Document Title: Branch-AJSM-Jan89.shtml
Article Title: Dynamic EMG analysis of anterior cruciate deficient legs with and without bracing during cutting
Authors: Thomas P. Branch MD, Robert Hunter MD, and Max Donath, PhD
Publication: American Journal of Sports Medicine, Baltimore, Maryland
Date: January 1989
Volume 17, Number 1, pages 35-41
Keywords: electromyographic (EMG) analysis

(Reference-denoting numbers appear in the same font and point size as the document text. As with all Knee Library documents, this article is provided in full-text form, complete with all figures and tables.)

Comments: In this intriguing study involving the use of electromyography (i.e. measuring the electrical signals at the skin arising as a consequence of muscle activity) during gait, these authors found that functional bracing did not alter the proportions of EMG activity (i.e. ratios stayed the same) nor muscle-activation-timing patterns. (You might wish to compare these results with those of subsequent studies focused on functional knee bracing. See, for example, DeVita-AJSM-Nov98 and Beynnon-AJSM-Jan03, both available here in the Knee Library.) Bracing did appear to generate an overall reduction in muscle activity. Keep in mind that these findings apply to completely ACL-deficient knees, hence with no ACL reconstruction. The discussion section of this study is especially insightful, and should be read very carefully. The authors set out to investigate the effect of knee bracing on both normal gait (running) and cutting (planting and twisting). Accordingly, each subject was asked to run to a force plate, and side-step cut using the ACL-deficient leg, both with and without knee bracing being worn. The side-step cut is perhaps one of the least riskiest plant-and-twist manoeuvres for an ACL-compromised knee, since it produces an external rotation moment at the tibia, which decreases the pivot-shift phenomenon. (The pivot-shift sign, like forwards tibial sliding [anterior drawer], is a hallmark of ACL deficiency.) For unbraced ACL-deficient people, the authors observed that, during the stance phase of gait, activity in the medial portion of the hamstring group increased simultaneously with a decrease in firing of the quadriceps; during the swing phase of gait, the increase in hamstring activity was especially noticeable. Because tensing the hamstrings counteracts anterior drawer, and because tensing the quadriceps exacerbates the anterior-drawer problem, the motor-control centre of the brain is automatically modifying muscle activity in order to reduce anterior drawer in the ACL-deficient knee. When a knee brace is added, an overall decrease in muscular activity was noted. The authors conclude that this could be an indication that a braced ACL-deficient knee requires less muscular stabilization -- and this decreased need for muscular stabilization is desirable, and it demonstrates bracing to be advantageous. Because no changes in muscle timing occurred, the authors note that bracing does not seem to be exerting a direct proprioceptive effect. They conclude that the effect of the brace may arise due to mechanical action; for example, the brace may subtly alter limb position in such a way as to reduce the need for muscular stabilization during the cutting movement. (Knee braces can exert anterior-drawer-counteraction forcing, if the straps above and below the knee are tightened appropriately; however, there are practical limits to the amounts of such forcing that can be exerted. One point to keep in mind is that, as with most knee-bracing studies, no standardization of brace-strap tightness was done. Ideally, strap tightness would be calibrated on the basis of tissue compression generated, not on the basis of strap tension itself.) The authors emphasize the importance of strong hamstring musculature in the context of rehabilitation (and reinjury prevention) for any ACL-injury-history knee.

ABSTRACT

The purpose of this research was to determine if bracing altered muscle firing amplitude, duration, or timing, creating improved dynamic stability. We hypothesized that a derotational knee brace improved the stability of an ACL deficient knee by augmenting limb proprioception, causing hamstring muscles to increase in activity and/or to contract earlier during a side-step cut. Ten subjects with documented unilateral isolated ACL deficient knees and five normal controls participated. A strap-dominant brace (Lenox Hill, Lenox Hill Brace, Inc., Long Island City, NY) and a shell-dominant brace (CTi, Innovation Sports, Irvine, CA) were selected for study. Using footswitches and dynamic EMG, we tested each subject during performance of a side-step cutting maneuver. Subjects completed 15 trials: 5 without bracing, 5 with the strap-dominant brace, and 5 with the shell-dominant brace. Normals cut 10 times each on their dominant limb.

In swing phase, subjects had 38% more and 32% higher lateral hamstring EMG activity than normals; in stance phase, subjects had less quadriceps and gastrocnemius activity but more medial hamstring activity. When braced during stance phase, the ACL deficient legs demonstrated a further reduction of 18% in quadriceps total activity and 14% in peak activity compared to the unbraced situation. The hamstrings showed a concomitant decrease of 18% in total activity. No timing differences were noted between the braced and unbraced conditions during swing or stance phase.

Clinically, the increase in hamstring activity and decrease in quadriceps activity found during the subjects’ stance phase could have a protective effect on the unstable knee due to the increased activity of muscles that work synergistically with the ACL to control anterior tibial translation. The reduced activity of antagonist muscles could enhance this effect. Bracing did not alter the relative EMG activity nor did it change firing patterns compared to the unbraced situation. All muscles showed a similar reduction in activity, suggesting that these braces did not have a proprioceptive influence.

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