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Balance in Single-Limb Stance in Patients With Anterior Cruciate Ligament Injury -- Relation to Knee Laxity, Proprioception, Muscle Strength, and Subjective Function, Eva Ageberg et al.; American Journal of Sports Medicine, Baltimore; October 2005, Vol 33, p. 1527-1535. Comments: This study looks specifically at the proprioceptive and neuromuscular consequences which flow from ACL deficiency (in particular chronic ACLlessness; note that all the subject in this study had no ACL reconstruction). When a natural ACL is torn, the embedded nerve endings are severed, and so the proprioceptive function (i.e. position-in-space feedback to brain's motor-control center) is lost, as is the protective hamstring reflex. (If the ACL is reconstructed, there is some hope that nerve endings will regrow into the graft, but this takes several years in any case. Moreover, there are still doubts as to whether or not such regrown nerve endings can be expected to become usefully connected to the brain, even in the long term [i.e. over 10 years post-operative].) The authors briefly discuss the proprioceptive role of the ACL, and they note that impaired postural control has been reported in people with chronic ACL tears as well as in people with reconstructed-and-rehabilitated ACL tears. (Note that single-limb stance, hence standing on one leg, is not only a static posture. It is also an integral part of normal walking. This means that the same mechanisms involved in maintaining static posture are also essential to the dynamic process of gait and movement in general.)

Neuromuscular Control of the Knee During a Resisted Single-Limb Squat Exercise, Richard K. Shields et al.; American Journal of Sports Medicine, Baltimore; October 2005, Vol 33, p. 1520-1526. This study shows that adding resistance to the standard single-limb squat exercise brings a number of benefits, most notably improved co-contraction of the quadriceps and hamstring muscles -- henceforth engendering reduced anterior-posterior shearing across the ACL-injury-history knee undergoing rehabilitation, as well as better hamstring control and improved dynamic (muscle-based) stabilization of the knee joint (and thus reduced likelihood of future knee injury).

Effects of Knee Bracing on the Sensorimotor Function of Subjects with Anterior Cruciate Ligament Reconstruction, Gloria K.H. Wu et al.; American Journal of Sports Medicine, Baltimore; Sep/Oct 2001, Vol 29/5, p. 641. Comments: This article (present in the Functional Knee Bracing section) discusses proprioception aspects germane to functional knee bracing. It also brings up some interesting points regarding proprioception in general.

Postural control after anterior cruciate ligament reconstruction and functional rehabilitation , Marketta Henriksson et al., American Journal of Sports Medicine; Baltimore; May/Jun 2001. Vol 29/3, p.359. Comments: This study found that people with ACL-reconstructed-and-rehabilitated knees generally had normal postural control. Only in two areas (reaction time and delay prior to onset of response to maximal sagittal-plane sway) were significant differences noted. This substantiates the conclusion that good rehabilitation (including not only muscle-strengthening and endurance training, but also prioprioception exercises and plyometrics and agility training) is of paramount importance in regaining full functionality after ACL reconstruction. This article also provides a good overview of ACL-proprioception issues, and shows the importance of the tension-sensitive nerve endings embedded in the ACL.

Direct evidence of ACL-hamstring reflex arc in humans , Eiichi Tsuda; The American Journal of Sports Medicine, Baltimore; Jan/Feb 2001, Vol 29/1, p. 83. Comments: This article discusses the ACL-protective hamstring reflex, thus highlighting the importance of strong hamstring musculature in protecting said ligament. (This article can also be found under the Knee Biomechanics, Functional Anatomy of ACL section.)

Longitudinal effects of anterior-cruciate-ligament injury and patellar-tendon autograft reconstruction on neuromuscular performance , Edward M. Wojtys; The American Journal of Sports Medicine, Baltimore; May/Jun 2000, Vol 28/3, p. 336. Comments: This penetratingly insightful article is one which everyone who has ever fully torn an ACL should read, as it is both a carefully-done study and an excellent overview of previous research. This study clearly illustrates the protracted neuromuscular consequences of ACL injury. (When the ACL is torn, the embedded nerve endings are lost as well. These nerve endings normally keep the brain apprised of the goings-on in the ligament and, by implication, in the limb in general. Although some regrowth of these nerve endings may occur in a reconstructed ACL over time, one wonders how likely it is for these nerve endings to actually become reconnected to the brain in a meaningful fashion.) Wojtys found that even at 12-18 months post-ACL-reconstruction, significant deficits in muscle performance (most notably reaction time) remained. It was found that quadriceps and hamstring reaction times were the best objective indicators of subjective knee functioning. (Also noted was that the central nervous system seems to limit affected-leg muscle output to about 70% of that of the contralateral limb. Because the knee-surrounding musculature (in particular the hamstrings) is so essential in protecting the knee against certain modes of abnormal forcing (most notably twisting and anterior-drawer forcing), these deficits likely mean that the ACL-reconstructed athlete is effectively more vulnerable to knee reinjury (as compared to a never-injured knee). Wojtys quotes the 1996 study by McDonald, which found that even at 50 months post-op, proprioception had not fully returned. Wojtys points out that "it remains to be seen if these deficiencies can be rectified over time." Note that because Wojtys' findings indicate that the full-ACL-tearing-and-subsequent-reconstruction affects both the hamstring and quadriceps groups, it is reasonable to conclude that the neuromuscular-functioning deficits he studied will exist with any type of ACL reconstruction. Wojtys also notes that in certain sports the knee can be exposed to impact forces 20 times the body's static weight (BW), and that the fact that 85% of ACL injuries are accompanied by articular-cartilage damage (i.e. bone-bruising) could have repercussions with regards to how well the ACL-injury-history knee can deal with the dynamic loadings of high-impact activities.

Evidence of reinnervation of free patellar tendon autograft used for anterior cruciate ligament reconstruction Robert L Barrack et al.; American Journal of Sports Medicine, Baltimore; Mar/Apr 1997, Vol 25/2, p. 196. Comments: This article, based on research in dogs (given the relative ease with which invasive histological investigations can be performed), shows that with the patellar-tendon autograft, there is good evidence that reinnervation occurs in the surrogate ligament. While it is still uncertain whether such regrown nerve endings enable regainment of pre-injury-style proprioception, at least it is realistic for useful nerve-ending regrowth to occur. Indeed, Barrack did observe that the newly developed nerve endings did respond to stimulation, and that this strongly implies that the neural pathway between the ACL and the central nervous system is eventually reestablished. This, in turn, shows that the grafted ACL does have the potential for providing meaningful sensory input. Therefore, a successful ACL reconstruction provides not only a mechanical restraint to abnormal knee movement, but could also reestablish some degree of propriopceptive (or quasi-proprioceptive) feedback to the brain. (Note: This is still an area of active research. The nerve-ending-regrowth-and-reconnection aspects would, in any case, take several years to occur. Remember, too, that there is no guarantee that such regrowth-and-reconnection will occur...and in fact, some later studies clearly show how uncertain occurrence of the regrowth-and-reconnection phenomenon is. In this regard, the only certainty seems to be that the outcome varies from person to person.) Barrack was one of the first to note the proprioceptive function of the ACL, in 1989. (Anyone who still thinks the ACL is completely devoid of nerve tissue would do well to read Barrack's studies!)

Proprioception in anterior cruciate ligament-deficient and reconstructed knees MacDonald, Peter et al.; American Journal of Sports Medicine; Baltimore; Nov/Dec 1996; Vol 24/6, p. 774. Comments: This study provides a good overview of the ACL's structure, including the various types of neurological detectors present in the ligament. It also notes the ACL-protective hamstring reflex. MacDonald also notes the chilling fact that in 90% of ACL-tear patients, other structures are also damaged, and that this also contributes to proprioceptive loss. (Even the effusion [swelling] resulting from each and every giving-way incident (of a chronically ACL-deficient knee) causes a reduction in neurological output of the sensors in the knee tissues. MacDonald also noted that the well-known fact that chronically ACL-deficient knees are prone to progressive instability [and eventual disability] is partly due to the proprioceptive loss.) In this study, the proprioception testing on the reconstructed knees was done 24 months after surgery (range, 17 to 30 months)...but note that more recent studies (in humans) show that the time frame for nerve-ending-regrowth-and-reconnection is longer than this. (Furthermore, in some people, such regrowth-and-reconnection might not occur at all.) MacDonald concludes (at the post-op time-frames noted) that ACL reconstruction does not improve proprioception in the ACL-deficient knee. (Note that a few of the subjects in this study had ACL reconstructions using hamstring autografting combined with synthetic graft back-up [known as ligament-augmentation devices]. The use of LADs has now fallen into disfavour, given the strain-shielding effect they cause. Conversion of an ACL graft into a bona-fide surrogate ligament requires carefully planned exposure to gradual increases in stresses; only this way can the normal helical parallel orientation of collagen strands [which is essential to the surrogate ACL developing good tensile-load-bearing capabilities] be developed in the nascent ligament. Here we see the importance of a good post-op physiotherapy programme, overseen by a knee-experienced physiotherapist. Note that variations in post-op rehab training were not accounted for by this study.) MacDonald concludes that because of the inherent difficulties and complexities associated with proprioception testing, this study may have missed some proprioceptive-type improvements. (This provides another possible explanation for the fact that Barrack's studies found prioprioceptive improvements following ACL reconstruction and rehabilitation, while MacDonald's study did not.)

Awareness of Knee Joint Angle Under Static Conditions K.W. Horch et al.; Journal of Neurophysiology; November 1975; Vol 42, p. 1436-1447. Comments: This is one of the first articles dealing with research into proprioception (formerly known as kinesthetic sense). It provides interesting background insight, and it shows how the field has progressed. The authors first looked at skin-subsurface nerve endings via a simple indentation experiment. Then, they investigated how passively flexing and extending the knee very slowly was detected. They concluded that fairly accurate and non-fading receptors are present in the knee (although at that time little was known about the proprioceptive role of the knee ligaments). Today, it is known that the knee ligaments contain tension-sensitive nerve endings which are extremely important in keeping the brain apprised of goings-on in the knee (and by implication, in the leg as a whole). The human brain's motor-control system, including not only the muscle-activating nerves but also the sensory ones, is directly analogous to the servomechanisms found in all types of electromechanical equipment.

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