After a total knee arthroplasty (TKA), the predominant impairment is a characteristic reduction in quadriceps strength that has been associated with a limitation in postoperative functional activity.1–4 Recent studies have highlighted an inability to fully activate the quadriceps muscle during a voluntary effort as an important factor in postoperative weakness.1,5–7 This activation deficit is ubiquitous and pronounced (> 20% activation failure) one month following surgery.5,7,8 While the ability to volitionally activate the quadriceps muscle improves over time, activation deficits persist as muscle dysfunction can last for years after TKA.1
A loss in quadriceps muscle strength prior to surgery can also be related to decreased muscle mass. The maximal quadriceps' crosssectional area (CSA) of older patients who are awaiting surgery for their painful osteoarthritic knees is approximately two‐thirds that of age‐matched individuals.9–12 This atrophy worsens in the first month after TKA surgery with a loss of 5% to 20% of quadriceps mass reported.6,8,13 One month following surgery, however, it is the loss of voluntary activation that contributes more to quadriceps weakness: activation deficits having 2‐fold the impact on quadriceps strength when compared to muscle atrophy.5,8
While the relative contributions of activation deficits and muscle atrophy are relatively well understood as it relates to the loss of quadriceps strength early after TKA surgery, their influence on residual long‐term weakness is not yet known. Therefore, the purpose of this short report is to describe the contribution of quadriceps muscle activation and muscle size to impaired muscle strength in older individuals more than 10 months following a TKA.
MATERIALS AND METHODS
Seventeen older individuals (mean age = 68 years ± 8.7), who had received either a unilateral or bilateral TKA more than 10 months prior to enrolling in this study (mean time following TKA surgery = 21 months ± 11.3), were recruited from an orthopaedic surgeon's (CP) list of follow‐up patients in the Department and Orthopedics at the University of Utah. A total of 24 TKA knees were included from this cohort of 13 women and 4 men. Though more symptomatic than a similar cohort of older TKA recipients, the participant's pain, activity, recreation levels, and health‐related quality of life are similar to that previously reported14,15 (Table 1).
Study procedures were approved by the University of Utah Health Sciences Center Institutional Review Board. All participants provided written consent prior to beginning the study. The TKA surgical procedures were performed via a “mini” medial parapatellar arthrotomy with minimal patella eversion.16,17 A conventional tri‐compartmental knee replacement was performed in all cases with the Biomet Vanguard knee system (Biomet, Warsaw, IN). Following TKA, patients received in‐patient care and home health physical therapy visits for the first 2 weeks following surgery. Thereafter guidelines for outpatient physical therapy and independent exercises were provided to the patient.18 The 8‐week comprehensive postoperative rehabilitation guidelines are provided in Table 2.
Maximal voluntary isometric contraction (MVIC) of the quadriceps muscle was measured on a Kin Com dynamometer (KinCom, Chattanooga, Hixon, TN).5,7,19 Participants were seated with their hips flexed to 85° and their knees flexed to 75°. Prior to testing, participants practiced using submaximal contractions at 50% and 75% of their maximal effort to familiarize themselves with the testing procedure. Participants then performed a MVIC of knee extension lasting 5 seconds. The subjects received visual feedback on a computer monitor and were verbally encouraged to exert maximal effort. The MVIC value was entered into a custom‐written software program using Labview V 6.0 (Labview, National Instruments) so that the MVIC values could be used for the burst superimposition technique to assess quadriceps activation.
The burst superimposition technique was used to determine quadriceps activation (QA). This technique involves superimposing a train of high voltage pulses with rapid frequency during a MVIC effort5,14,20,21 with 2 (7.8x12.7cm) self‐adhesive neuromuscular stimulation electrodes (CONMED Corporation, Utica, NY) secured to the thigh on the limb with TKA. The cathode was placed over the motor point of the vastus medialis and the anode was placed over the motor point of the rectus femoris.7 During a maximal knee extension effort, and when torque values reached >94% of peak MVIC, the participants held their MVIC effort for 0.5s at which time a burst of electrical stimulation (Grass S88000 with a Grass model SIU8T stimulus isolation unit, (Grass Instruments, West Warwick, RI) consisting of an electrical current (10 pulses, 100pps train, 600 μsec pulse duration, 135 Volts) was delivered to the quadriceps. Torque data was digitized at 200 samples*s‐1 and analyzed with custom written software. Voluntary activation of the quadriceps was reported as an index known as the central activation ratio (CAR).8,22,23 The CAR index is derived from the volitional MVIC divided by the total torque; with the total torque being calculated as a combination of the MVIC and a superimposed burst of electrical stimulation (E) (Figure 1). A CAR of 1.0 denotes complete activation of muscle.23,24
CAR = MVIC/MVIC + E
Quadriceps volume (QV), a measure of muscle size, was determined by magnetic resonance imaging (MRI). Participants were positioned in a standardized fashion; using in‐house MRI protocol procedures. All participants were asked to lie supine in the MRI magnet with their lower extremities in a pillow‐supported, resting knee extension posture with their feet comfortably strapped to avoid movement. A 1.5T GE Signa LX MRI instrument and body coil was used to obtain a coronal scout and axial T1‐weighted images of the quadriceps muscle between the inguinal crease and the proximal pole of the patella. The axial MRI images were digitized through DICOM Works and MATLAB (Mathworks, Natick, MA) software packages, and analyzed to determine the CSA of the quadriceps muscle (independent of bone and fat). The MRI produced 18–22 axial slices to evaluate (slice thickness of 8 mm and an interslice distance of 15 mm). Volume measurements were estimated by summing the volumes from each slice to give total volume as described previously.25,26
Data were analyzed with SPSS 12.0 (SPSS, Chicago, IL). Descriptive statistics were calculated for demographic variables and dependent measures. For each dependent measure, the assumptions for parametric statistical analysis were met. Pearson correlation coefficients were calculated to determine the relationship between quadriceps strength, QV, and QA. Stepwise multiple regression analysis was performed to determine the relative contributions of QV and QA to quadriceps strength. The alpha level was set at 0.05.
The mean quadriceps strength produced by participants during their TKA knee extension effort was 107.3 Nm ± 36.4 (range: 43.22 ‐ 205.2). The mean QA was 0.97 ± 0.04 (range: 0.83 ‐ 1.00). The average QA was 0.97; only 13% of the knees with a TKA exhibited activation levels less than healthy older adults with no known knee pathology.20 The mean QV was 1093 cm3 ± 311.80 (range: 653.66 ‐ 1706.56).
A significant correlation existed between quadriceps strength and QV (r = 0.88; p < .001) and a nonsignificant correlation between quadriceps strength and QA (r =.04, p =0.85) was observed (Figure 2).
The results of the linear regression analysis indicated that muscle volume and muscle activation explain 85% of the variance in quadriceps strength (R2 = .85, p < 0.001). Quadriceps volume was the greatest contributor to quadriceps strength, explaining 77% of the variance (R2 = .77, p < 0.001).
The novel finding of this short report is that QV is a much stronger predictor of quadriceps strength than QA in individuals approximately 1 to 3 years following TKA. The low contribution of volitional activation failure in explaining the variance in quadriceps strength is the reverse of what is found early after TKA, when activation deficits are the primary contributors to quadriceps strength.5 When combined with existing evidence it is clear quadriceps strength remains impaired over time,1–3,27 yet quadriceps activation deficits present early after TKA seem to resolve in most individuals. This is encouraging as previous findings report older individuals with knee osteoarthritis who have not received a knee replacement have deficits in QA.28 When coupled with our findings, it seems that if these older individuals were to choose to replace their osteoarthritic knee with a TKA their QA deficits would likely resolve over time. Therefore, the relative impact of volitional activation on quadriceps strength approximately 1 to 3 years following TKA is overshadowed by the impact of residual quadriceps atrophy.
It should be noted that several limitations of this study warrant caution when interpreting the results. Considering the variability in the amount of time since TKA surgery, our participants should be considered a heterogeneous cohort with very few deficits in activation. Moreover, despite a programmatic approach to rehabilitation during the first 3 to 4 months (see Table 2) following TKA, it is not clear what fraction of participants were compliant with this program. This TKA cohort's overall functional status does, however, resemble that of TKA recipients typically depicted in the literature one year following surgery.29–31 As well, considering that impairments in muscle and mobility peak 6 to 9 months following surgery14 and the remaining deficits continue to be present over subsequent years,1–3,27,32,33 we made an apriori decision to include only those older individuals > 9 months following TKA surgery.
The results of our study do suggest that rehabilitation efforts should include countermeasures that will improve quadriceps muscle size, thereby addressing the long‐term quadriceps weakness present after TKA. Prospective longitudinal research of this evolving relationship between quadriceps weakness and impairments in volitional activation and atrophy is warranted to determine the optimal rehabilitation approach that can predictably mitigate the long‐term quadriceps strength deficits in individuals following TKA.
1. Berth A, Urbach D, Awiszus F. Improvement of voluntary quadriceps muscle activation after total knee arthroplasty. Arch Phys Med Rehabil.
2. Silva M, Shepherd EF, Jackson WO, Pratt JA, McClung CD, Schmalzried TP. Knee strength after total knee arthroplasty. J Arthroplasty.
3. Walsh M, Woodhouse LJ, Thomas SG, Finch E. Physical impairments and functional limitations: a comparison of individuals 1 year after total knee arthroplasty with control subjects. Phys Ther.
4. Yoshida Y, Mizner RL, Ramsey DK, Snyder-Mackler L. Examining outcomes from total knee arthroplasty and the relationship between quadriceps strength and knee function over time. Clin Biomech (Bristol, Avon).
5. Mizner RL, Stevens JE, Snyder-Mackler L. Voluntary activation and decreased force production of the quadriceps femoris muscle after total knee arthroplasty. Phys Ther.
6. Perhonen M, Komi P, Hakkinen K, von Bonsdorff H, Partio E. Strength training and neuromuscular function in elderly people with total knee endoprosthesis. Scand J Med Sci Sports.
7. Stevens JE, Mizner RL, Snyder-Mackler L. Quadriceps strength and volitional activation before and after total knee arthroplasty for osteoarthritis. J Orthop Res.
8. Mizner RL, Petterson SC, Stevens JE, Vandenborne K, Snyder-Mackler L. Early quadriceps strength loss after total knee arthroplasty. The contributions of muscle atrophy and failure of voluntary muscle activation. J Bone Joint Surg Am.
9. Ferri A, Scaglioni G, Pousson M, Capodaglio P, Van Hoecke J, Narici MV. Strength and power changes of the human plantar flexors and knee extensors in response to resistance training in old age. Acta Physiol Scand.
10. Frontera WR, Hughes VA, Fielding RA, Fiatarone MA, Evans WJ, Roubenoff R. Aging of skeletal muscle: a 12-yr longitudinal study. J Appl Physiol.
11. Gur H, Cakin N. Muscle mass, isokinetic torque, and functional capacity in women with osteoarthritis of the knee. Arch Phys Med Rehabil.
12. Gur H, Cakin N, Akova B, Okay E, Kucukoglu S. Concentric versus combined concentric-eccentric isokinetic training: effects on functional capacity and symptoms in patients with osteoarthrosis of the knee. Arch Phys Med Rehabil.
13. Rodgers JA, Garvin KL, Walker CW, Morford D, Urban J, Bedard J. Preoperative physical therapy in primary total knee arthroplasty. J Arthroplasty.
14. Mizner RL, Petterson SC, Snyder-Mackler L. Quadriceps strength and the time course of functional recovery after total knee arthroplasty. J Orthop Sports Phys Ther.
15. Roos EM, Toksvig-Larsen S. Knee injury and Osteoarthritis Outcome Score (KOOS) - validation and comparison to the WOMAC in total knee replacement. Health Qual Life Outcomes.
16. Lombardi AV, Jr, Viacava AJ, Berend KR. Rapid recovery protocols and minimally invasive surgery help achieve high knee flexion. Clin Orthop Relat Res.
17. Peters CL. Soft-tissue balancing in primary total knee arthroplasty. Instr Course Lect.
18. Ranawat AS, Ranawat CS. Pain management and accelerated rehabilitation for total hip and total knee arthroplasty. J Arthroplasty.
2007;22(7 Suppl 3):12-15.
19. Stevens JE, Binder-Macleod S, Snyder-Mackler L. Characterization of the human quadriceps muscle in active elders. Arch Phys Med Rehabil.
20. Stackhouse SK, Stevens JE, Lee SC, Pearce KM, Snyder-Mackler L, Binder-Macleod SA. Maximum voluntary activation in nonfatigued and fatigued muscle of young and elderly individuals. Phys Ther.
21. Stevens JE, Mizner RL, Snyder-Mackler L. Neuromuscular electrical stimulation for quadriceps muscle strengthening after bilateral total knee arthroplasty: a case series. J Orthop Sports Phys Ther.
22. Stackhouse SK, Dean JC, Lee SC, Binder-MacLeod SA. Measurement of central activation failure of the quadriceps femoris in healthy adults. Muscle Nerve.
23. Kent-Braun JA, Le Blanc R. Quantitation of central activation failure during maximal voluntary contractions in humans. Muscle Nerve.
24. Miller M, Flansbjer UB, Downham D, Lexell J. Superimposed electrical stimulation: assessment of voluntary activation and perceived discomfort in healthy, moderately active older and younger women and men. Am J Phys Med Rehabil.
25. Dibble LE, Hale TF, Marcus RL, Droge J, Gerber JP, LaStayo PC. High-intensity resistance training amplifies muscle hypertrophy and functional gains in persons with Parkinson's disease. Mov Disord.
26. Tracy BL, Ivey FM, Jeffrey Metter E, Fleg JL, Siegel EL, Hurley BF. A more efficient magnetic resonance imaging-based strategy for measuring quadriceps muscle volume. Med Sci Sports Exerc.
27. Berman AT, Bosacco SJ, Israelite C. Evaluation of total knee arthroplasty using isokinetic testing. Clin Orthop Relat Res.
28. Petterson SC, Barrance P, Buchanan T, Binder-Macleod S, Snyder-Mackler L. Mechanisms underlying quadriceps weakness in knee osteoarthritis. Med Sci Sports Exerc.
29. Bert JM, Gross M, Kline C. Outcome results after total knee arthroplasty: does the patient's physical and mental health improve? Am J Knee Surg.
30. Dawson J, Fitzpatrick R, Murray D, Carr A. Questionnaire on the perceptions of patients about total knee replacement. J Bone Joint Surg Br.
31. Lingard EA, Katz JN, Wright RJ, Wright EA, Sledge CB. Validity and responsiveness of the Knee Society Clinical Rating System in comparison with the SF-36 and WOMAC. J Bone Joint Surg Am.
32. Huang CH, Cheng CK, Lee YT, Lee KS. Muscle strength after successful total knee replacement: a 6- to 13-year follow up. Clin Orthop Relat Res.
33. Rossi MD, Brown LE, Whitehurst M. Knee extensor and flexor torque characteristics before and after unilateral total knee arthroplasty. Am J Phys Med Rehabil.
Key Words:: total knee arthroplasty; quadriceps strength; voluntary muscle activation; quadriceps muscle volume