Deficits in strength, rate of force development, and functional tasks persist even after the expected time point for return to sport after ACL reconstruction 1,2,3,4. During late stage ACL rehab, it is important to match the interventions with the tissues’ tolerance as well as the goal adaptation (ROM, strength, speed-strength, power, coordination, skill etc.). Not only does the intervention need to match the intended impairment, it must be of adequate intensity and complexity to induce progressive adaptation. Therefore, it is integral that clinicians understand underlying physiological factors and training considerations that may affect strength development and progression. Current evidence suggests that increasing muscle cross sectional area and work capacity followed by phasic progression can induce superior strength-power gains than non-progressive training 5.
This occurs through mechanical changes of muscle cross sectional area and increased musculotendinous stiffness to improve force transmission which may play a role in rate of force development. In addition to the morphological changes that occur, neuromuscular qualities adapt to improve strength, rate of force development and performance. These include an increase in motor unit recruitment (Henneman size principle), motor unit synchronization to improve rate of force development, rate coding and decreasing neuromuscular inhibition 5.
These adaptations are accepted to occur with specific training principles using a targeted approach along the speed-strength continuum. As the typical course of rehabilitation follows this training progression principle, current evidence suggests rehabilitation may fall short in fully restoring muscular strength, particularly rate of force development after ACL reconstruction 1,2,6,7. Therefore, training principles are needed to improve these impairments provided the athlete has a foundational level of strength and overall activity tolerance. These training principles appear to hold true not only athletes training for performance, but also in athletes during late stage ACL rehab6.
Although the study by Angelozzi et al. demonstrated successful maximal isometric contraction 6 months post ACL reconstruction, they found persisting deficits in quadriceps rate of force development. The subjects proceeded to perform an additional 20 weeks of training with an emphasis on rate of force development improvement. This 20-week program incorporated training principles including a combination of high force/low velocity, low force/high velocity, and high force/high velocity exercises (see figure below for relationship between force and velocity). Examples of each of these are shown in the video below as well as an the graph demonstrates an athlete’s velocity with an array of given loads 8.
The biggest theme within each of these places along the force velocity curve is that maximal intent is applied to the given load as well as minimizing fatigue so our reps are performed with the highest quality. This can be done in a variety of ways including monitoring rest times, using cluster sets, prescribing an appropriate amount of repetitions, and beginning with an appropriate warm up. Without maximal intent each and every rep, our planned adaptations to improve rate of force development using exercises along the speed-strength continuum are less likely.
The findings of the additional 20 weeks of rate of force development focused training resulted in further improvements in maximal strength and improvements in rate of force development that returned to pre-injury levels 6. Therefore, specific rate of force development training programs appear beneficial and necessary in order to return an athlete to pre-injury levels of neuromuscular performance. These results, however, should not yet be generalized to populations other than the one tested (professional male soccer athletes) based on this study alone. General strength and conditioning principles should be applied to athletes prior to return to sport to ensure adequate muscular performance and athlete readiness.
Over the past few blogs we have examined persistent deficits in maximal strength, rate of force development, functional performance and biomechanics following ACL reconstruction. The included population includes males and females across all age groups but a higher proportion of athletes between the ages of 20 and 30 involved in various sports and at different time points after ACL reconstruction. The data suggest that athletes, overall, are returning to sporting activities too early and are not passing current recommended guidelines including strength measures, hop testing, and movement assessment. There is a need for athletes to participate in a comprehensive, data driven late stage ACL rehab program.
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Data further suggest that the current guidelines may not be rigorous enough to ensure athlete readiness and that a greater focus on rate of force development and comparison to pre-injury values or normative data should be implemented prior to allowing an athlete to return to sport. Training should always be appropriate based upon athlete readiness and specific to their impairments while also being specific to their sport demands. Ultimately, the hope is that more appropriate training and assessment strategies will not just allow safer for safer return to sport, but a return to performance and a decreased risk of future injury.
Are you an athlete who recently suffered an ACL injury, is currently undergoing ACL Rehabilitation or are looking for that next step to bridge the gap from ACL physical therapy to Return-to-Play? We work with all types of athletes recovering from ACL injuries, taking them through a detailed process to return them back to sport safe and prepared to play! We focus on areas such as strength restoration, movement re-education and objective testing with the latest strength and force plate technology to assess progress and readiness for return-to-play. Learn more about our approach to ACL Rehabilitation, or reach out today to schedule a free consultation to see if we can help you.
Schmarzo M, Van Dyke M. Applied Principles of Optimal Power Development.