So in phases 2 & 3 your goal was to achieve greater maximal strength as well as decrease some major asymmetries between limb strength. By this point, the anthropometrics of each limbs quadriceps/hamstring cross-sectional area should be within the same range. If you have achieved those goals, then the athlete is on the right track to begin to work on applying their strength towards higher velocities.
In phase 4, the focal point on the training program is to efficiently work towards generating higher peak velocities, greater rate of force production, and continued strength development. The purpose of this phase is to return to player back to practice...THAT'S IT. Once the athlete is able to participate in practice (with a given capacity to begin and gradual increase over time), they can begin to enhance their conditioning with practice. And let's face it, to be honest, the only sport-specific is the sport itself, and I never found it necessary to put an athlete returning from injury through a conditioning protocol, that's what practice is there for.
For this phase, I monitor two things:
1) Repeat Sprint Ability (RSA)
2) Peak Power Output (bilateral and unilateral)
Repeat Sprint Ability
The RSA has been shown to be an effective return to sport test for hamstring and ACL injuries. The two data points you want to track over time are Time Drop Off and Speed Increase.
Through the research I've read, the test is run at 30 meters, however I have been running my athletes at 30 yards. A long enough distance to work and acceleration, and not long enough to over stress the hamstring musculature. The athlete will sprint (however fast they feel is comfortable for them at that moment) for 30 meters. The coach will time the sprint by either hand timer or laser (best option). The athlete will get 30 seconds rest after each sprint and will repeat this eight times.
Time Drop Off is measured by subtracting the athletes last sprint time from their first sprint time (i.e., 4.25 - 3.15 = 1.10), and monitoring the trend week by week. Speed Increase (obviously) is just monitoring the athletes speed week by week.
From my findings, athletes have decreased their Time Drop Off week-by-week, and have improved their running speeds. This tells us that the athlete is not fatiguing as quickly and can sustain repetitive high velocity movements. My belief is also that, after the initial run, the athlete becomes more confident in their ability to begin to run faster and faster, eventually leading to their ability to WANT to move faster. Also, they are beginning to 'wake up' their nervous systems ability to increase muscle rate-coding. Here is what you want that graph to look like over time:
This chart does not tell us that the athlete is able to return back to playing their sport, it just gives coaches and the athlete visual feedback on their improvement and that they are on the right track to return back to practice. Because this is just a linear movement, and sports are playing in a multidirectional plane of movement, I would advise to wait until this training phase has been completed prior to testing the athletes multidirectional times in the Pro Agility shuttle and other Level 2 assessment tools.
Measuring Peak Power Output
This is the trickiest part of it all, as you will need some hardware to help do this part of the job. I used a Keiser pneumatic power rack as well as the Keiser A300 Squat Machine to measure Peak Power. Now I'm suggesting you go out and spend $6,000 on a squat machine, there are also other less expensive tools that can help you do the job such as GymAware (which is what I currently use), Tendo's, Push Band, and many other brands that are out there.
I measured peak power on the Squat Machine for bilateral, and on the Keiser Rack for the unilateral step up to compare limb differences.
Over-time, the goal was to improve peak power output to assist in improving the athletes CNS' ability to fire off fast. An increase in peak velocity shows apparent improvement of muscle firing rate (speed of contraction), as well as ability to overcome inertia. Phase 1-3 last about 9-12 weeks (depending on the individual's progress) and that is were you will see most of the neurological and muscular strength gains. It is at this point where the athlete should now begin to shift the training adaptation to moving weight faster. The purpose of this phase is to allow the athlete to move the weight as fast as they can, and increase the speeds at which they move that weight over time. Improvements of RFD have been shown to return as late a 12 months in post-op athletes who spend too much time in PT and heavy strength training. Our goal is to achieve Peak RFD sooner so the athlete is able to tolerate high rates of force efficiently and return to practice.
Now I'll be quite honest, a force plate will give you all the detailed metrics needed to say "YES" the athlete is able to return safely because they can produce and absorb force efficiently. Unless you can shell out some dollar bills to get one, then good for you, use that as your RTS guide.
However, based off of my personal work and experiences, this method has worked, and I was able to return mostly all my athletes back to practice by month 7/8.
Here is a graph of the Bilateral Squat over a phase of 3 weeks:
As you can see from this graph, this athlete was able to produce higher outputs over 3 weeks with the same weight. This is evidence that the athletes neuromuscular system is able to generate faster contraction speeds over time. Notice how on the athletes last set, the highest force is being produced, I cannot give you any scientific reason as to why this is happening, but possibly because the athlete knows this is the last set and I might have told them to give ti everything they got.
Now let's take a look at uni-lateral power outputs:
Again, this was done in the same phase over 3 weeks. Notice how the athlete has begin to produce more uni-lateral velocity with their reconstructed limb over time. Now, say what you want, but my belief is that the athlete probably has more confidence in them to move faster over time. In week one you can tell that there is a 30% difference in uni-lateral power output. BIG RED FLAG. But this is why we train, to mitigate instances like this and improve on the deficits. In Week two, the deficit is now cut in half to 16%, and again in half to 8% in week-three. I would say the goal is to keep all imbalances under 10-12% to mitigate any chance of re-injury.
Now, I am not a magician, this is simply an athlete put through proper training and developing confidence over time to being to perform at higher speeds and intensities. To learn more about training in this phase, please feel free to reach out to me through the website, and I will be glad enough to talk some more because this phase can be very complex depending on your athletes situation.