FEATURE ARTICLE: ACL reconstruction return to soccer: Strength or Func

ACL reconstruction return-to-soccer: Strength or Function?

Budiman Pranjoto (Budi), BBiomedSc (FUHB), BPhty, PGDipPhty (OMT)

Introduction

Anterior cruciate ligament (ACL) injury is one of the more common and serious injuries sustained by football players, some even requiring surgical reconstruction (Bizzini, Hancock, & Impellizzeri, 2012). However, the rehab protocol for soccer players returning to sport after ACL reconstruction is not well defined (Bizzini, et al. 2012). A concern for athletes who have had an ACL reconstruction is the incidence of reinjury and surgical revision (Nagelli & Hewett, 2016). There are multiple aspects to return-to-sport after an ACL reconstruction, such as nutrition, general health, biomedical and psychological readiness. However, as physiotherapists, we generally “clear” an athlete to return-to-sport after they achieve good mobility (range of motion) with good strength and/or function relevant to their sport. For the purpose of this written assignment, we will primarily discuss rehabilitation goals and tests that are grounded around strength compared to function.

This essay will discuss recent evidence around strength vs. function for return-to-sport for soccer players after an ACL reconstruction. I will discuss pros and cons for each of them and when strength or function is more appropriate if one of them is superior to one another. For clarification, I define strength as a performance measure (e.g. peak torque) to produce an isolated movement (e.g. knee extension), while function is the ability to produce movement involving multiple joints in a functional pattern (e.g. hopping). When are strength or functional measures more appropriate?

Does strength affect function?

While strength is defined as an isolated movement measure, it is important to consider that strength may result in better function as well. Schmitt, Paterno, and Hewett (2012) demonstrated this very well. Ninety soccer players were involved in their study, 55 had undergone ACL reconstruction and 35 were uninjured. Maximum voluntary isometric contraction (MVIC) of quadriceps femoris was measured on both legs to give a quadriceps index for symmetry (involved/uninvolved × 100%). The 55 players with ACL reconstruction were then sub-grouped to those with high quadriceps index (≥90%), and low (<80%). The functional measurement outcome was a hop test.

The players that had ACL reconstruction and high quadriceps index performed similarly in the hop test compared to uninjured controls. However, the players that had ACL reconstruction but had low quadriceps index (asymmetry), performed significantly worse in the hop test compared to uninjured controls. The result of this study brings evidence to what is intuitively understood that strength and functional outcome measures may not be two independent variables –but one can affect another. It is important that in this study, causality was not established between the two variables, correlation is found but strength does not necessarily lead to function and

function does not lead to strength.

A similar study by Clagg, Paterno, Hewett, and Schmitt (2015) was completed with modified Star Excursion Balance Test (mSEBT) and isokinetic dynamometry. Sixty-six participants who have had a unilateral ACL reconstruction and 47 uninjured participants (controls) were tested. The participants with ACL reconstruction were tested at the time of return to sport. mSEBT included the anterior, posterolateral and posteromedial reaches and isokinetic dynamometry was used to quantify strength. The participants with ACL reconstruction had significantly lower anterior reach distance in the mSEBT on the involved and uninvolved limbs compared to the uninjured participants (controls). Furthermore, they found an association between the lower mSEBT anterior reach distance and lower extremity muscle strength measured in the isokinetic dynamometry. This demonstrates once again that strength and function are not two independent variables but may be associated with each other.

Can a treatment or rehabilitation protocols affect both?

A study by Taradaj, et al. (2013) investigated the use of neuromuscular electrical stimulation (NMES) for soccer athletes post-ACL reconstruction. Eighty soccer athletes post-ACL reconstruction were involved in this study, who were then divided into NMES (n=40) and control (n=40) groups. Both the control and NMES group underwent the same treatment rehabilitation protocols, including functional and progressive resisted exercises. The NMES group had NMES applied three days a week for a month. Both the control and NMES groups increased quadriceps muscle strength measured by tensometry and quadriceps circumference. However, the NMES group had increased significantly more in quadriceps strength and circumference compared to the control group. This study showed that NMES was an effective treatment to improve strength gain.

Another study by Paillard, Noe, Bernard, Dupui, and Hazard (2008) also investigated the effects of NMES with a vertical jump test –a functional measure. The study included 27 healthy trained students. They were divided into three groups: controls, NMES for strength and NMES for endurance (different parameters). The participants performed vertical jump tests at the beginning of the study, at one week and at five weeks at the end of the study after completing their NMES programme or control. The groups that received NMES for strength or endurance significantly increased their vertical jump height in the test, showing NMES was effective in improving a functional measure.

These two studies showed that NMES was an effective treatment that improves both strength and function. This is consistent with the first point of the discussion: strength and function may not be two independent variables, and physiotherapy may be able to improve both at the same time. In the scope of this essay, I will not delve into different treatments that would affect both or one but not the other –I will focus on strength vs. functional goals/tests.

A point to note is that in Paillard, Noe, Bernard, Dupui, and Hazard’s 2008 study, they did not have soccer players or athletes as participants, and they were healthy participants. That may mean that their results may not be directly applicable to soccer players post-ACL reconstruction. However, these two studies still point to the finding that a single treatment may have effects on strength and on function.

Good function and poor strength

It is generally accepted that if the soccer players are not ready to return to sport if they have poor strength and function –(Arundale, Silvers-Granelli, & SnyderMackler, 2018). The fact that strength and function are not necessarily two independent variables as demonstrated above are also convenient in making return-to-sport protocol and tests. However, what happens when the function test does not reflect the strength test?

Herrington, Ghulam, and Comfort (2018) demonstrated this phenomenon exactly in their study. Fifteen full-time professional soccer players who have undergone ACL reconstruction were involved in this study. Eccentric, concentric and isometric peak torque strength were measured and compared to their distances in a single hop for distance and cross-over hop for distance –normalised to their leg length. The injured and uninjured leg were compared for symmetry with the goal of ≥90% for both strength and hop distance. Herrington, Ghulam, and Comfort (2018) found that more than 80% of the players did not achieve ≥90% symmetry for strength, while 67-73% achieved ≥90% symmetry for the hop tests. This showed that majority of participants had good functional outcomes, but poor strength measures.

Mayer, et al. (2015) found similar results with a different set of measurements. Clinical impairments, including isokinetic quadriceps strength, were measured and 98 patients post-ACL reconstruction were group in either cleared or not cleared to return to sport by the orthopaedic surgeon performing the clinical impairment measurements. A blinded tester then performs the Functional Movement Screen (FMS) and the Y Balance Test for Lower Quarter (YBT-LQ) as the functional measures. The results showed that the cleared and not cleared groups did not show a statistical difference in the YBT-LQ scores (both independent and composite scores). Similarly, the cleared and not cleared groups did not show a statistical difference in the FMS composite scores. This suggests similar results to Herrington, et al. (2018) that people post-ACL reconstruction can “pass” their functional measures, but “fail” the strength measures. A criticism for this study however, FMS is composed of seven separate tests and their composite score will not be as valid as the individual scores on the seven tests which can each be a “pass” or “fail” on their own.

This brings us to the question then: how do these people and athletes perform well functionally, but have poor strength? The answer may lie in the original definition of strength. Strength was measured in an isolated single joint performance, while function usually involves multiple joints working together. This may mean contribution from other joints and muscle groups may hide strength deficits that are present in isolation. Hence, functional tests are not sensitive enough to detect isolated strength deficits which was also described by Thomeé, et al. (2011) in their review –asking for more sensitive tests.

Most of the evidence investigating functional measures usually also include one or two functional measures (e.g. single hop distance, mSEBT, crossover hop distance), while this is easily reproducible and consistent, real-life soccer players do not just hop or reach on the spot. Soccer players perform wide and diverse functional movements with their movements (e.g. running, change in direction, kicking), which should be clinically checked before returning to sports. The other functional movements may then show a strength deficit that was not detected from a hop or reach test alone.

Good strength and poor function?

Surprisingly at the time of this writing, there are no evidence and examples in the literature of any athlete having good strength but poor function, especially soccer athletes post-ACL reconstruction. However, there is no evidence of their absence either. It would not be too surprising for anecdotal evidence and clinical evidence of strong and healthy individuals or athletes being unable to perform simple functional tasks such as single leg stance. Similarly, there would be healthy individuals who are unable to perform functional tests such as the FMS well. Which would give a clear example of good strength but poor function.

There is a prospective study by Paterno, et al. (2010) which investigated landing and postural stability function using a 3-D motion analysis and Biodex SD Stability System. A drop vertical jump manoeuvre and postural stability assessment was analysed on 56 athletes who have had an ACL reconstruction and followed for occurrence of a second ACL injury. Valgus movement, asymmetry in internal knee extensor moment and a deficit in single-leg postural stability of the involved limb were specific predictive parameters. On top of that, hip rotation moment independently predicted second ACL injury (C = 0.81) with high sensitivity (0.77) and specificity (0.81). The authors concluded that altered neuromuscular control of the hip and knee during the functional tasks are predictors of a second ACL injury. The study by Paterno, et al. (2010) demonstrated the reason why clinicians perform functional tests in the first place: neuromuscular control. Neuromuscular control is the ability to use the separate isolated parts (quadriceps and glute muscles) to perform together in a functional task. While there is no evidence of athletes having good strength but poor function or neuromuscular control post-ACL reconstruction, Paterno, et al. (2010) showed the importance of neuromuscular control to predict a second ACL injury. There may be cases where soccer players who have had an ACL reconstruction have good strength but poor function due to poor neuromuscular control, unable to utilise separate isolated muscular strength together to perform a functional movement. The equipment used by Patreno, et al. (2010) would not be available in most clinical settings, but the principle in testing function to detect neuromuscular control deficit which may predict a second ACL injury is still applicable.

Conclusions: Strength and function

It has been established that strength and function may not be separate independent variables to test or achieve, but one may affect another. Some treatments such as NMES may also result in improved both strength and function, though this may not be the case for all treatments and rehabilitation protocols. However, in the context of return-to-sport tests and criteria, these two are not independent variables.

Despite strength and function being associated, should we test one, or both? It appears that functional tests are not sufficiently sensitive to detect isolated strength deficits, possibly because other joints and/or muscles can come in to compensate for the strength deficit. However, on a similar note, neuromuscular control seems to be a strong predictor of a second ACL injury in athletes which cannot be ignored. There is an absence of evidence showing that athletes can have good function but poor strength, hence appropriate functional tests should still be performed to detect neuromuscular control deficits. In conclusion, until evidence proves otherwise, strength and function should both be tested and soccer athletes who have

had an ACL reconstruction need to be able to pass both tests to return-to-soccer. and isokinetic dynamometry was used to quantify strength. The participants with ACL reconstruction had significantly lower anterior reach distance in the mSEBT on the involved and uninvolved limbs compared to the uninjured participants (controls). Furthermore, they found an association between the lower mSEBT anterior reach distance and lower extremity muscle strength measured in the isokinetic dynamometry. This demonstrates once again that strength and function are not two independent variables but may be associated with each other.

Can a treatment or rehabilitation protocols affect both?

A study by Taradaj, et al. (2013) investigated the use of neuromuscular electrical stimulation (NMES) for soccer athletes post-ACL reconstruction. Eighty soccer athletes post-ACL reconstruction were involved in this study, who were then divided into NMES (n=40) and control (n=40) groups. Both the control and NMES group underwent the same treatment rehabilitation protocols, including functional and progressive resisted exercises. The NMES group had NMES applied three days a week for a month. Both the control and NMES groups increased quadriceps muscle strength measured by tensometry and quadriceps circumference. However, the NMES group had increased significantly more in quadriceps strength and circumference compared to the control group. This study showed that NMES was an effective treatment to improve strength gain. Another study by Paillard, Noe, Bernard, Dupui, and Hazard (2008) also investigated the effects of NMES with a vertical jump test –a functional measure. The study included 27 healthy trained students. They were divided into three groups: controls, NMES for strength and NMES for endurance (different parameters). The participants performed vertical jump tests at the beginning of the study, at one week and at five weeks at the end of the study after completing their NMES programme or control. The groups that received NMES for strength or endurance significantly increased their vertical jump height in the test, showing NMES was effective in improving a functional measure.

These two studies showed that NMES was an effective treatment that improves both strength and function. This is consistent with the first point of the discussion: strength and function may not be two independent variables, and physiotherapy may be able to improve both at the same time. In the scope of this essay, I will not delve into different treatments that would affect both or one but not the other –I will focus on strength vs. functional goals/ tests.

A point to note is that in Paillard, Noe, Bernard, Dupui, and Hazard’s 2008 study, they did not have soccer players or athletes as participants, and they were healthy participants. That may mean that their results may not be directly applicable to soccer players post-ACL reconstruction. However, these two studies still point to the finding that a single treatment may have effects on strength and on function.

Good function and poor strength

It is generally accepted that if the soccer players are not ready to return to sport if they have poor strength and function –(Arundale, Silvers-Granelli, & Snyder-Mackler, 2018). The fact that strength and function are not necessarily two independent variables as demonstrated above are also convenient in making return-to-sport protocol and tests. However, what happens when the function test does not reflect the strength test?

Herrington, Ghulam, and Comfort (2018) demonstrated this phenomenon exactly in their study. Fifteen full-time professional soccer players who have undergone ACL reconstruction were involved in this study. Eccentric, concentric and isometric peak torque strength were measured and compared to their distances in a single hop for distance and cross-over hop for distance –normalised to their leg length. The injured and uninjured leg were compared for symmetry with the goal of ≥90% for both strength and hop distance. Herrington, Ghulam, and Comfort (2018) found that more than 80% of the players did not achieve ≥90% symmetry for strength, while 67-73% achieved ≥90% symmetry for the hop tests. This showed that majority of participants had good functional outcomes, but poor strength measures.

Mayer, et al. (2015) found similar results with a different set of measurements. Clinical impairments, including isokinetic quadriceps strength, were measured and 98 patients post-ACL reconstruction were group in either cleared or not cleared to return to sport by the orthopaedic surgeon performing the clinical impairment measurements. A blinded tester then performs the Functional Movement Screen (FMS) and the Y Balance Test for Lower Quarter (YBT-LQ) as the functional measures. The results showed that the cleared and not cleared groups did not show a statistical difference in the YBT-LQ scores (both independent and composite scores). Similarly, the cleared and not cleared groups did not show a statistical difference in the FMS composite scores. This suggests similar results to Herrington, et al. (2018) that people post-ACL reconstruction can “pass” their functional measures, but “fail” the strength measures. A criticism for this study however, FMS is composed of seven separate tests and their composite score will not be as valid as the individual scores on the seven tests which can each be a “pass” or “fail” on their own.

This brings us to the question then: how do these people and athletes perform well functionally, but have poor strength? The answer may lie in the original definition of strength. Strength was measured in an isolated single joint performance, while function usually involves multiple joints working together. This may mean contribution from other joints and muscle groups may hide strength deficits that are present in isolation. Hence, functional tests are not sensitive enough to detect isolated strength deficits which was also described by Thomeé, et al. (2011) in their review –asking for more sensitive tests.

Most of the evidence investigating functional measures usually also include one or two functional measures (e.g. single hop distance, mSEBT, crossover hop distance), while this is easily reproducible and consistent, real-life soccer players do not just hop or reach on the spot. Soccer players perform wide and diverse functional movements with their movements (e.g. running, change in direction, kicking), which should be clinically checked before returning to sports. The other functional movements may then show a strength deficit that was not detected from a hop or reach test alone.

Good strength and poor function?

Surprisingly at the time of this writing, there are no evidence and examples in the literature of any athlete having good strength but poor function, especially soccer athletes post-ACL reconstruction. However, there is no evidence of their absence either. It would not be too surprising for anecdotal evidence and clinical evidence of strong and healthy individuals or athletes being unable to perform simple functional tasks such as single leg stance. Similarly, there would be healthy individuals

who are unable to perform functional tests such as the FMS well. Which would give a clear example of good strength but poor function.

There is a prospective study by Paterno, et al. (2010) which investigated landing and postural stability function using a 3-D motion analysis and Biodex SD Stability System. A drop vertical jump manoeuvre and postural stability assessment was analysed on 56 athletes who have had an ACL reconstruction and followed for occurrence of a second ACL injury. Valgus movement, asymmetry in internal knee extensor moment and a deficit in single-leg postural stability of the involved limb were specific predictive parameters. On top of that, hip rotation moment independently predicted second ACL injury (C = 0.81) with high sensitivity (0.77) and specificity (0.81). The authors concluded that altered neuromuscular control of the hip and knee during the functional tasks are predictors of a second ACL injury.

The study by Paterno, et al. (2010) demonstrated the reason why clinicians perform functional tests in the first place: neuromuscular control. Neuromuscular control is the ability to use the separate isolated parts (quadriceps and glute muscles) to perform together in a functional task. While there is no evidence of athletes having good strength but poor function or neuromuscular control post-ACL reconstruction, Paterno, et al. (2010) showed the importance of neuromuscular control to predict a second ACL injury. There may be cases where soccer players who have had an ACL reconstruction have good strength but poor function due to poor neuromuscular control, unable to utilise separate isolated muscular strength together to perform a functional movement. The equipment used by Patreno, et al. (2010) would not be available in most clinical settings, but the principle in testing function to detect neuromuscular control deficit which may predict a second ACL injury is still applicable.

Conclusions: Strength and function

It has been established that strength and function may not be separate independent variables to test or achieve, but one may affect another. Some treatments such as NMES may also result in improved both strength and function, though this may not be the case for all treatments and rehabilitation protocols. However, in the context of return-to-sport tests and criteria, these two are not independent variables.

Despite strength and function being associated, should we test one, or both? It appears that functional tests are not sufficiently sensitive to detect isolated strength deficits, possibly because other joints and/or muscles can come in to compensate for the strength deficit. However, on a similar note, neuromuscular control seems to be a strong predictor of a second ACL injury in athletes which cannot be ignored. There is an absence of evidence showing that athletes can have good function but poor strength, hence appropriate functional tests should still be performed to detect neuromuscular control deficits. In conclusion, until evidence proves otherwise, strength and function should both be tested and soccer athletes who have had an ACL reconstruction need to be able to pass both tests to return-to-soccer.