Importance of Attenuating Quadriceps Activation Deficits After Total Knee Arthroplasty

Abbey C. Thomas; Jennifer E. Stevens-Lapsley, M.P.T., Ph.D.

Disclosures

Exerc Sport Sci Rev. 2012;40(2):95-101. 

In This Article

Attenuating Deficits

Neuromuscular Electrical Stimulation

The presence of large CAD may impair the ability to train the quadriceps at sufficient intensities to promote strength gains.[30] Neuromuscular electrical stimulation (NMES) may override CAD, thereby allowing for restoration of normal quadriceps muscle function more effectively than voluntary exercise alone.

How NMES improves muscle strength is unclear, although some theories have emerged. First, the intensity of the muscle contraction produced during stimulation may be greater than that without NMES. Training programs require intensities of at least 30%–50% of maximal voluntary effort to overload the muscle sufficiently to induce strength gains,[27] which may not be possible volitionally in muscles with CAD. Similar to higher intensity voluntary muscle contractions, electrically elicited muscle contractions at high intensities produce muscle hypertrophy and corresponding increases in force production if used for a sufficient length of time. Second, NMES may alter motor recruitment. Electrically elicited muscle contractions allow for activation of a greater proportion of Type II muscle fibers than volitional exercise at comparable intensity. Type II muscle fibers are larger than Type I, so greater activation of type II fibers maximizes force production. Type II fibers are typically only activated during higher intensity voluntary contractions. With voluntary contractions, smaller motoneurons have lower activation thresholds than larger motoneurons; therefore, smaller motoneurons and Type I muscle fibers are recruited before larger motoneurons and Type II muscle fibers. With electrically elicited muscle contractions, factors, such as the size of the axonal branches and their orientation to the current field, influence motor units at lower contraction intensities. Finally, NMES also may influence functional measures of motor performance via peripheral afferent inputs that alter motor cortex excitability. Stimulation of peripheral afferent nerves can induce prolonged changes in the excitability of the human motor cortex. A study investigating the effects of stimulation of the hand afferents on cortical activity after subthreshold peripheral stimulation in healthy individuals found an increase in functional magnetic resonance imaging signal intensity in the primary and secondary motor and somatosensory areas.[6] Similar results have been demonstrated in individuals following stroke, further supporting the notion that NMES may play an important role in enhancing cortical excitability to allow for improved motor function.

NMES has been used effectively in a variety of patient populations, including individuals after stroke, to both re-educate muscle and facilitate hypertrophy.[18] In individuals after stroke, a 77% improvement in quadriceps force and nearly 20% improvement in motor unit recruitment were achieved through NMES treatment compared with only 31% improvement and no change, respectively, without NMES.[18] Similarly, NMES may improve quadriceps strength in individuals with knee OA. Specifically, Talbot et al.[34] noted a 9% increase in muscle strength in patients receiving NMES compared with a 7% loss of strength in patients who did not receive NMES.

Results of studies applying NMES to the quadriceps muscle of patients after TKA are promising. Gotlin et al.[7] noted that NMES applied within the first week after TKA reduced the knee extensor lag (i.e., deficit in the ability to actively extend the knee through the full available range of motion) from 7.5 to 5.7 degrees compared with control subjects who had an increase in extensor lag from 5.3 to 8.3 degrees in the same time frame. As such, early NMES treatment after TKA may translate to better quadriceps function. Similarly, Avramidis et al.[2] demonstrated a significant increase in walking speed in patients after 6 wk of daily NMES treatment (4 h per day) compared with controls 6 wk after TKA. There was a carryover in faster walking speed with NMES at 12 wk postoperatively, which is likely secondary to an initially faster recovery of quadriceps muscle force and subsequent ability to participate more fully in the voluntary exercise program.[2] Finally, when unilateral NMES was initiated 3–4 wk after bilateral TKA and continued for 6 wk, quadriceps muscle activation and force increased 431% in the limbs that received NMES plus voluntary exercise and only 182% in the contralateral limbs, which received voluntary exercise alone.[31] Despite the augmented response, force production in NMES-treated limbs remained below expected levels for healthy adults 6 months after surgery.[31]

A recent investigation in our laboratory examined the effects of NMES delivered twice per day (15 repetitions per session; biphasic waveform at 50 pps; 250− μs pulse duration; duty cycle 15 s on and 45 s off). Treatment began within 48 h after surgery and continued for 6 wk. Patients also received standardized rehabilitation for 8 wk (inpatient, home, and outpatient). The control group received only standardized rehabilitation. Results indicated that individuals receiving NMES demonstrated greater quadriceps and hamstrings strength, knee range of motion, and functional performance than controls (Fig. 2).[30] Three and a half weeks after TKA, activation trended toward greater improvements with NMES treatment compared with preoperative values (Fig. 2). Furthermore, strength and functional performance differences between groups were maintained 52 wk postoperatively.[30] Early restoration of quadriceps strength by countering activation deficits likely contributed to the long-term improvement in functional performance in patients receiving NMES.

Figure 2.

Differences in quadriceps strength (A) and CAD (B) between neuromuscular electrical stimulation (NMES) and control groups. Data are mean ± SD. *Statistically significant difference in quadriceps strength between groups (P < 0.05). [Adapted from [30] PTJ. 2012;92(2):210–26, with permission of the American Physical Therapy Association. This material is copyrighted, and any further reproduction or distribution requires written permission from American Physical Therapy Association.]

Although the results of several investigations indicate that NMES may be beneficial after TKA, a recent randomized controlled trial comparing exercise and exercise + NMES suggests that NMES (10 contractions, twice per week for 6 wk) initiated 1 month postoperatively may not be any more beneficial than exercise alone.[22] Specifically, the authors noted no differences between the exercise and exercise plus NMES groups in quadriceps strength, CAD, or function 3 or 12 months postoperatively.[22] However, both groups had better strength, CAD, and function 12 months after TKA compared with a cohort receiving less intensive rehabilitation in the community.[22] These results suggest that the timing and frequency of NMES treatment may be critical to patient outcomes. Specifically, early use of NMES (i.e., before 1 month after TKA) and NMES delivered greater than twice per week may be necessary.

Limitations and Future Directions The effectiveness of NMES is limited by patient tolerance to the treatment. Data from our laboratory and others examining the use of NMES after anterior cruciate ligament reconstruction[26] indicate that higher intensity stimuli (i.e., a greater dose of the treatment) are needed to achieve greater gains in strength and activation (Fig. 3). Yet the ideal studies and patient populations. After TKA, our results suggest that doses as little as 10%–20% of daily maximal voluntary isometric contraction may be effective when applied early (2 d after surgery) and frequently after TKA (2 times/day).[30] We found a significant association between NMES dose and change in quadriceps strength at 3.5 wk (R2 = 0.68) and at 6.5 wk (R2 = 0.25) after TKA. In contrast, less frequent NMES application after TKA (2 times/week) has not been effective even with a minimum 30% treatment dose.[22] Although high intensities result in greater gains in strength and activation, stimuli delivered at high intensities often are uncomfortable for patients. Furthermore, both electrode size and placement need to be considered during NMES treatment to improve patient comfort. Smaller electrodes have a higher current density and, thus, may increase patient discomfort compared with larger electrodes. We have found that because of the lower current density with 7.6 × 12.7 cm (3 × 5 inch) rectangular electrodes, these electrodes provide better patient comfort with NMES to the quadriceps muscle. As patient tolerance is critical to the success of NMES, future investigations would benefit from determining stimulation parameters and doses to optimize comfort and attenuate CAD. Furthermore, although some patients do not tolerate NMES well, others are capable of exceeding the maximal capability of the stimulators, which also limits the effectiveness of NMES application. NMES also may have limited effects in patients without CAD because studies of NMES applications in healthy individuals demonstrate fewer benefits than applications in patient populations with activation deficits. As will be discussed in the next section, patients with limited activation deficits may benefit from other forms of treatment, including aggressive voluntary strengthening exercises. A better understanding of who will benefit most from NMES treatment still is necessary to target the most appropriate patients.

Figure 3.

Relation between dose of neuromuscular electrical stimulation (NMES) treatment and quadriceps strength recovery after anterior cruciate ligament reconstruction. Percentage maximal voluntary isometric contraction = percentage of maximal voluntary contraction (%MVC). (Reprinted from [26]. Used with permission from the American Physical Therapy Association. This material is copyrighted, and any further reproduction or distribution requires written permission from American Physical Therapy Association.)

High-intensity Rehabilitation

Although NMES offers a promising strategy to target quadriceps CAD, it also is possible that using a more intensive, progressive rehabilitation program may be effective. Evidence from a variety of sources, including studies conducted on individuals after periods of both detraining and immobilization, supports the possibility that strength and CAD can be improved by progressive resistance exercise. Henwood and Taaffe,[8] for example, demonstrated a 17% improvement in strength after retraining exercise in older adults. Similarly, individuals demonstrated improvements in both muscle strength and functional performance following rehabilitation after immobilization subsequent to ankle fracture despite CAD.[24,32] Suetta et al.[33] induced CAD in older and younger male subjects via cast immobilization. After cast removal and 4 wk of rehabilitation, quadriceps strength improved by 23% in older male subjects and 32% in younger male subjects.[33] Furthermore, older male subjects demonstrated a 10% increase in central activation, whereas younger male subjects improved their activation by 5%.[33] Finally, previous research suggests that strengthening exercises for individuals on bed rest may attenuate strength loss and decrease CAD.[10]

Despite this evidence suggesting activation deficits can be improved with intensive, progressive resistance training, less information is available for patients after TKA. In the presence of profound CAD after TKA, initial evidence suggests that progressive exercise may help attenuate quadriceps CAD. A recent investigation in our laboratory compared a highintensity rehabilitation program with a lower intensity rehabilitation program after TKA. High-intensity training consisted of 22 outpatient physical therapy visits beginning within 1 wk after surgery. Exercises included general lower extremity exercises with emphasis placed on the quadriceps. These exercises were progressed from seated and side lying exercises to more aggressive, eccentrically focused functional exercises. Resistance was increased from ankle weights to machine-based exercises. Patients progressed through exercises if they could complete two sets of eight repetitions without fatigue and with pain less than 5/10 using a numeric pain rating scale of 0–10. The lower intensity training group participated in 10 outpatient physical therapy sessions over 6 wk, beginning 2 wk after surgery. Exercises consisted of seated and side lying hip and knee muscle strengthening exercises as well as step-ups, step-downs, and wall slides. The maximal weight used by patients in the lower intensity training group was 4.5 kg. Results indicated that high-intensity rehabilitation produced greater quadriceps strength and a trend toward smaller quadriceps CAD compared with lower intensity rehabilitation (Fig. 4).[3]

Figure 4.

Quadriceps strength (A) and CAD (B) after high-intensity and traditional rehabilitation. Data are presented preoperatively and 3.5, 12, 26, and 52 wk postoperatively. Data are mean T SD. *Statistically significant from preoperative time point (P < 0.05).

Limitations and Future Directions Although high-intensity rehabilitation has been shown to be beneficial after TKA, its benefits may be more limited in patients whose weakness is centrally mediated. These patients may require NMES or other modalities aimed at over-riding CAD before they can optimally benefit from strength training. Future studies should investigate the specific causes (e.g., spinal, cortical, peripheral) of quadriceps weakness and CAD so that targeted interventions can be implemented. As the origins of weakness and CAD may not be the same in every patient, researchers need to develop simple, cost-effective ways to determine the origins of these impairments within individual patients so that patient-specific rehabilitation strategies can be used.

Minimally Invasive TKA

In addition to rehabilitation strategies to attenuate quadriceps CAD after TKA, modifying the surgical technique may minimize postoperative quadriceps impairments. Compared with traditional approaches, minimally invasive surgery (MIS) for TKA uses smaller instrumentation and generates smaller incisions while avoiding patellar eversion and joint dislocation. Most importantly, this technique avoids disruption of the knee extensor mechanism and suprapatellar pouch and limits extreme knee flexion during surgery. Combined, these modifications have been postulated to minimize damage to the quadriceps muscle, thereby decreasing long-term, postoperative muscle weakness and functional impairments.

Early results from retrospective, cohort comparisons indicated that MIS reduced hospital stays, decreased postoperative pain, and enabled patients to return to functional activities more quickly than traditional TKA.[35] However, more recent, small-scale randomized trials universally are not as supportive of MIS, with some studies noting that the benefits of MIS are temporary, whereas others have found no differences between surgical techniques postoperatively.[11] Within the first few days after surgery, Seon and Song[23] demonstrated that MIS yielded less pain, shorter time to achieve 90-degree knee flexion and straight leg raise, and a smaller knee extension lag, although these differences were not present 2 wk postoperatively. Similarly, in individuals receiving simultaneous bilateral TKAs, with one knee receiving MIS and the other a conventional TKA, the knee receiving MIS had better Hospital for Special Surgery and Western Ontario McMaster Universities Osteoarthritis Index (WOMAC) scores for up to 6 months postoperatively as well as lower WOMAC pain scores for up to 9 months postoperatively.[28] These differences were not present 1 yr after surgery.

Recent results from a prospective, randomized controlled trial in our laboratory agree with those of other researchers. MIS resulted in greater hamstring strength and trends toward greater quadriceps strength 4 wk after surgery; however, there were no differences between groups in quadriceps CAD, range of motion, functional performance, or WOMAC scores.[5] By 12 wk postoperatively, strength was not different significantly between groups.[5] Additionally, 12 wk after surgery, patients in the MIS group had better WOMAC scores, but differences between groups were smaller than previously established clinically meaningful differences. Furthermore, differences between groups did not persist at 26 wk.[5] Therefore, MIS led to a faster recovery of muscle strength in the first 4 wk after TKA compared with traditional TKA, but this effect dissipated by 12 wk after surgery. Importantly, functional performance was not influenced by surgical approach. Although the use of MIS may lead to faster recovery of strength in patients undergoing TKA, there is no apparent benefit of MIS on the longer-term recovery of strength or functional performance.

Limitations and Future Directions MIS reduces visualization compared with traditional surgical approaches. As such, this technique requires specialized training to learn and perfect. An increasing body of evidence suggests that the purported benefits (e.g., reduced trauma to the knee joint complex) do not outweigh the cost of potentially longer operating times and advanced skills required to perform the surgery.[23,28] Furthermore, the limited, short-term benefits of MIS may not outweigh the risks associated with poor surgical visualization, which could result in a greater number of surgical complications, such as poor alignment. Although research should continue to focus on advancing surgical techniques to avoid excess trauma to the extensor mechanism, techniques should be refined to improve visualization.

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