Developing Rate of Force Development in ACL Rehab - Case Example

Restoring Strength in ACL Rehab

Throughout ACL rehabilitation, there are a host of physical qualities that need to be restored: range of motion, strength, cardiovascular conditioning, psychological preparedness, power etc. These qualities are often related, but very distinct and various phases of the ACL rehab plan may emphasize a specific quality over another, as some are building blocks for others. For example, in early stage rehab, range of motion is a high priority. Then as we move towards mid stage rehab we emphasize strength training, followed by power training in late stages. 

Strength, by definition, is the ability of a muscle or group of muscles to produce force. This can be measured using a dynamometer that reads a force output. Typically peak force can be achieved about 300ms after onset, as it takes time to ramp up to full intensity. Achieving full strength is a huge priority in ACL rehab as higher strength levels have been correlated with improved return to sport outcomes. For every 1% asymmetry improvement in quad strength after 90% symmetry there has been shown to be a 3% decreased risk of reinjury. 

That all being said, another important quality to restore after ACL reconstruction is called Rate of Force Development (RDF). This is simply the rate of force production, or how quickly force can be produced. This is an important metric to address as most athletic movements involve ground contact times of 30ms to 200ms. Additionally, ACL tears often happen within about 30ms after their foot hits the ground, thereby necessitating restoration of the ability to rapidly produce force to stabilize the knee.

The purpose of this article will be to overview a case study example of how we were able to measure these deficits, create a targeted plan to improve these deficits and measure progress to ultimately return this particular athlete as best as possible back to sport. If you are looking for more in depth education surrounding strength and rate of force development in general,  read our article How Do I Get My Speed Back at the End of ACL Rehab?

Additionally, if you would like more information surrounding earlier stages of rehab, please read our other article surrounding early stage ACL rehab: How to Nail Your First 6 Weeks of ACL Rehab.

Case Example Testing

Below is a graph showing the results of this athlete during late-stage rehab of ACL reconstruction on her right knee, about 9 months post-op.

Interpreting the Findings

If you read the graph and table above documenting our athlete’s rate of force produced during a jump, we notice the following:

• Strength
  • Symmetry: Great
  • Relative to body weight: Great
• Rate of Force Development
  • Significantly lower on the right side than the left
• Overall Interpretation
  • If you look at the graph read out you can see the blue curve looks very different from the orange. The x-axis is time, the y-axis is force in pounds. You can see both achieve very similar peaks represented where the dots are, however, they achieve it very different ways. Notice the orange curve is very steep in the beginning and tapers off. The blue slowly keeps increasing until its peak at the end. The steepness of the line is going to equate to the rate of force development. Steeper line = higher Rate of Force Development. 

Notable Jump Testing Results

Goals of Home Exercise Program

• Braking and propulsive phase Rate of Force Development
• Single leg output
• High number of exercises with high neuromuscular recruitment

General Strategy Ideas to Improve Rate of Force Development

Based on these findings above, we want to develop some strategies and a plan to get this athlete’s RFD and overall strength improved through the ACL rehab stages. 

Here are some general strategy ideas we implemented:

• Depth drops - RFD - up to 48”
• Band resisted jumps - accentuated braking RFD
• Elastic plyo’s - short GCT
• Oscillatory movements - decreasing inhibition, coordination
• Cuing fast ECC - trap bar SL snap down
• ISO pushing with emphasis on pushing hard and FAST
• Very high load, low rep strength work to increase neuromuscular recruitment
• Deep plyo’s - emphasizing braking RFD in deeper ranges of motion

One question we need to ask ourselves at this point is, “Why are deficits in RFD present even in late stage ACL rehab?” This will be an important question to ask and understand to best figure out what our plan needs to address.

• Changes in cortical brain areas after injury - This is something that is changed after the injury, however with training it can be trained to improve the signaling from the brain to the muscles. 
• Increased hamstring co-contraction - This is a coordination overshoot to improve joint stability but ultimately can limit performance and output. We want the hamstrings to contract to help to stabilize the knee, but at the right time and at the correct intensity. Again, a trainable quality. 
• Loss of afferent signaling  - This leads to compensatory protection in activating fast twitch motor units meaning our brain doesn’t like to contract the muscle fibers responsible for the high output/fast contractions. Again, this can be trained!
• Loss of muscle size, type IIa, increased type IIx/hybrid - Due to the surgery/injury, inability to train in early stages of rehab, our bodies change their proportion of “power” muscle fibers and use more “endurance” fibers. Training and adaptation can alter this. 
• Insufficient rehab - Lacking maximal and ballistic training, maximal intent

Example Training Week

Here is an example week of what this athlete’s specific plan looked like. We were planning on her training 3x/week. She had been accustomed to this volume of training and her total fatigue was well managed. Training focused on RFD isn’t often very fatiguing as she was having very high efforts but instructed to give plenty of rest so each and every rep was at very high quality and high intensity. 

That being the case, each workout didn’t feel very hard, thus higher frequency for her was beneficial. Each day always started with a consistent warm up using the RAMP protocol - Raise, Activate, Mobilize, Potentiate. This was important to prepare her body to hit the first working set with as high an output as possible. 

The first working set was typically an elastic movement - quick but low fatigue. We prioritized single leg movements as we didn’t want her to compensate onto her other side too much and we were finding the most deficits during single leg tests. The goal was to have a variety of movements targeted at different ground contact speeds, different depths and different planes of movement. Each day also concluded with some isolated strength work to the lower body. 

Retesting 2.5 Months Later

Below you can see the results after retesting 2.5 months after the late-stage rehab testing. The image on the left is the initial test, the right is most recent.

Looking at the strength curves above, you can note several important things.

First, peak force on both sides has actually improved. More importantly, the shape of the curve has also changed. The blue now ramps up quicker and has a steeper start. This means the RFD has also improved. Additionally, when looking at when the peak force occurs (the dot), it happens earlier than the first testing 2.5 months earlier. 

When looking at our single leg jumping tests, here were the results:

Jump height improved on both sides. The depth of the jump remained the same from the first testing date, however the braking RFD (how quickly the athlete was able to apply the brakes) significantly improved on both sides. Further, when looking at the drop jump, the athlete was able to leave the ground in less time and jump higher. 

Case Study Summary

In conclusion, achieving symmetrical peak force is not the end-all-be-all for testing criteria, but it is a prerequisite for developing a high rate of force development and sport performance. With proper assessment, we can identify deficits, brainstorm reasons for why they exist, and develop a targeted plan to improve these deficits.

This is not a quick fix, nor is it an easy one. This case example took almost 3 months to go from the first set of testing results to the last, which consisted of 3 days/week of targeted, hard training, as well as the athlete being more involved in their sport. 

Achieving high strength levels and RFD has been shown to reduce risk of re-injury and help performance. It is all too common athletes return to sport without feeling themselves yet, and therefore don’t perform at their desired level. Restoring high RFD can be a huge help in performance once stepping back on the field, but if it’s not assessed, you’re simply guessing.