ACL rehabilitation: When should athletes start running?!



Running is an activity present in most of the sports we know (Tongen & Wunderlich, 1994).  Every healthy person experienced 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.

Running foundations underpin several sports skills as acceleration or sprinting mechanics that highly impact performance, being also an activity that has evidence to be beneficial to weight management or cardiovascular and fitness improvements (Barnes & Kilding, 2014). Therefore, considering the injured player under rehabilitation, the start of running activities seems to be a psychological barrier, after which the return-to-training seems closer.

Considering the mentioned benefits, often people involved in sports assume the start of running as a way to improve athletes’ fitness levels and even to help achieving rehabilitation outcomes in a fastest way, which take us to a relevant question when considering the athlete in rehabilitation process: Should we use running to physically prepare the athlete or should we be more concerned in physically preparing the athlete to run?

Anterior cruciate ligament (ACL) injuries are one of the most common severe knee injuries in football (Ekstrand, 2016), being responsible for 5791 days absent in European leagues between 2001-2009 (Waldén, Hägglund, Magnusson, & Ekstrand, 2011). Normally, an ACL reconstruction rehabilitation takes an average of 6-8 months to return-to-competition, which represents a considerable part of the football season duration (Waldén, Hägglund, Magnusson, & Ekstrand, 2011). This fact might explain the anxiety experienced by players, technical and managing staff regarding players’ return to training timings (Rambaud, Ardern, Thoreux, Regnaux, & Edouard, 2018).

Traditionally, long-term knee injuries were managed in a conservative way, being the rehabilitation process predominantly passive until symptoms and mobility limitations were overcome, what could take several months (Rambaud, Ardern, Thoreux, Regnaux, & Edouard, 2018). Muscle trophism was considered in a way, however without many precisions regarding the development of the different strength qualities. As soon as knee symptoms and signs allowed, “functional rehabilitation” was taking place until the athlete had clearance from the orthopedic surgeon to start training/competing, usually extremely dependent on time (Rambaud, Ardern, Thoreux, Regnaux, & Edouard, 2018).

Nowadays, with the improved knowledge surrounding sports medicine and sports science, this passive and conservative rehabilitation scenario gave a way to a more active and aggressive treatment approach that resulted in the development of several accelerated rehabilitation protocols, leading to an early start of functional activities compared with the previous ones (Rambaud, Ardern, Thoreux, Regnaux, & Edouard, 2018).

This accelerated approach is based on progression criteria markers that are more or less established for each rehabilitation stage, as opposed to the time-based approach used in previous years (Rambaud, Ardern, Thoreux, Regnaux, & Edouard, 2018). Despite those improvements in rehabilitation management, some functional activities still seem to lack some well-grounded criteria to be initiated, considering the current state of the knowledge on physiology, biomechanics and pathology. Running can be pointed as one of those activities.

Running 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 that, this article will focus on running biomechanics, capacities and skills required to perform it, hoping to make the bridge to some basic and foundational criteria that should be met before starting this activity during an ACL reconstruction rehabilitation. Ultimately, this could help for a better judgement if running is being initiated too soon or not during the rehabilitation.


Considering the detail and extent of this article, we show you below the key-points/take home messages that should be present whenever a medical team is preparing an athlete with an ACL reconstruction surgery for running. For more detailed information and reasoning regarding Football Medicine® Criteria to initiate running after an ACL reconstruction surgery (Table 5), please have a look to the different article sections below.

Definition and characteristics of running

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).

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


Considering Figure 1, and besides the obvious kinematic differences between walking and running, significant kinetic differences in respect to vertical GRF in between them are shown, with the magnitude of peak force being the double from walking to running (Tongen & Wunderlich, 1994). 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.


Table 1 – Biomechanical characteristics of running


From the analysis of Table 1, 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 motionand 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 (e.g.) prevent or minimize exaggerated articular cartilage compressionthat 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).


Capacities and Skills for Running

As highlighted above, 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). Those are important qualities to take in consideration before starting to run once, besides increasing running economy and efficiency, might also lead to a decreased risk of injury (Lin, Lin, Lin, & Jan, 2009; McGinty, Irrgang, & Pezzullo, 2000; Nagahara, Mizutani, Matsuo, Kanehisa, & Fukunaga, 2017; Salzmann, et al., 2016; Storen, Helgerud, Stoa, & Hoff, 2008; Turner, Owings, & Schwane, 2003).



Although it might be relevant for the athlete to have unrestricted mobility to be able to run properly, those levels of mobility are not that demanding to achieve as shown in Figure 3.

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


When considering an athlete that performed an ACL reconstruction surgery, it is easily perceived, according to Table 2, that he/she will be able to access those amounts of mobility soon in the rehabilitation process without requiring additional copying strategies that may compromise the musculoskeletal system with compensations, associated with secondary injuries or complaints rather than the injured knee. However, if other capacities such as strength are taken into account, the same rationale might not be applicable.

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



As stated before in this article, proper running implies the production and carry over of significant amounts of forces to the musculoskeletal system (Donoghue, Shimojo, & Takagi, 2011; Dugan & Bhat, 2005; Grabowski & Kram, 2008; Johnson, Golden, Mercer, Mangus, & Hoffman, 2005; Skals, 2005; Yamin, Amran, Basaruddin, Salleh, & Rusli, 2017). Those forces are certainly higher during the acceleration phases and also during top speed phases of running, however, even when jogging, the GRF produced are not negligible, as shown in Table 3.

Table 3 – GRF during different phases of running and different velocities


Considering Table 3, while jogging, an athlete might have to handle and produce a force equivalent to 1,4-2,4 times his BW (Grabowski & Kram, 2008; Keller, et al., 1996), however, those forces can increase up to around 3 times his BW if we consider the acceleration phase of a sprint or running at maximum speed (Bezodis, Kerwin, & Salo, 2008; Hunter, Marshall, & McNair, 2005; Mero, Komi, & Gregor, 1992; Nagahara, Mizutani, Matsuo, Kanehisa, & Fukunaga, 2017; Nummela, Rusko, & Mero, 1994). Only these values must already make us think how fit an athlete should be before he can handle those forces without increasing the risk of injury. Even so, you might still be thinking – “Ok, in practical terms, how much do those bodyweight equivalents represent in terms of effort?” – later on this article we will show you how massive can be those forces in comparison to frequently used gym-based exercises.

Figure 4 – Braking (A) and propulsive phases (C) of running; Adapted from Manzer et al., (2016)


As shown in Figure 4, as soon as the foot contacts with the ground, force must be produced to, first, absorb the force from the landing/body in movement; for then produce enough force to overcome his weight (vertical force) and still be able to jump off the ground (vertical and horizontal forces) to the next flight phase (Moir, 2015).

As it is widely known, contact times during running are significantly short, ranging from 99-172 ms, meaning that the athlete not only needs to produce a significant amount of force during ground contact, but must also be able to access those levels of force quickly (Kugler & Janshen, 2010; Moir, 2015; Morin, et al., 2015; Slawinski, et al., 2010). This lead us to understand the importance of the rate of force developmentwhen 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.


In practical terms, and for the reader to have an idea of the huge forces involved during running, those might be as high as the forces produced by athletes 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


At this point, and considering the demanding forces involved in running, some of the readers might be asking: “If such high forces must be produced, how do sedentary people run? And, how can a human continuously produce those forces for several kilometers?”

Considering the examples above – hang power clean vs running – the type of muscle work between both is fundamentally different. While during the weightlifting derivative exercise 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 (Garhammer, 1984; Haug, Drinkwater, & Chapman, 2015; Hennessy & Kilty, 2001; Moir, 2015).

This particularity turns running a much more economic activity than the hang power clean, once, instead of using a concentric muscle action (high metabolic demand), during the stance phase of running leg extension muscles are virtually, exclusively, contracting isometrically (less metabolic demand) being the joint range of motion given by the stretching that happens on the connective tissue, when absorbing the energy during the braking phase and reutilizing it during the propulsive phase (Barnes & Kilding, 2014; Hennessy & Kilty, 2001). This might be one explanation for the difference in the perceived effort between both activities.

Based on the above argument, it should be clear that previously to start running, athletes’ SSC mechanisms should be optimized, in order to achieve a proper economy during running and decrease the risk of overuse injuries during the rehabilitation process (Barnes & Kilding, 2014; Donoghue, Shimojo, & Takagi, 2011; Dugan & Bhat, 2005; Grabowski & Kram, 2008; Hennessy & Kilty, 2001; Turner, Owings, & Schwane, 2003). For that to be possible, 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).

Considering the above criteria, to be able to adequately perform fast SSC activities (as it is running), 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). When considering an athlete that undergone ACL reconstruction, and considering all the muscle atrophy and strength decrease due to the surgical intervention, together with the inflammatory process that lasts several weeks, it can be challenging to fulfill those strength qualities criteria before three months after surgery. Therefore, if there are concerns (as we would recommend) to achieve balanced strength between lower limb muscles as other criteria to start with running, this goal will be quite difficult to achieve during at least the first 2,5 months post-surgery, according to our experience.

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 ACL reconstruction


Motor Control

Even in the presence of all the capacities needed to perform running with the proper technique, sometimes athletes lack the skillto 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).

Video 3 – Example of exercises to improve running skills


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 highlight 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.


When considering an ACL reconstruction rehabilitation

From the considerations above, it is easily perceived that the most demanding capacities needed to achieve before starting to run is the ones related with strength as well as musculoskeletal system tolerance to the significant forces implied in running (load), once those will be the ones that will take longer to acquire.

As said before, an athlete with an ACL reconstruction 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). According to NSCA guidelines, before starting with plyometric activities, athletes should be able to lift 2 times their bodyweight in a back squat (Boyle, 2016). Those recommendations, although with some relevance, are extremely general, especially if we consider that, normalizing to the bodyweight with a ratio model, will always privilege athletes with less bodyweight. Besides, how difficult is it to find a football player that is able to lift that load when he is fit?? So, how hard and time consuming would it be to develop those amounts of strength on a player recovering from an ACL injury? Anyhow, from that, it is already possible to understand that considerable amounts of maximum strengthare needed to be able to adequately perform plyometrics, together with an ability to access those levels of force quickly once, during SSC activities, when time to produce force should not be higher than 250ms, which implies significant levels of rate of force development (Hennessy & Kilty, 2001).

Taking the limitations above in consideration, other authors have been suggesting the initiation of plyometric exercises with less demanding criteria – as being able to perform 3×15 repetitions of single-leg squat; adequate balance in a single-leg (Comfort & Abrahamson, 2010). However, in our perspective, plyometric and ballistic training should not be initiated before injured lower limb maximum strength is balancedconsidering the uninjured one (which might take around2-2,5 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 reutilize it during the following jump.

By now, it should be clear that in order for an athlete to run properly, he must be able to produce high amounts of force and expressing it in a demanding fashion. It was also already discussed the significant amounts of forces that the body must handle during running, given the GRF produced in each step. With this in mind, 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.

As stated before on this article, while running, GRF might vary between 1,5-3 times bodyweight, which means that athletes musculoskeletal system should be prepared to handle those forces. Through the analysis of Table 3, representing traditional gym exercises and activities with the respective GRF when performing it, 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 motions may vary between those exercises and running, leading to a more or less dynamic correspondence which might lead to different musculoskeletal repercussions.

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 history. 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


Authors: Ruben Ferreira; Micael Moreira; João Pedro Araújo




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Join the discussion 2 Comments

  • Andreas Bjerregaard says:

    Hi. Thank you for a very informative article. Can you help with your opinion about running before jumping exercise or vice versa. Another thing I was wondering was your H/Q ratio. Do you get any reference for this because we use it as discharge criteria, but not as a criteria for return to running? Can the quad be to strong in comparison to hamstring, if it less than 10% quad to quads difference?

    • FM says:

      Good morning Andreas,

      First, thank you very much for commenting.

      In regards to your questions:
      We use it as a criteria for running once the conventional ratios might have implications on knee ago/ant provided stability, on the other hand, if higher speed are requested. to the athlete, the functional ratio can play a role on the hamstring muscle injuries risk, and so, to avoid concomitant injuries (in this case a hamstring injury due to sprinting) during the rehab, before starting to run at speeds higher than 5,5m/s we advise you have already the functional ratios corrected.

      In what concerns quadriceps and hamstrings strength, it is normal and advisable that quads are stronger than hamstrings in a concentric fashion, however, hamstring concentric strength shouldn’t be less than 60% of quads strength because as you know, hamstrings play a role on knee stability and anterior drawer prevention, if its strength is decreased and with massive quads, you might be having a knee stability issue.

      I hope we matched your doubts and questions, let us know if we were not clear!

      All the best!

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