearly of indoor rowing, is that its power production is a continuum of just three interrelated factors: Force, Stroke Rate, and Stroke Length. Theoretically you can increase your boat speed by increasing any one of these factors while maintaing each of the others. Or you could produce the same speed by means of varying proportions of each of the three. Although various physical limitations of human anatomy and boat construction constrain these differences within a range, they can play a significant role in any race, for example where a strong low rating crew overcomes a fatiguing high rating crew. Annual surveys of rigging and stroke rates of elite crews result in recommended ranges of stroke length, stroke rate, snd gearing. But, although anecdotal wisdom regarding training as distinct from racing at a given Stroke Rate or Stroke Force (or Work per Stroke) abounds, there has not been much research into its physiological implications. Some work has been done on the efficacy of low cadence training in Cycling but it too remains inconclusive.

I am puzzled by many coaches recommendations to always row at SR16 for long rows and SR28 for Threshold pace etc. These usually neglect to mention in what type of boat you are rowing and at what Stroke Pressure, or why. There is a huge difference between 28 in a Single and 28 in an VIII. And how fast you are moving at each stroke rate determines how hard you have had to pull each stroke. On an ergometer, rowing at easy pace, for me say 2:24 splits, at  Stroke Rate 16 results in the same work per stroke as I would put in during a flat out 2k at stroke rate 33. The ratio of work to recovery, another sacred cow of many coaches, is simply a result of pace and stroke rate. Unless pushed to extremes, stroke rate intrinsically is only a minor contributer to training load (although also greater than we assume, with implications for recovery slide training). The main considerations regarding general workload are boat speed or power (work / time) and its duration, which stresses the cardiovascular an metabolic systems, and the force you need to apply in each individual stroke, which stresses the peripheral muscular system.

So to gauge the load or the potental training and fatigue effect of a piece, as well as the average power generated and the duration to complete the piece, we need to know the extent and duration of muscle tension that it required. Now, the cardiovascular stress to produce that power extends over the entire duration of the piece, but the muscle tension only occurs while you are pulling the oar through the water and depends on the force with which you do that in order to maintain a certain power at a given stroke rate. So at a lower stroke rate, for the same power output or boat speed, the force or muscle tension is higher. The muscular effect is therefore more strength oriented and, as the total number of these strokes increases, it becomes more strength-endurance oriented. This is apart from the fitness gained through just maintaining a level of power regardless of stroke rate. 

You can quantify the muscular load by counting the number of strokes whose work is greater than a given percentage of your work per stroke in a flat out 2k.  And Concept2 use a measure they call SPI (Stroke Power Index). This is simply Watts of power divided by stroke rate that can be used as a proxy for Work per Stroke which otherwise would be expressed as a less managable number of Jules. If you know that your SPI for 2k at SR33 is 8 then any stroke at a submaximal pace, SR16 or SR20 or whatever, that exceeds SPI of  7.5 can count as a Hard Stroke and any stroke less than SPI 7.5 but more than, say, SPI 6 can count as a Firm Stroke. Tracking the number of Hard and Firm strokes can give you a gauge of muscular load in each session as distinct from cardiovascular and metabolic load which is a product of only training duration and intensity or pace.  

This makes intuitive sense, but we still need better research to determine what precise effect different proportions of each training type might have.