INTRODUCTION

Rehabilitation after knee injuries, especially anterior cruciate ligament (ACL) ruptures, has significantly changed since the mid 1990s. In the past, rehabilitation focused on protection with a period of immobilization, guarded range of motion (ROM), strengthening exercises, and a delay in return to function. The return to functional activities and sports after ACL reconstruction is now more rapid, partly owing to the shift in rehabilitation philosophy and improved surgical techniques. Modern therapy focuses on immediate motion^41,42 and weight-bearing, functional exercises, and proprioception and neuromuscular control,^35,55 thus allowing an earlier regain of functional dynamic stability and return to functional activities.

One of the most significant changes in rehabilitation after ACL injury is the greater emphasis on neuromuscular training. In this chapter, proprioception and neuromuscular control are defined, the implications after ACL injury and surgery are discussed, and techniques to restore neuromuscular control and functional dynamic stability after ACL injury and/or surgery are described.

TERMINOLOGY

Although often used interchangeably, proprioception, kinesthesia, and neuromuscular control have different definitions. Proprioception is the sense of awareness of the joint position, whereas kinesthesia is the sensation of joint movement.³⁵ Wilk⁵⁴ defined neuromuscular control as the afferent sensory recognition of joint position and the efferent response to that of awareness. Furthermore, neuromuscular control provides the functional component referred to as dynamic stabilization.

Proprioception is achieved by a sensory pathway response that is first triggered by mechanoreceptors found in the synovial joints of the body. These mechanoreceptors include pacinian corpuscles, Ruffini endings, the muscle spindle, and Golgi tendon organs (GTO). These receptors are found in muscles, capsules, and ligaments. All of these components act together in a healthy joint to transmit sensory information concerning joint position, movement, and strain via afferent pathways to the central nervous system (CNS). The CNS in response sends electrical signals through efferent pathways to corresponding muscles surrounding the joint to alter muscle joint tone and function.

The joint and muscle receptors play a significant role in the mediation of extrinsic muscle stiffness³² and thus contribute to dynamic joint stability. The receptors in muscles contribute to muscle stiffness through two mechanisms: (1) force feedback, which is modulated by the GTO,²⁹ and (2) length feedback, which is mediated by the muscle spindle pathway and facilitates motor activity.^37,38

Two other possible mechanisms have been proposed for the joint receptors’ contribution to joint stability. The first mechanism is a direct ligamentous-muscular protective reflex. An example of this is the reflex between the ACL and the hamstring muscles. When tension is generated onto the ACL, the mechanoreceptors are stimulated, causing a reflex inhibition of the quadriceps and a facilitation of the hamstrings. This protects against increasing strain on the ACL. The second mechanism is the joint receptors’ indirect contribution to dynamic joint stability. In this mechanism, the joint receptors contribute to preparatory adjustments of muscles stiffness and dynamic joint stability. This is referred to as presetting.³¹ The authors believe this concept of presetting is critical in providing joint stability during functional activities such as preparation for deceleration or cutting.

Motor responses depend on the level of processing of afferent inputs within the CNS. The processing may occur at three different levels: the spinal cord, the brainstem, and the cerebral cortex.⁹ The site of processing affects the speed of motor responses. Spinal reflexes represent the shortest neuronal pathways and, consequently, the most rapid response to afferent stimuli.^11,48,58 Theoretically, these spinal reflexes are faster than ligamentous failure.^46,58 Conversely, sensory information mediated at the brainstem, cerebellum, and cortical levels include longer pathways and slower response times. Numerous studies^11,21,48 have documented that sensory input processed at the CNS above the spinal cord level can be modified with training. Thus, owing to the adaptability of the responses at the brainstem and the cerebellum, these pathways are believed to be important in providing dynamic knee stability.¹⁸

After injury, the complex interactions within the neuromuscular system are disrupted, which can result in diminished proprioception and kinesthesia, abnormal patterns of muscle activity,¹⁵ and reduced joint dynamic stability.^4,7,34 Furthermore, injury to one knee can affect the proprioception on the contralateral (uninvolved) extremity.²⁸ Therefore, immediately after injury or surgery, the rehabilitation program must be directed toward creating an environment that promotes the restoration and development of motor responses and proprioception for both extremities.

Several other terms require classification. Open-loop control is defined as a movement that is brief, predictable, and produced in an unchanging environment that does not require sensory information for modification. Thus, these are movements that do not require feedback.⁴⁸ Conversely, movements that rely on feedback from the sensory system, such as reflexive movements, are considered closed-loop control.

Chmielewski and coworkers¹² defined motor skills in respect to the environment. Some motor skills, such as walking, stairclimbing, and extending the leg, although performed in a relatively stable environment, are referred to as closed skills. Conversely, open skills are performed in an environment that is changing and unpredictable. Examples of open skills include running in the woods, downhill skiing, or balancing on a wobble board. Other examples are playing sports such as basketball, soccer, or football.

EFFECTS OF INJURY ON PROPRIOCEPTION

Numerous authors4–7,²⁸ have shown a decrease in proprioception and kinesthesia after ACL injury. After ACL injury, deafferentization of peripheral sensory receptors occurs.^6,49 Changes in proprioception happen quickly. Lephart and associates³⁵ reported that these changes occur within 24 hours from the injury. Alterations in proprioception may persist for as long as 6 years.¹⁷

After an injury, changes occur within the joint that affect normal recruitment and timing patterns of the surrounding musculature.²⁰ Several theories regarding this deterioration of musculature activation have been proposed. One theory is that after an acute injury, an alteration occurs in the ratio of muscle spindle to GTO activity, leading to interference of the proprioception pathway. Another theory suggests that joint effusion after an acute injury alters the ability of the musculature to contract and, therefore, leads to decreased proprioception. A study by Palmieri-Smith and colleagues⁴⁴ showed that effusion of the knee of just 30 ml significantly decreased the activation of the vastus medialis and lateralis muscles during a single-leg drop landing. Joint effusion of 30 ml is barely palpable by most clinicians.

Injury to the ACL can lead to significant problems for the athlete. One study by Wojtys and Hutson⁵⁹ showed there was a significant decrease in muscle activation timing and recruitment order in the medial and lateral quadriceps, medial and lateral hamstrings, and gastrocnemius in response to anterior tibial translation in individuals with ACL-deficient knees compared with an uninjured control group. This delay in muscle recruitment can lead to decreased stability of the joint because the musculature is the prime joint stabilizer owing to loss of ACL function. Beard and coworkers⁷ examined the effects of applying 100 N of anterior shear force on ACL-deficient knees and noted a reflexive activation of the hamstring muscles. Paterno and associates⁴⁵ reported a significant difference in force production during a drop vertical jump in ACL-reconstructed knees compared with the contralateral limbs a mean of 27 months after ACL reconstruction. This study is one of many that show continued differences between ACL-reconstructed and uninvolved limbs for an extended period of time after surgery.^28,30,53,59

Wilk and colleagues²⁸ reported that 24 to 48 hours after ACL injury, proprioception was altered bilaterally according to measurements on a stability system. The uninvolved lower extremity’s ability to stabilize on a sway board (Biodex Stability System, Shirley, NY) was compromised for 6 to 8 weeks, with a gradual improvement in sway balance thereafter.

EFFECTS OF INJURY ON GAIT

After ACL injury, patients exhibit an alteration in gait patterns. Andriacchi and Birac¹ and Berchuck and coworkers⁸ coined the term “quadriceps avoidance gait pattern” (see Chapter 6, Human Movement and Anterior Cruciate Ligament Function: Anterior Cruciate Ligament Deficiency and Gait Mechanics). Patients walk with greater hamstring activity, a flexed knee, and minimal to no quadriceps electromyographic activity. It has been clinically observed that these protective neuromuscular adaptations (quadriceps-avoidance gait) may persist for several months if not appropriately addressed in rehabilitation.

DURATION OF INJURY EFFECTS

Many theories exist regarding the length of time that a patient with an acute ACL injury experiences a decreased sense of proprioception and neuromuscular control; the exact duration remains unclear. Most sources cite anywhere from 1 to 3 years as the timeframe for altered proprioception of the knee joint. Harrison and associates²⁵ studied the differences between the reconstructed and the uninvolved legs during single-leg stance in patients after ACL reconstruction. These researchers found no significant differences in postural sway (with eyes both open and closed) between the involved and the uninvolved lower extremities during single-leg stance 10 to 18 months after surgery. This finding suggests that proprioception can be restored in a shorter timeframe than expected. Other studies propose that proprioception and joint position sense take much longer to be reestablished. Fremerey and colleagues²⁴ examined the differences of joint position sense at different time intervals and compared those data to information gathered preoperatively. This study showed that joint position sense of the knee was almost completely restored at the near-end range of knee flexion and knee extension 6 months postoperatively. However, the study also reported that proprioception at the midrange of knee motion was not fully restored at 6 months. In fact, some patients took over 3½ years to fully recover their joint position sense at midrange positions. This is critical to an athlete because a majority of activities that occur during competition do so at these midrange of motion positions. The lack of joint position sense at these levels may have a significant impact on the probability that the athlete will sustain a second injury.

Another theory also considered when studying the duration of decreased proprioception in patients with acute ACL injury is the preinjury level of activity of the individual. Roberts and coworkers⁴⁷ demonstrated that patients whose preinjury activity levels were high had a faster recovery of joint position sense after ACL reconstruction.

CLINICAL RELEVANCE

When designing a rehabilitation program for a patient who has sustained an ACL injury, the clinician must remember several critical components. One is the restoration of neuromuscular control almost immediately after ACL reconstruction to prevent deafferentation of the joint. The progression of the patient must be increased gradually, and therefore, it is the responsibility of the therapist to find a balance between a detrimentally slow progression and advanced techniques prematurely that could have dangerous results. The therapist must consider the additional stresses that neuromuscular training places on the joint and factor those stresses into the overall volume of work that the patient performs. One must find a delicate balance to maximize rehabilitation benefits but prevent fatigue without recovery that could lead to delays in rehabilitation and setbacks.

The therapist can use several techniques to progress the patient that do not involve the actual injured joint. One such technique is to incorporate neuromuscular training to the uninvolved side. Studies have shown that there can be a carry-over effect from training the uninvolved extremity; challenging the uninvolved side can lead to improvements in the involved side. Performing passive/active joint repositioning is valuable in restoring joint awareness in the patient. This technique can be performed immediately after surgery or injury. Another technique is to challenge the core, ideally, while the patient is in an athletic stance to produce the most relevance to the patient’s sport. Core activities also help prepare the entire body for a return to activity when the patient is ready.

STAGES OF MOTOR SKILL DEVELOPMENT

Individuals progress through four stages when learning a new motor skill: mobility, stability, controlled mobility, and skill.⁵²Mobility refers to the availability of ROM to assume a posture or position and the presence of sufficient motor unit activity to initiate a movement. Individuals cannot perform a movement or skill if they do not exhibit adequate mobility. Stability refers to the ability to produce a co-contraction to provide tonic holding. Controlled mobility refers to movement added to a stable posture. Examples include standing and rocking back and forth, weight-shifting, ambulation, and balancing on an unstable platform. Skill is the highest level of motor control and refers to function and the ability to manipulate and perform tasks in an unstable environment, such as playing soccer or tennis.

Chmielewski and coworkers¹² classified three stages of motor skill development: cognitive, associative, and autonomous. The cognitive stage exists when a new task or drill is introduced. During this time, errors are made, movement is rigid or stiff, and the individual requires more training to learn the new task. Next is the associative stage in which less time is spent thinking about the task/drill and the movement is improved but is still not automatic. The third stage is the autonomous stage, defined as when, after practice, the movement becomes automatic and efficient and, with time, more skilled. In sports, skills such as shooting a basketball, hitting a tennis serve, or swinging a golf club require these three stages of motor skill. There also appears to be a fourth stage, which the authors refer to as the refining stage. This occurs when the individual refines the task/drill to a level of perfection.