Anterior Cruciate Ligament Deficiency: The Need for Muscle Strengthening

The knee joint’s location in the middle of the lower limb kinetic chain is imposed to high loads, which reach multiple the body mass, particularly in the single stance phase of sport activities.^1,2 Rupture of the anterior cruciate ligament (ACL) destabilizes the knee joint,^3–5 thus making ACL deficient knees prone to repeated subluxations, which form a potential cause for secondary damage to the joint.^6–8 Subsequently, dynamic stabilization through the quadriceps and hamstrings becomes very crucial for the protection of the injured knee.^9,10 Apart from its mechanical role, the ACL functions as a sensory organ due to the mechanoreceptors within its substance.¹¹ After its rupture, this function is lost, and therefore optimization of the lower limb muscle properties becomes increasingly important in order to compensate for the resulting anterior and rotational knee instability.

Exercise in ACL deficient patients aims at the improvement of various aspects of muscle properties including reflexes, strength, endurance, and coordination with other muscles. Functional exercise that reeducates the neuromuscular coordination holds the central role in rehabilitation programs, as growing evidence supports the development of preprogrammed compensatory muscle activation strategies for efficient shear force dissipation during injured knee loading.^12–16 However, the fundamental issue of the need for strength testing and the value of strengthening exercises still leaves room for investigation.

Among the criteria for progression of ACL rehabilitation is the level of quadriceps and hamstring weakness.^17,18 Strength testing and exercise have been traditionally incorporated into musculoskeletal rehabilitation regimens. Although the connection between the level of quadriceps strength and functional status has been disputed,^19–22 some studies support the interrelation between functional performance of the knee and thigh muscle strength. It is of clinical importance for ACL rehabilitation that patients with greater than normal strength in the injured limb seem to reduce abnormalities during low- and high-stress activities.²³ Quadriceps strength appears to determine the functional ability of the ACL deficient or operated limb to a great degree.^24,25 Its weakness coincides with low functional performance²⁶ and pathological gait pattern.²⁷ In addition, functional improvement in ACL deficient athletes after training followed the same pattern as the strength of both the quadriceps and hamstrings.^23,28

Likewise, increase in hamstring strength after functional exercise incorporating strengthening, stretching, and plyometric drills paralleled a decrease in peak landing forces, and hence safer landing.²⁹ Hamstring strength has also been associated with the level of knee function^10,30 and performance,³¹ and increasing the hamstring–quadriceps (H:Q) strength ratio has be come a rule in order to promote dynamic control of the ACL deficient knee.^32,33 Even more, this improvement has been connected with the return to physical activity after ACL injury,³⁴ and the strength of both thigh muscle groups reflects the functional improvement^23,28 and the ability to return to physical activity.³⁴

It appears that changes in muscle strength might be a global reflection of muscle properties, including neural changes.³⁵ It seems that adequate strength ensures that a solid basis is built for other refined neuromuscular properties. In other words, adequate strength secures the proper background for the development of global muscle properties. Therefore, it appears that objective evaluation of strength has a valuable position in the functional assessment after ACL injury, and in combination with our findings, it could be suggested that therapeutic intervention should minimize strength weakness, which persists over time when not addressed.

Importance of the Hamstrings, Especially in Soccer Players: Our Research

Our group recently investigated the connection of thigh muscle strength with the level of knee performance and the chronic stage of the injury in ACL deficient athletes. In order to reveal the net effect of ACL rupture on muscle strength, we examined amateur soccer players who abstained from structured rehabilitation in an attempt to exclude interference of exercise with the results.

The first study focused on revealing a possible connection of quadriceps and hamstring strength deficits with the level of knee function determined by Lysholm score.³⁰ Three groups of ACL deficient amateur soccer players were examined at different levels of knee function and were compared with a group of controls matched for the preinjury level of activity. The median Lysholm scores of the low, intermediate, and high knee functioning groups were 64.5, 76, and 86 points, respectively. Weakness depicted by the contrast to the healthy condition was significant in all cases and ranged from 19% to 35% according to the muscle or the patient group. Regarding the side-to-side deficit, these major muscle groups did not follow the same pattern. The strength asymmetry of the quadriceps was consistently significant even in the high functioning knees, being greater than 14%, in contrast to the hamstrings, which revealed acceptable symmetry within the normal levels (about 2% to less than 6%) at the high and intermediate knee function groups. Only the poorly functioning athletes had a significant 19% deficit (Fig. 7-1), which places hamstring strength asymmetry (H asymmetry) as a discriminating factor for knee functionality.

Fig. 7-1 The percentage of extensor and flexor deficit in each experimental group formed according to the level of knee function (L1, L2, and L3: high, intermediate, and low Lysholm score, respectively) and the percentage of asymmetry in the control group (dominant versus nondominant knee).

The importance of assessing H asymmetry is highlighted in our recent study that examined different groups of amateur athletes involved in cutting and twisting sports such as soccer, basketball, and handball at different times since ACL rupture.³⁶ We tested the quadriceps and hamstring strength of 36 patients with unilateral ACL deficiency who were divided into three equal groups with mean times for chronicity of about 4, 11, and 57 months for short term, intermediate term, and long term, respectively. We investigated how the strength weakness evolved with time, using the strength of matched healthy controls as a baseline score. Additionally, we questioned whether the quadriceps’ and hamstrings’ side-to-side asymmetry in strength would be consistently significant in all stages of chronicity.

As in the previous study, significant weakness was evident in both muscles in all patient groups, ranging from 21% to 32%. Considering the side-to-side asymmetry of ACL deficient knees, the quadriceps deficit persisted through time, whereas the hamstrings regained symmetry even after 1 year without organized rehabilitation. Regarding the side-to-side strength differences, they tended to lower with time, but in the case of quadriceps, they varied from 10% to 23%, whereas the hamstrings were significantly asymmetric only in the short-term group (14%) and acquired acceptable symmetry within 1 year postinjury (Fig. 7-2).

Fig. 7-2 The percentage of extensor and flexor deficit in each experimental group (injured versus intact knee: short term, intermediate, long term) and the percentage of asymmetry in the control group (dominant versus nondominant knee).

Both studies show a trend for hamstring symmetry much more emphatically than the quadriceps as function improved or as the distance from the incidence of rupture increased. The strength asymmetry evident only in the worst-functioning group and the short-term group might reveal a natural compensatory reaction for ACL deficiency because no patients followed a structured rehabilitation program. A supplementary finding of both studies was that the strength of the healthy side was considerably affected by the disuse, which was depicted on the mean reduction to the level of performance by a mean of 3 to 3.5 degrees Tegner. This raises the issues of ensuring not to neglect the intact side as well and counseling patients to maintain activity with safe exercise after injury.

The quadriceps muscle is affected to a greater degree after ACL injury possibly because of (1) postinjury neural inhibition due to the loss of afferent feedback from ACL to gamma motor neurons^37,38 and (2) the adaptation toward a “quadriceps avoidance gait” pattern^39,40 to prevent anterior subluxation,^41,42 which unloads the limb, promoting quadriceps weakness in ACL deficient patients.¹² The greater atrophy of the quadriceps (10% versus 4%) reported even 1 year postinjury⁷ may also add to the explanation of their higher deficit compared with the hamstrings. In contrast, evidence exists that the hamstrings are recruited in weight-bearing activities in a subconscious attempt to counteract anterior shear forces.^5,43 This stimulus might have assisted with the improvements in our patients. Evidence in the literature also supports the development of subtle electrophysiological modifications in ACL deficient patients that retune the hamstrings and preprogram their muscle activation strategies to optimize shear force dissipation during injured knee loading.^14–16

In another study, we investigated the quality of muscle contraction when ACL deficient patients performed maximal exercise via the smoothness of the torque curve throughout knee extension and flexion.⁴⁴ Our methodology comprised transformation of each torque-time curve pattern into the frequency domain (power spectrum) via fast Fourier transform in order to quantify the smoothness of the isokinetic curve (Fig. 7-3). Each curve of biological signal that is not a perfect sine is actually the sum of other curves, and therefore it can be analyzed into its fundamental components. Our biological interpretation of this method is based on the notion that disturbed motion is generally connected to poor level of joint functionality. Irregular torque output has been connected to other pathologies such as anterior knee pain.^45,46 In contrast, smoothness of torque generation is indicative of enhanced force control.⁴⁷ The frequency contained at three levels of the total power of the signal (90%, 95%, and 99%) was calculated in order to exclude noise from the 100% power level but still include enough harmonics. Both extension and flexion isokinetic curves demonstrated increased irregularities as expressed by the higher-frequency contents by 18.8%, 10.6%, and 40.0% for knee extension and 49.5%, 24.5%, and 16.3% for knee flexion, according to the power level of assessment (Fig. 7-4). Although the results regarding quadriceps were expected on the basis of previous reports using different methodologies, the hamstrings’ increased irregularity had not been reported elsewhere. This finding might be of functional importance and open a future area of investigation.

Fig. 7-3 A characteristic extensor (A) and flexor (B) isokinetic curve of the intact knee (blue line) and the anterior cruciate ligament (ACL) deficient knee (red line), demonstrating the side-to-side difference to the torque-time curve smoothness.

Fig. 7-4 Example of transformation of the isokinetic data of knee extension from the time domain (A) to frequency domain (B). The arrow shows the frequency content calculated for 99% of the signal power.

The higher oscillations characterizing the isokinetic curve of the ACL deficient knee, which is expressed in increased frequency contents, may be attributed to mechanical and/or neuromuscular factors. Increased anterior gliding of the tibia during knee extension might account for the mechanical part. Quadriceps inhibition^37,38 and poorly coordinated activation within the hamstrings^43,48 must explain the neural aspects of abnormalities of mechanical output. This loss of smoothness in extension-flexion might be clinically important and should be investigated further. Quantification of irregularity of the extension-flexion curve is an innovative approach and could be a valuable tool in the assessment of ACL deficient knees.

Review of the Literature on the Role of the Quadriceps and Hamstrings in Anterior cruciate ligament Deficient Knees

The quadriceps is the muscle group suffering the most dramatic effects after ACL tear.^19,30,36,49 For this reason, in addition to its functional importance for normal gait, it attracts most of the attention from clinicians and researchers. Quadriceps torque deficit is more than double hamstring deficit, which is attributed to its susceptibility for quick atrophy due to disuse^10,18 and neural inhibition.^37,38 Marked weakness of the quadriceps prevents the knee from functioning normally, and given that this weakness is exaggerated in many cases,³⁰ it should be managed adequately. If the voluntary deficit measured via superimposed electrical burst to the maximal voluntary contraction exceeds 5%, treatment with electrical stimulation effectively ameliorates loss of the quadriceps strength and should be implemented from the early stages.⁵⁰ Although in ACL deficient knees there is no graft to be stressed due to the anterior instability of the tibia caused by quadriceps contraction particularly near extension,^9,42,51 this might be harmful for other capsuloligamentous structures.

In contrast, the hamstrings are properly located to counteract anterior tibial instability at flexion angles exceeding 30 degrees.^52–54 However, doubts exist regarding the efficacy of the hamstrings to counterbalance shear loading of the knee,^53,55 based on two concerns: first, whether the magnitude of the posteriorly directed muscle force is enough to counteract shear forces in the functionally more important knee angles near extension,^41,54,56 and second, whether reflex activation of the hamstrings during abrupt perturbations of the knee is fast enough to develop tension in time with the peak external destabilizing moment.^57,58

Considering the development of the properly directed stabilizing force, studies on cadavers,^55,59 animals,^54,60 and mathematical models^53,61,62 support that beyond 30 degrees of knee flexion, the posteriorly directed vector of hamstring force becomes adequate in stabilizing the ACL deficient knee (Fig. 7-5). In addition, it should not be underestimated that even when the line of pool of the hamstrings is inefficient, co-contraction could increase joint stability due to joint compression⁶³ and widening of the pressure distribution along the articular surfaces of the knee.⁶⁴ Additionally, it has been reported that the hamstrings cause greater stiffness to the ACL deficient knee than they do to the intact knee.⁶⁵

Fig. 7-5 As the knee flexes from A to B, the anti-shear vector of the hamstring force (solid line) increases.

Regarding the question of the timely activation of the hamstrings, an overfocus on their reflex latency of 40 to 50 ms, which is a medium latency response,^66–68 may be misleading in regard to their efficacy to prevent instability. Growing evidence in the literature supports the development of preprogrammed compensatory muscle activation strategies.^12–16 These strategies suggest that subtle electrophysiological modifications of the subjects are implemented by deficient patients after ACL injury to optimize shear force dissipation during injured knee loading. Hence, feed-forward mechanisms can be adopted that initiate hamstring co-contraction during the expectation of knee loading, not only as a reflex response.^13,16,69,70 Therefore the hamstrings should be well conditioned in order to have a greater potential to enhance knee stability.

Bracing in anterior cruciate ligament Deficient Patients: Is It Effective?

The use of functional knee braces is a common practice for enhancing knee stability after rupture of the ACL or reconstruction, with contradictory opinions about their importance in knee unloading.^71–73 A favorable change of firing pattern for the hamstrings was observed more often when ACL deficient patients performed single-leg landings wearing a brace⁶⁹ and in skiers during periods of increased knee flexion.⁷⁴ The greater biceps femoris activity was exhibited by the more unstable knees. Lam et al⁷⁵ found that wearing a functional brace improved hamstring reflex responses in ACL deficient knees after fatigue induced by repeated extension and flexion against spring resistance. Although their protocol did not replicate a functional weight-bearing condition, it gives a potentially useful message that bracing in ACL deficient knees may enhance protection. Wojtys et al⁷⁶ showed that braces can decrease anterior tibial translation by a large margin.

In contrast, other findings have shown a slowing of hamstring muscle reaction times with bracing⁷⁶ or decreased activation.^71,77

Research into the utility of functional knee bracing for ACL protection is inconclusive due to a great heterogeneity in the electromyography applied, experimental maneuvers, and characteristics of the participants. Moreover, whether the influence of braces on muscle function and proprioception is favorable for knee stability remains unclear.

However, patients with torn ACLs who do not cope well with their injury and do not choose surgery might benefit from bracing.⁷⁸

Rehabilitation

Rehabilitation after ACL rupture is a multifactorial issue. However, the general goals consist of gaining good functional stability, optimizing the functional level, and minimizing the risk for reinjury.⁷⁹ Interventions follow a general scheme depending on chronicity and are generally grouped into the acute phase focusing on range of motion, pain management, regaining ambulation, and retarding atrophy; the advanced phase, which aims to increase strength and endurance; and the return-to-play phase, with the final neuromuscular optimization of knee function.⁸⁰ Progression from one stage to the other follows certain criteria, one being the achievement of adequate strength for the demands of each stage.

Rehabilitation should aim at limiting thigh muscle weakness after ACL rupture, reducing the quadriceps side-to-side deficit to clinically acceptable levels, and assisting the hamstrings to regain strength faster. These benefits could enhance the natural reaction of the body against anterior knee instability. The effect of organized rehabilitation protocols on the time course of strength adaptations after ACL injury has yet to be examined. A promising research area is the investigation of whether early and intensive postinjury strengthening of the hamstrings will allow more ACL deficient patients to cope with their injury. Nevertheless, only a small fraction of ACL deficient athletes who choose not to have a reconstruction return to their previous level of exercise, and those who do so progress through rehabilitation through specific training and by achieving certain criteria.⁸¹ The majority follow a more conservative lifestyle attributed either to knee problems⁷⁹ or to social reasons and fear of reinjury.⁸²

Excessive atrophy and weakness of the quadriceps are evident in ACL deficient patients and, in cases of quadriceps inhibition, might not be reversible without electrical stimulation, which should commence from the early stages.³⁸ Quadriceps activity potentially leads to knee instability, but it can be exercised safely either through a closed kinetic chain or in angles flexed more than 45 degrees. This deficit should not be neglected because it persists through time.³⁶

Hamstring strength has been connected with the functional outcome of the ACL deficient³⁰ and ACL reconstructed knee10 or the possibility of engaging in higher levels of sports participation.³² Strengthening of the hamstrings holds a key position in the conservative treatment of the ACL deficient knee,^32,83 and it appears that strength training of a specific muscle also increases its coactivation level.⁸⁴ A 3-month, high-resistance training program with four sets of 8 RM (repetitions maximum: load that permits the completion of only eight repetitions) performed 3 times per week may increase the capacity of the hamstrings to provide stability to the knee joint during fast extension.⁸⁵ This improvement was expressed through a significantly elevated H eccentric:Q concentric ratio.

As a rule, ACL rupture leads to a decrease in the level of physical activity, especially in those who were involved in cutting and twisting sports.^86,87 ACL deficient athletes appear to reduce physical activity to 4 degrees Tegner from an initial 9 and 10 degrees.⁸⁸ Characteristically, Lysholm et al³ reported that although the activity of 62% of their sample was higher than 7 degrees Tegner prior to ACL rupture, after the injury only 10% remained above this limit. Reduction of physical activity markedly affects the healthy side as well,^30,36 complicating the return to high-demand activities. To counteract that, closed kinetic chain exercises with great hip flexion angles are an evidence-based method to re-educate coordination and physiological properties with safety, along with open chain single-joint exercise for more specific effects.

Hip control is crucial for the control of knee rotation and proper lower limb alignment, but it is important to remember that the knee is not only directly interrelated with the hip joint but is also connected with the ankle joint. Rudolph et al⁸⁹ found that coping patients partly relieved the ACL deficient knee by demonstrating a higher contribution of the ankle extensors to the total moment production of the lower limb. Likewise, hamstring tasks to control knee instability can be reinforced by synergists including the anterior tibialis, soleus,^90,91 and muscles of the deep posterior compartment muscle group.⁹⁰ Those synergists have the potential to control the anterior translation and internal rotation of the tibia. This occurs via their reverse action on the ankle and the subtalar joints during closed kinetic chain activities, when the foot is planted on the ground and subsequently the peripheral movement is restricted. Concerning the gastrocnemius, its origin above the knee joint complicates the effect of its action. Fleming et al ⁹² implanted the more precise and accurate differential variable reluctance transducer (DVRT)^93,94 on the anteromedial bundle of the ACL and showed that isometric gastrocnemius muscle contraction strained the ACL within the last 15 degrees of knee extension. Additionally, gastrocnemius muscle contraction combined with quadriceps or hamstring muscle contraction increased the strain up to the last 30 degrees of flexion in comparison with isolated contractions of these muscles. This study ensured muscle specificity in contraction by applying transcutaneous electrical muscle stimulation in subjects under spinal anesthesia. However, there are studies indirectly showing that gastrocnemius contraction could stabilize the tibia under certain circumstances. O’Connor,⁹⁵ using a two-dimensional mathematical model, found that simultaneous contraction of the quadriceps, hamstrings, and gastrocnemius totally unloaded the ACL at 22 degrees of flexion. Kvist and Gullquist⁹⁶ showed that when the gastrocnemius is coactivated with the quadriceps in a closed chain exercise (squatting) with the center of gravity over or posterior to the foot, joint compression is increased but anterior tibial translation is not. They also suggest that ACL deficient patients should enhance the spontaneous coactivation of quadriceps with certain neuromuscular training. However, Fleming et al⁹⁷ questioned the safety of closed kinetic exercises, leaving the area still open for investigation.

A safe compromise among those findings could be that isolated contractions of the gastrocnemius should be avoided after ACL rupture and their coactivation with other muscles in open chain exercise can be avoided by performing hamstring curls with the foot plantar flexed. Protective exercise for the ACL should be global and include exercises for the joints adjacent to the knee, and strengthening of those muscles could create a background for improvement of the overall muscle performance. Strengthening should combine open chain exercises for all lower limb joints, executed in a safe manner according to the current research, as well as closed chain activities. Generally, neuromuscular training with closed chain exercises appears beneficial when performed with caution and progressively, as it provides more sophisticated muscle interrelations that enhance safety but are still difficult to define.

Summary

ACL rupture deprives the knee of a major stabilizing structure, and therefore the role of the surrounding musculature becomes crucial for dynamic protection of the joint. The quadriceps suffer the greatest deficits in performance, which affects the normal lower limb kinematics and should be treated, even with the use of electrical stimulation if necessary. The hamstrings tend to recover earlier; this might be attributed to a natural reaction to promote stability. This trend should be facilitated and reinforced because it appears that neurophysiological adaptations include reprogramming of muscle activation, during which hamstrings are activated earlier in anticipation of perturbations. Even in knee angles in which the magnitude of force is small and inefficient to counterbalance anterior tibial subluxation, hamstrings contraction still has a stabilizing potential for the knee through joint compression. Adequate strength levels provide the basis for further functional improvement during rehabilitation of ACL deficient knees. Organized and specific exercises should incorporate muscle strengthening, functional plyometric drills, and joint perturbations advancing in steps under certain criteria. The use of knee braces, although their degree of ACL deficient knee protection is still unclear, might help in poorly functioning patients.