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Introduction

Anterior Cruciate Ligament (ACL) injuries is, by far, one of the scariest injuries to be sustained by a professional athlete, not only because of the time needed to return to play, but especially due to the possibility of not returning at the same level of competition, or in the worst-case scenarios, not returning to competition at all. In fact, it is reported that only nearly 65% of athletes that undergo an ACL injury are not able to return to play at the top level 3 years after the surgery (Waldén et al., 2016).

With that in mind, it is understandable that the start of the running activity is an extremely relevant milestone for a professional athlete once it gives the impression of being closer to accomplish the rehabilitation process or is helping to improve its fitness quicker due to the known benefits of running to the cardiovascular system.

Every healthy person experience running in life (Dugan & Bhat, 2005), even the most sedentary, meaning that people are extremely familiar with that activity – under the scope of the user – which leads to certain assumptions about it. Although complex, people tend to attain a feeling of understanding about its indications, contra-indications, as well, as benefits. This phenomenon is well present in nutrition as well – every person eats, so everybody has a feeling of understanding about nutrition – ultimately leading to dogmas, that once in a while might cause troubles, as it happens sometimes during ACLR rehabilitations.

This might explain why, during the rehabilitation process, athletes usually have a rush to start running even when the foundation to perform it safely are not met yet. When this phenomenon happens, the therapist that is conducting the rehabilitation has the responsibility of educating the patient towards the importance of building the foundations of running to avoid future complications either on the rehabilitation process or even after the athlete returns to training with the team.

However, this idea of accelerating the start of the running and its relationship with a better/faster outcome, successful rehabilitation progression and enhancement of the self-esteem (both from the athlete and therapist) are dogmas present not only on the patient’s mind, but, unfortunately, is also present on many professionals that conduct ACLR rehabilitations.

Running foundations underpin several sports skills as acceleration or sprinting mechanics that highly impact performance. In fact, it is a complex activity, involving considerable amounts of ground reaction forces (GRF) – up to around 3-4 times bodyweight (BW) –, thus, leading to a significant loading of the musculoskeletal system (Donoghue, Shimojo, & Takagi, 2011; Dugan & Bhat, 2005; Grabowski & Kram, 2008; Johnson, Golden, Mercer, Mangus, & Hoffman, 2005; Nagahara, Mizutani, Matsuo, Kanehisa, & Fukunaga, 2017; Skals, 2005; Yamin, Amran, Basaruddin, Salleh, & Rusli, 2017).

Considering the arguments above, the present article will discuss, in light of the available evidence, the importance of establishing clinical criteria to start running, developing a rationale on the criteria of exercise progression taking in consideration the GRF implied both on running and also on the traditional exercises used in ACLR rehabilitations.

Figure 1 - Vertical GRF of Running opposed to walking; adapted from Tongen (1994)

Although, GRF play a significant role on musculoskeletal loading, keep in mind that ACLR rehabilitation rationale of exercise progression must go way deeper than only focusing on this variable (e.g. strain induced on the graft). For some more information regarding this subject you can have a look at THIS ARTICLE.

Running Biomechanics – Capacities and Skills needed

According to Tongen (1994), running can be defined as a “gait in which there is an aerial phase – no limbs are touching the ground – as opposed to walking” (Tongen & Wunderlich, 1994).

Table 1 – Biomechanical characteristics of running

Table 1 shows some of the biomechanical characteristics of running that should be taken in consideration and put in context whenever someone wishes to start with running activities. It is easily perceived that a significant amount of GRF are produced during running. There are two moments in which it is possible to observe peak GRF. One occurs in the braking phase during ground contact – Impact Peak Ground Reaction Force (iGRF) – and the other during mid-stance – Active Peak Ground Reaction Force (aGRF) – that may vary with the striking pattern as shown in Figure 2 (Dugan & Bhat, 2005; Grabowski & Kram, 2008; Tongen & Wunderlich, 1994; Wannop, Worobets, & Stefanyshyn, 2012)

Figure 2 - GRF during different foot striking patterns when running: bold line – rearfoot strike; dashed line – midfoot strike; adapted from Kelly et al., (2018)

Continued expose to high levels of GRF – mainly iGRF – have been associated with the development of injuries in sports, especially overuse ones, such as stress fractures (Grabowski & Kram, 2008). Although these forces are important for tissue remodeling and adaptation to load, attenuating its excessive impact is of highly importance. That capability is carried out by eccentric muscle contractions/work, joint motion and articular cartilage compression (Dugan & Bhat, 2005), together with soft tissue vibration (Friesenbichlern, Stirling, Federolf, & Nigg, 2011).

Therefore, and considering the role of active elements such as the muscle-tendon complex in load absorption during running, the existence of adequate muscular strength, leg stiffness and range of motion may be crucial before the start of running related activities during rehabilitation. Appropriate levels of strength will tend to prevent or minimize exaggerated articular cartilage compression that might lead to knee joint overloading and consequent development of degenerative injuries (Lin, Lin, Lin, & Jan, 2009; McGinty, Irrgang, & Pezzullo, 2000; Salzmann, et al., 2016).

With that said, there are some fundamental capacities and skills that underpin a proper running performance, as it might be the mobility, strength and/or motor control (Barnes & Kilding, 2014; Dugan & Bhat, 2005).

            Mobility

As you can understand from Figure 3, from a kinematic point of view, mobility requirements to perform a run are not that much difficult to achieve, even considering an athlete during ACLR rehabilitation. However, it is never enough to keep in mind the average range of motion needed on the main lower limb joints to perform running – Table 2 – , in order to avoid developing copying strategies that may compromise the musculoskeletal system with compensations, associated with secondary injuries or complaints, sometimes in a location rather than the injured knee.

Figure 3 – Athlete’s body position from start acceleration phase to top speed; Adapted from Nagahara et al., (2014)

Table 2 - Lower limb joints sagittal plane range of motion during jogging and running (4 m/s); Adapted from (Orendurff, et al., 2018)

From the information above the reader can already understand that, in most cases, mobility issues won’t be the limiting factor to start the running activity during ACLR rehabilitations, however, the same cannot be said for strength and/or motor control related issues.

            Strength

Table 3 shows us the magnitude of the GRF that the musculoskeletal system must carry over when running at different velocities, that can reach up to 4,4 times BW. It is possible to analyze that the forces are significantly higher during acceleration and top speed phases, however, the GRF that our bodies must handle even when jogging (2,4x BW) are not neglectable at all (Donoghue, Shimojo, & Takagi, 2011; Dugan & Bhat, 2005; Grabowski & Kram, 2008; Johnson, Golden, Mercer, Mangus, & Hoffman, 2005; Skals, 2005; Yamin, Amran, Basaruddin, Salleh, & Rusli, 2017).

Table 3 - GRF during different phases of running and different velocities.

Anyway, the high levels of force that must be produced during running are just the tip of the iceberg in what concerns the strength related capacity issues. As highlighted before, contact times during running are extremely short in duration (99-172 ms), meaning that there is a considerable restriction in time to produce the above-mentioned amounts of force at the moment of ground contact. This leads us to understand the importance of the rate of force development when considering an efficient and proper running mechanics. Just as a curiosity, according to the literature, when running at top speed, athletes might have to produce force at a rate of 12172 N/s, which is the equivalent to 1242 kg/s (Slawinski, et al., 2010).

Figure 5 - Force/time graphics of two athletes (A and B) reaching the same level of peak force but with different rates of force development. Athlete A is able to access high levels of force quicker than athlete B, thus having a higher rate of force development.

While running, forces might be as high as the ones produced by trained individuals when performing a hang power clean with 90% RM (see Video 1) – around 2512-3544 N –   with the remarkable particularity that, while running, those amounts are produced in a single leg (Donoghue, Shimojo, & Takagi, 2011; Dugan & Bhat, 2005; Grabowski & Kram, 2008; Johnson, Golden, Mercer, Mangus, & Hoffman, 2005; Hori, et al., 2007; Kilduff, et al., 2007; Skals, 2005; Suchomel, Wright, Kernozek, & Kline, 2014; Yamin, Amran, Basaruddin, Salleh, & Rusli, 2017).

Video 1 - Hang Power Clean

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Although both activities produce similar amounts of GFR the way those levels of force are produced are biomechanically different, especially in what concerns the type of muscular work. While during the hang power clean most of the force is produced through the concentric contraction of the recruited muscles during the leg extension mechanism, when running, most of the force is produced through a stretch shortening cycle (SSC) mechanism in the musculotendinous unit, turning running a much more economic activity than the mentioned weightlifting derivative exercise (Garhammer, 1984; Haug, Drinkwater, & Chapman, 2015; Hennessy & Kilty, 2001; Moir, 2015).

Based on the above arguments, it should be clear that, previously to start running, athletes SSC mechanisms should be optimized. This way, athletes not only must possess adequate levels of strength, as they must also have adequate rate of force development (RFD) associated with adequate leg extension stiffness (Horita, Komi, Nicol, & Kyrolainen, 2002; Folland, Allen, Black, Handsaker, & Forrester, 2017; Storen, Helgerud, Stoa, & Hoff, 2008; Turner, Owings, & Schwane, 2003). Indeed, athletes should first be exposed to maximum strength and power programs to improve the qualities above, being after exposed to plyometric training programs to improve SSC mechanisms (Boyle, 2016).

It is not the scope of this article to explain how to develop maximum strength and power, neither how to diagnose potential deficits on those qualities, however, you can review those topics on our previously published articles regarding maximum strength, power and strength diagnosis.

Video 2 – Example of exercises to improve strength qualities after an ACLR

            Motor Control

Even in the presence of all the capacities needed to perform running with the proper technique, sometimes athletes lack the skill to do it. Meeting the proper technical model while running is relevant, especially when considering an athlete that is coming from an injury that is gradually increasing the loading of his musculoskeletal system, once most of the details will imply more economy during running with less musculoskeletal impact (Barnes & Kilding, 2014; Dugan & Bhat, 2005; Grabowski & Kram, 2008).

As an example, proper timing and range during the triple flexion observed on the swing leg might decrease the force and energy expense needed during the propulsion phase on the stance leg (Folland, Allen, Black, Handsaker, & Forrester, 2017). A typical technical mistake that has direct implication on knee joint loading is the overstriding or insufficient leg recovery during the last period of the leg swing, which will lead to an excessive anterior ground contact relative to the center of mass, increasing the braking forces during running. When running at moderate velocities, those technical errors often lead to heel strike, which is proven to have impact on lower limb overuse injuries (Grabowski & Kram, 2008).

It is widely known that some movement patterns (e.g. dynamic knee valgus) during close kinetic chain activities might increase the risk of acute and overuse injuries in field sports athletes, which highlights why matching an adequate motor control during running should be tried (Kozinc & Sarabon, 2017).

Nevertheless, anatomical, functional and/or past movement history might unable or even turn inadequate for an athlete to run fulfilling all the parts of the “ideal” technical model. Said that, it is important to respect the individuality of the athlete unless he is not able to cope with his own individual variations from the “ideal” technical model.

Video 3 – Example of exercises to improve running skills

Matching Science with ACLR rehabilitations progression

An athlete that undergone ACLR will be subject to significant muscle atrophy, considerably decreasing his strength, besides having to deal with the inflammatory process that, to some extent, will limit load increments, as well as impact activities.

As seen before, plyometric training or, at least, adequate plyometric performance underpins proper running performance (Barnes & Kilding, 2014; Turner, Owings, & Schwane, 2003). Literature clinical/functional criteria to start with plyometric training is controversial, ranging from being able to perform 3x15 repetitions of single-leg squat with adequate balance (Comfort & Abrahamson, 2010) to NSCA guidelines that advise the initiation of plyometric training only after being able to lift 2xBW in a back-squat (Boyle, 2016). Those recommendations, although with some relevance, are extreme positions and may not reflect the truly needs for plyometrics. Anyhow, from that, it is already possible to understand that considerable amounts of maximum strength are needed to be able to adequately perform plyometrics, together with the ability to access those levels of force quickly once, during SSC activities, time to produce force should not be higher than 250ms, implicating significant levels of rate of force development (Hennessy & Kilty, 2001).

In our perspective, plyometric and ballistic training should not be initiated before injured lower limb maximum strength is balanced considering the uninjured one (which might take around 2-2,5 months to achieve according to our experience), in order to decrease the risk of overloading the knee joint due to that lack of capacity to absorb impacts, thus worsening rehabilitation’s outcome.

The capacity of absorbing energy and then applying it quickly into the ground might be assessed through the calculation of the Reactivity Strength Index (RSI), that, for sports requiring fast SSC, it is widely accepted that RSI values should be above 2,0. This can be a good metric to use to understand how well is the injured athlete being able to use his leg extension strength to absorb energy from the landing and then quicly reutilize it during the following jump.

When conduction an ACLR rehabilitation, physiotherapists and coaches should not only be concerned about preparing the athlete to produce the levels of force needed during running, but also to create adaptations on the musculoskeletal system in order to guarantee that it will handle the forces implicated in running, especially when considering a knee that comes from a surgical procedure and is still gradually adapting to load acceptance again. This bring us to the importance of selecting and progressing in the gym exercises and activities considering the forces that will be involved on the musculoskeletal system of the athlete.

Through the analysis of  Table 4, representing traditional gym exercises and activities with the respective GRF, it can be noted that, to equal running GRF, after an ACL reconstruction, athletes should undergo serious strengthening programs in order to create musculoskeletal adaptations that allow them to perform running without an increased risk of overuse injuries due to gradual overloading. However, precautions should be taken when analyzing those numbers once, as highlighted before, the type of muscular work, the range of motion and other parameters may vary between those exercises and running, leading to a more or less dynamic correspondence which might lead to different musculoskeletal adaptations.

Table 4 – GRF during traditional exercises

Table 4 may be useful to realize the adaptations needed in terms of GRF production and handling in order to achieve proper running with decreased risk of injury, highlighting the use of some reasoning in order to progressively increase the demands in GRF for the athlete in rehabilitation through a graded exposure strategy (see Figure 6). Nevertheless, the presented progression is only considering the GRF, which may not be totally adequate taking in consideration all the details and precautions of an ACL reconstruction rehabilitation, as it should be the precautions regarding the shear forces on the tibia and stress on the ACL graft produced with different exercises – you can review this on our previous published article regarding the use of the leg-extension machine.

Figure 6 - Vertical GRF during skipping and running; Besides not having a dynamic correspondence in what concerns the RFD and magnitude of force, form skipping might still be useful to use as a graded exposure strategy considering a preparation for the GRF in running; Adapted from Johnson et al., (2005).

In our perspective, and considering the knowledge, evidence and clinical experience regarding ACL reconstruction rehabilitations, we consider that, in several situations, athletes are starting to run too early. Proof of that are cases of athletes that initiate the in-field rehabilitation (Return-to-Play – RTP) before 3 months after surgery (Rambaud, Ardern, Thoreux, Regnaux, & Edouard, 2018), once, as showed in this article, the capacities and skills needed to run adequately will be extremely difficult to achieve until the third month.

As stated in the literature, early start of impact activities without adequate criteria may increase the risk of the athletes to develop comorbidities during the rehabilitation process, and possibly increasing the risk to attain injuries that can be functionally related with the limitations still present at the time of returning to competition (Rambaud, Ardern, Thoreux, Regnaux, & Edouard, 2018). At Football Medicine^®, we believe that those cases may be a consequence of inadequate progressions during the rehabilitation stage.

With that in mind, we share below on Table 5 our clinical criteria regarding the initiation of running after an ACL reconstruction surgery. Football Medicine^® professionals underline that this represents a general approach, that should be taken in consideration with the specificities and needs of each athlete, together with his injury background. However, bear in mind that those are the minimums to achieve before starting to run, taking in count the complications that might come in short- and/or long-term due to a premature initiation of running/impact activities.

Table 5 - Football Medicine® Clinical Criteria to initiate running after an ACL reconstruction surgery

“Practice doesn’t make you perfect. Perfect practice does!”

by Red Holzman

If you want to learn more about ACL rehab, you can subscribe and watch the lectures of ACL experts Kieran Richardson, Bart Dingenen, Lee Herrington here: Click here

About the author:

Ruben Ferreira 

• Co-Founder & Director of Football Medicine^® (link www.footballmedicine.net)
• Head of rehabilitation at Sporting Clube Portugal (Sporting Lisbon)
• Master in Orthopedic Manual Therapy
• Ongoing Master in Strength & Conditioning at St. Mary’s University Twickenham – London
• Certified Kinesio Taping Instructor

Football Medicine^® (www.footballmedicine.net)

                Football Medicine^® is a company/brand founded in 2015 by one Sports Medicine Doctor – Dr. João Pedro Araújo – and two Physiotherapists – Paulo Barreira and Ruben Ferreira – at the time they were part of the medical department of Al Ahli Football Club in Dubai. This project’s aim was to show, discuss and promote what best was being done in the sports medicine universe, as well as fighting some dogmas existent in this community.

            The project that before was covering mostly areas related with rehabilitation and medicine has now grown and Football Medicine^® Team is, nowadays, constituted by Sports Medicine Doctors, Physiotherapists, S&C Coaches, Sports Scientists, Sports Nutritionists and Researchers, that act in World-class football clubs and organizations as Arsenal FC, PSG, FC Porto, Sporting Clube Portugal, Iranian National Team, Aspetar, FIFA Medical Centre of Excellence and even Editors of the British Journal of Sports Medicine.

            Although it is having an exponential growth with the newly subjects covered by our team, the mission of Football Medicine^® remains the same: Promote scientific grounded discussion, delivering free contents related to sports medicine and sports science targeting some of the dogmas that exist inside the football community.

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