Most arm injuries occur at the start of the season(https://mikereinold.com/mlb-tommy-john-injuries/). It is reasonable to assume that the strain during this time is often too high for the players' fitness level. For this reason, it is very important not to throw too much too quickly. If you do, it very often leads to arm pain.
But what is the best way to build up the arm? How much should you throw, how often and how quickly should you increase? To answer this, we need to expand a little.
To know how much strain you can put on an athlete without risking injury, I need to know how much strain they are used to. This is called the chronic load. It indicates how much stress the athlete has been exposed to on average (simplified) in the last four weeks. For a runner, for example, this would be kilometers. They should therefore be able to easily cope with this load today (the acute load), plus a little more. The acute load is the load of the last 7 days. The interesting question is: how much is "a little more"?
If you divide the acute workload by the chronic workload, you get a ratio. This is known as the acute to chronic workload ratio (ACWR).
Across all sports, it has been found that the increase in load can be controlled very well via the ratio of chronic to acute load (A/C ration). This makes it relatively easy to subsequently observe how the injury frequency changes with the ACWR.
In order to do the same for throwers, it is best to know the load of a throw. If you know this, you can simply multiply it by the number of throws and you will get your daily load. The problem, however, is that not every throw causes the same amount of strain.
By measuring a large number of throws, from a large number of players, at different distances(Modeling Elbow Valgus Torque From Throwing Distance With 627,925 Baseball Throws by Competition Level, 2019, Ben Hanson), you can roughly assign load zones to distances. If you control the number of throws per distance/loading zone, you can also control the total load of the throwing day.
That's exactly what I did. To keep the ratio of the acute load to the chronic load (the A/C ratio) around or below 1.5 over the 6-8 week on-ramp phase (see next page), I adjusted the load for each day and calculated the ACWR. This is how I arrived at the number of throws and distances for each day.
After the onramp phase, the arm is fit enough to have sufficient scope to work on the mound at higher intensities and to continue building up the chronic load.
The graph shows a few important values. The green line is ACWR. If possible, it should lie between the two red lines.
In the first few days, the ACWR is significantly higher than the target - there are mathematical reasons for this. Since the chronic load is zero at the beginning (or very low even after a few adjustments and weighting), the ratio must be correspondingly high. In reality, however, the arm/body of a healthy athlete is never completely unfit for exercise. For this reason, the starting point is also the most "arbitrary" in this calculation. Even after that, the ACWR keeps jumping over the red line. I consider this to be justifiable for the following reasons:
- If you don't allow this, you have to increase the program over many more weeks. A 12-week onramp program before pitchers go to the mound is not realistic and is more appropriate after injuries. You may lose more pitchers to boredom than to injury 😉
- The shorter the break from throwing, the fitter the arm is. Even after one (or more) month(s) break(s) from throwing, the arm load capacity does not drop completely to the level of untrained people. However, since the chronic load is measured over 4 weeks, this would be the assumption in the model and therefore never works optimally in the first few days.
- The lower the absolute daily load for these overloads, the less drastic the effects of the overload. In the beginning, a handful of litters to 120 can mean a calculated overload. I think we can agree that "one more litter" is unlikely to be the cause of overload dislocation. In this model, a few extra throws at the beginning of the program can give exactly this impression. But I think that's an exaggeration.
- Exceeding the ACWR does not mean an immediate injury - merely an increase in risk. As already indicated, the amount and duration of the overrun must also be taken into account.
- The goal is to minimize the risk of injury at an acceptable cost, not eliminate it at all costs. An overly cautious approach might still be a little "safer," but it costs more in performance gains and, most importantly, is extremely monotonous. What pitcher wants to throw slowly with hardly any progress for many many weeks?
The blue bars represent the daily load. These will be considerably higher later during matches than during this onramp phase. The orange line represents the chronic load - this will also continue to increase after the onramp phase.
The higher the chronic load, the higher the fitness level. A higher fitness level enables more pitches per day and week without risking overload. This in turn enables more, and above all better quality, training on the mound and more or longer matches.
The model has some limitations that could be reduced, for example, by continuous individual measurements of each pitch (for example with a PULSE sensor) or at least by creating further and more accurate approximations. However, as a basic framework for an on-ramp phase for healthy pitchers, it seems to be a sensible first step.
You can find detailed instructions for implementation here and here.