Quadriceps and Hamstring Muscle Strength Recovery After a Multiligament Knee Injury: A Long-term Follow-up

Abstract

Background:

Multiligament knee injuries (MLKIs) are serious and heterogeneous injuries that often lead to long-term functional limitations. There is limited literature regarding objective muscle strength and functional recovery after surgical treatment of MLKIs.

Purpose:

To investigate long-term quadriceps and hamstring muscle strength and functional recovery after surgical treatment of MLKI and to compare postoperative outcomes between single cruciate MLKI and bicruciate MLKIs.

Study Design:

Cross-sectional study; Level of evidence, 3.

Methods:

Patients surgically treated for an MLKI at a single level 1 trauma center between January 2013 and December 2020 were included. Isokinetic quadriceps and hamstring muscle strength were assessed using a Biodex dynamometer, and physical function was measured by single-leg hop, triple hop, crossover hop, and 6-m timed hop tests, with the limb symmetry index (LSI) calculated.

Results:

Of 124 eligible patients, 90 (72%) participated at a mean follow-up time of 7 years (SD, 2.1) since surgery to evaluation. The median age at the time of testing was 49 years (range, 21-72) and 59% were male. In total, 54% of the patients had not regained full quadriceps strength (LSI <90%), and 46% had persistent hamstring deficits for peak torque/body weight (PT/BW) at 60 deg/s. Across the entire cohort, there was a significant difference between the uninvolved and involved sides for quadriceps and hamstrings for PT/BW (P = .01). The mean LSI for quadriceps PT/BW was 88% at 60 deg/s and 90% at 240 deg/s, while for the hamstrings, the mean LSI was 93% at 60 deg/s and 97% at 240 deg/s. Median LSIs for the hop tests ranged from 93% (range, 41%-187%) for single leg to 99% (42%-141%) for crossover; however, 44% of patients failed to reach the 90% threshold for the single-leg hop. The 6-m timed hop test and quadriceps muscle strength were significantly lower for patients with bicruciate injuries than those with single cruciate injuries (P < .05).

Conclusion:

Persistent deficits in quadriceps and hamstring muscle strength were observed 7 years after MLKI surgery, with a substantial proportion of patients failing to regain full functional symmetry in muscle strength and hop test performance. Patients with bicruciate injuries demonstrated significantly more pronounced deficits in muscle strength and hop test scores as compared with those with single cruciate injuries.

Multiligament knee injuries (MLKIs) represent some of the most devastating knee injuries and often affect patients’ ability to return to their previous levels of function in sports participation and at work. An MLKI usually affects the young and active population, involving the disruption of ≥2 major knee ligaments,^23,27,39 potentially leading to instability, pain, and long-term functional limitations. Owing to the severity of MLKIs, these injuries often present with concomitant injury to surrounding structures, including neurovascular injuries, which can significantly affect patient outcomes. The quadriceps and hamstring muscles play a critical role in knee stability and lower limb function, and adequate strength in these muscle groups is essential for daily activities and mobility after knee surgery. Weakness in these muscle groups can significantly impair activities such as walking, stair climbing, rising from a chair, and returning to sport or work.^16,29 Residual quadriceps weakness is associated with impaired knee stability, reduced function, and long-term disability. It may also contribute to excessive mechanical stress on articular cartilage, potentially accelerating degenerative changes and osteoarthritis progression.^1,30 These challenges are particularly pronounced after severe knee trauma such as an MLKI. ⁹

Isokinetic muscle strength testing and hop performance ² have been widely used for decades to assess quadriceps and hamstring strength in athletic and nonathletic populations.^6,21,38 However, such assessments are infrequently reported in the MLKI population. The limited studies available^7,10,11,16 have primarily focused on short-term follow-up, finding that strength and function may improve within the first 1 to 2 years postoperatively. These studies note that strength deficits frequently persist beyond 24 months, although data on whether these strength deficits are sustained over the long term are still lacking. Understanding muscle strength deficits in patients with MLKI will provide valuable insights into optimizing rehabilitation strategies, potentially improving long-term functional outcomes, and delaying degenerative changes. Furthermore, while information on functional recovery is reported for single cruciate MLKIs and bicruciate MLKIs,^7,11 it rarely provides direct comparisons between these groups, owing to small sample sizes and heterogeneous injury patterns. ¹⁰ Given the limited documentation on muscle strength and functional recovery in this population, the purpose of this study was to investigate intermediate-term quadriceps and hamstring muscle strength and functional recovery after surgical treatment of MLKI and to compare postoperative outcomes between single cruciate and bicruciate MLKIs.

Methods

Patient Population

This cross-sectional study consisted of patients who underwent surgical management for MLKI at the orthopaedic department of a level 1 trauma center between January 2013 and December 2020. The study was approved by the Regional Committee for Medical and Health Research Ethics (REK Nord-IRB206446). Informed consent was received from all included patients. MLKIs were defined as injuries to at least 2 of the 4 major knee ligaments requiring surgical intervention: the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), or fibular collateral ligament (FCL). Eligible patients had at least 24 months of postoperative follow-up. Exclusion criteria included patients who had undergone revision ligament reconstruction, tibial osteotomy, or total knee replacement before the time of follow-up, as well as those with concomitant intra-articular knee fractures requiring surgical treatment. Of the 191 patients treated during the study period, 124 (65%) agreed to participate in an initial study assessing self-reported function. ³⁶ From this group, 90 patients consented to participate in the present study, evaluating muscle strength and physical function.

Surgical Treatment

All patients underwent anatomic reconstructions based on established guidelines,^3,26,36 performed at a single center by surgeons experienced in treating MLKIs. The ACL reconstructions were performed with either a bone–patellar tendon–bone or hamstring tendon autograft. For PCL tears, a double-bundle anatomic PCL reconstruction was performed with Achilles, tibialis anterior, or tibialis posterior tendon allografts. The MCL was reconstructed via the LaPrade repair augmentation technique with autograft, ¹ and the LaPrade posterolateral corner reconstruction technique was used for posterolateral corner injuries with allograft. ²²

Postoperative and Rehabilitation Protocol

After surgery, patients with PCL involvement were immobilized in a knee range of motion (ROM) brace locked in full extension. Passive extension was initiated immediately, and after swelling subsided, the brace was replaced with a dynamic PCL brace. This brace was worn for 12 to 16 weeks, with initial settings that limited the knee ROM to 0° to 90°. For patients without PCL injuries, a ROM brace was used for 8 weeks, allowing full ROM. All patients followed a rehabilitation program that consisted of initial partial weightbearing with crutches, early isometric quadriceps training, and straight-leg exercises. After 8 weeks, full weightbearing and progressive strengthening activities, such as stationary cycling, were introduced. From 16 weeks postoperatively, rehabilitation focused on restoring full motion, improving muscular control, and advancing functional training, such as walking, stair work, and balance exercises. Neuromuscular training was gradually progressed from stable to dynamic conditions, and strength training was intensified with high-load and endurance-based protocols. Light jogging was introduced once patients demonstrated full mobility, adequate quadriceps control with a ≥75% quads limb symmetry index (LSI), and the ability to walk 30 minutes without pain or increased swelling. Return to full activity was permitted and recommended when patients achieved at least 90% quadriceps strength symmetry as compared with the contralateral limb and nearly full knee motion.

Baseline Characteristics

A retrospective chart review was conducted to obtain baseline variables: age at the time of injury, sex, date of injury, side of injury, type of injury, presence of meniscal and cartilage injuries, surgical procedures, treatment, and complications. The ligament injuries were categorized according to the modified Schenck classification, as outlined by Poploski et al, ³¹ which differentiates various MLKI patterns (MLK 1-AM to MLK 4) depending on the ligaments involved. For the analysis, cases were grouped into 5 types: type 1 (single cruciate with MCL or FCL), type 2 (bicruciate), type 3-L (ACL, PCL, FCL), type 3-M (ACL, PCL, MCL), and type 4 (all major ligaments involved). Injuries were further stratified by energy mechanism: high-energy events included motor vehicle accidents, significant falls (>5 ft), or downhill skiing, whereas low-energy injuries occurred during ball sports, cross-country skiing, minor falls (<5 ft), or daily activities.

Muscle Strength Testing and Physical Function

A Biodex System 4 dynamometer with Advantage BX software (Version 5.3.06) was used to assess isokinetic quadriceps and hamstring muscle strength. Before the test, participants performed a 5-minute warm-up on an ergometer cycle. After the warm-up session, the participant was secured to the apparatus with straps across the chest, pelvis, thigh, and ankle joint. Before data collection, the dynamometer's gravity correction procedure was performed according to the manufacturer's recommendations to account for the weight of the tested limb. Participants were then given 5 to 10 trial repetitions to familiarize themselves with the system. The test protocol consisted of 5 maximal repetitions at 60 deg/s to assess peak torque capacity, followed by a 1-minute rest period and 30 repetitions at 240 deg/s to evaluate muscular endurance and fatigue resistance, in accordance with the Biodex manufacturer guidelines. The uninvolved side was tested first, and the ROM was set from 90° of flexion to 0° of extension, depending on the patient's ROM. Body weight–corrected peak torque (PT/BW) and total work (TW) values were used in the analysis. Of the 90 patients included in the study, 88 completed the strength tests. To measure physical function, the participants performed 4 hop tests: single hop for distance, triple hop for distance, triple crossover hop, and 6-m timed hop. These tests serve as proxies for neuromuscular control, strength, and confidence in the limb^25,28,32,35 and are commonly used to assess ligament injuries. ²⁸ Of the 90 included patients, 15 to 17 were unable to complete the hop tests because of difficulties performing them. The 2 patients who did not complete the strength testing were nevertheless able to perform the hop tests.

Statistical Analysis

Statistical analyses were performed in Stata SE 18 (Stata Corp LLC). Patient characteristics and surgical details were presented as mean with standard deviation or median with range for continuous variables and as frequency and percentage for categorical variables. The groups were compared by t test for continuous variables and Wilcoxon rank sum (Mann-Whitney) test for continuous, nonnormally distributed variables. A P value of .05 was considered statistically significant, and all tests were 2-sided. LSI was calculated by dividing the result for the involved leg by that of the uninvolved leg and multiplying by 100, based on PT/BW and TW values. Normalizing the strength to body weight allows for more clinically meaningful comparisons among individuals rather than absolute torque values alone and is consistent with the primary strength outcomes of this study. An LSI ≥90% was considered normal.^5,28 To evaluate the balance between muscle groups, the hamstrings-quadriceps (H/Q) ratio representing the relationship between hamstring and quadriceps strength was calculated. The ratio is calculated by dividing peak torque of the hamstring muscle by peak torque of the quadriceps muscle. ³³ Additional post hoc correlation analyses were performed using patient-reported outcome measures and return-to-sport data from the same study cohort, previously reported elsewhere,^36,37 to examine associations with muscle strength measurements using Spearman ρ (P < .05).

Results

Patient Demographics

Of the 90 patients, 59% were male with a mean (SD) body mass index of 28 (8.1) at the time of testing. The median age was 49 years (range, 21-72), and the mean time from surgery to evaluation was 7 years (2.1). Most participants (n = 65, 72%) had sustained low-energy injuries versus high-energy injuries (n = 25; 28%). The injury mechanisms and injury patterns according to the modified Schenck classification are reported in Table 1. Concomitant chondral injuries were present in 32 knees (35%) and meniscal tears in 27 (30%). Peroneal nerve injury occurred in 6 patients (7%), with 5 (5%) undergoing a tibialis posterior tendon transfer for a foot drop.

Table 1.

Injury Mechanism and Injury Pattern ^a

	No. (%)
Injury mechanism
Motor vehicle accident	1 (1)
Motorcycle	7 (8)
Ball sports	14 (16)
Skiing sports	30 (33)
Other sports	14 (16)
Other b	24 (26)
Multiligament injury pattern
MLK 1-AM: ACL and MCL	19 (21)
MLK 1-AL: ACL and FCL	18 (20)
MLK 1-AML: ACL, MCL, FCL	0 (0)
MLK 1-PM: PCL and MCL	3 (3)
MLK 1-PL: PCL and FCL	4 (3)
MLK 1-PML: PCL, MCL, FCL	1 (1)
MLK 2: ACL and PCL	8 (9)
MLK 3-M: ACL, PCL, MCL	26 (29)
MLK 3-L: ACL, PCL, FCL	9 (10)
MLK 4: ACL, PCL, MCL, FCL	2 (1)

^aACL, anterior cruciate ligament; FCL, fibular collateral ligament; MCL, medial collateral ligament; MLK, multiligament knee; PCL, posterior cruciate ligament.

^bInjury mechanisms classified as “other” include falling on a slippery floor, falling from a height, getting run over by a dog, and tripping.

Quadriceps and Hamstring Muscle Strength

For the quadriceps PT/BW at 60 deg/s, the mean (SD) was 168 (57) N·m/kg for the involved limb versus 192 (63) N·m/kg for the uninvolved limb (side-to-side difference, 24 ± 39; P = .01), while at 240 deg/s, it was 105 (36) versus 114 (35) N·m/kg, respectively (side-to-side difference, 9 ± 18; P = .01). For hamstring PT/BW at 60 deg/s, the mean (SD) was 82 (30) N·m/kg for the involved limb versus 90 (31) N·m/kg for the uninvolved limb (side-to-side difference, 8 ± 16; P = .01), while at 240 deg/s, it was 53 (21) versus 58 (22) N·m/kg (side-to-side difference, 5 ± 12; P = .01) (Figure 1). Results for the TW capacity are presented in Table 2.

Figure 1.

Body weight–corrected peak torque for quadriceps and hamstring at 2 angular velocities comparing involved and uninvolved limbs. Error bars represent the standard deviation.

Table 2.

Total Work in Joules for Quadriceps and Hamstring Muscles at the 2 Angular Velocities (60 and 240 deg/s)

	Limb, J, Mean ± SD
Total Work: Muscle	Involved	Uninvolved	Difference	P Value
60 deg/s
Quadriceps	659 ± 241	743 ± 279	83 ± 154	<.001
Hamstrings	357 ± 165	398 ± 176	40 ± 84	<.001
240 deg/s
Quadriceps	1816 ± 758	1969 ± 748	144 ± 338	<.001
Hamstrings	808 ± 411	848 ± 403	40 ± 215	.03

The mean (SD) quadriceps PT/BW LSI was 87% (19%) at 60 deg/s and 92% (16%) at 240 deg/s, while the mean hamstring LSI was 91% (24%) and 91% (21%), respectively. Figure 2 presents the percentage of patients with normal limb symmetry (LSI ≥90%) for PT/BW for quadriceps and hamstrings at 60 and 240 deg/s. For the TW, the corresponding proportions were 42% (n = 37) for the quadriceps LSI at 60 deg/s, 56% (n = 50) for the quadriceps LSI at 240 deg/s, 52% (n = 46) for the hamstring LSI at 60 deg/s, and 59% (n = 52) for the hamstring LSI at 240 deg/s. Table 3 presents the mean H/Q ratio for PT/BW and TW.

Figure 2.

Percentage of participants with normal (LSI ≥90%) and abnormal (LSI <90%) limb symmetry for peak torque body weight for quadriceps and hamstrings at 60 and 240 deg/s (n = 88). LSI, limb symmetry index.

Table 3.

Mean Hamstring to Quadriceps Ratio

	Limb, Mean ± SD
Ratio	Involved	Uninvolved
Peak torque/body weight
60 deg/s	0.49 ± 0.11	0.51 ± 0.12
240 deg/s	0.51 ± 0.12	0.53 ± 0.13
Total work
60 deg/s	0.50 ± 0.14	0.53 ± 0.15
240 deg/s	0.44 ± 0.15	0.43 ± 0.15

Hop Tests

The median (range) LSI was 93% (41%-187%) for the single-leg hop test, 98% (52%-129%) for the 6-m timed hop test, 95% (47%-207%) for the triple-hop test, and 99% (42%-141%) for the crossover hop test. The percentage of patients with normal limb symmetry (LSI ≥90%) on the various hop tests is presented in Figure 3.

Figure 3.

Percentage of participants with normal (LSI ≥90%) and abnormal (LSI <90%) limb symmetry for the single-leg, 6-m timed, triple hop, and crossover hop tests. LSI, limb symmetry index.

Comparison of Strength Between Single Cruciate and Bicruciate Ligament Injuries

There were significant differences between the single and bicruciate MLKIs for quadriceps for PT/BW and TW at 60 deg/s, as well as for the 6-m timed hop test. No significant differences were found at 240 deg/s or in the other hop tests (Table 4).

Table 4.

Outcome Scores Comparing Type 1 and Types 2-4

	Score, Mean ± SD
Outcome Scale	Type 1	Types 2-4	P Value a
Quadriceps
Peak torque/body weight, N·m/kg
60 deg/s	182 ± 56	154 ± 55	.02
240 deg/s	110 ± 32	100 ± 39	.17
Total work, J
60 deg/s	719 ± 250	600 ± 219	.02
240 deg/s	1942 ± 744	1693 ± 760	.12
Hamstring
Peak torque/body weight, N·m/kg
60 deg/s	87 ± 29	79 ± 31	.23
240 deg/s	56 ± 19	52 ± 22	.43
Total work, J
60 deg/s	393 ± 153	336 ± 170	.11
240 deg/s	886 ± 377	757 ± 434	.15
Test
Single hop, cm	94 ± 45	93 ± 45	.95
6-m timed hop, s	293 ± 125	394 ± 278	.05
Triple hop, cm	343 ± 123	320 ± 129	.44
Crossover hop, cm	298 ± 126	272 ± 131	.40

^aBold indicates P≤ .05.

Patients with single-ligament injuries demonstrated higher quadriceps LSI at 60-deg/s PT/BW (mean ± SD, 94.9 ± 18.8 vs 82.1 ± 18.2; mean difference, 12.7; P = .002), 60-deg/s TW (97.6 ± 22.6 vs 83.6 ± 18.2; mean difference, 14.0; P = .002), and 240-deg/s PT/BW (97.2 ± 13.4 vs 87.0 ± 17.4; mean difference, 10.2; P = .003) as compared with bicruciate ligament injuries. No significant differences were observed in hamstring LSI at any velocity (all mean differences, −0.4 to −4.1; all P > .36).

Associations Among Muscle Strength, Patient-Reported Outcomes, and Return to Sport

Correlation analyses showed that quadriceps LSI at 60 deg/s was weakly but significantly correlated with International Knee Documentation Committee subjective knee form (ρ = 0.27; P = .011), Knee injury and Osteoarthritis Outcome Score (KOOS)–Sport and Recreation (ρ = 0.25; P = .022), and KOOS–Quality of Life (ρ = 0.28; P = .008) (Table 5). Hamstring LSI at 60 deg/s was not significantly correlated with any of the PROMs. Quadriceps LSI was not significantly associated with return to the same or higher preinjury activity level (Table 6).

Table 5.

Correlation Between Outcome Measures and LSI for Quadriceps and Hamstrings Strength at 60 deg/s ^a

	Quadriceps LSI, 60 deg/s		Hamstrings LSI, 60 deg/s
Outcome Measure	Spearman ρ	P Value	Spearman ρ	P Value
IKDC	0.27	.011	0.17	.121
KOOS
Sport and recreation	0.25	.022	0.11	.300
Quality of life	0.28	.008	0.12	.282

^aSpearman correlation coefficient (ρ) is reported. IKDC, International Knee Documentation Committee; KOOS, Knee injury and Osteoarthritis Outcome Score; LSI, limb symmetry index.

Table 6.

Quadriceps LSI (60 deg/s) According to RTS Status ^a

	RTS = 1 (n = 34)	RTS = 0 (n = 53)	P Value
Mean ± SD	90.9 ± 23.1	86.3 ± 16.6	.281 b
Median (IQR)	90.6 (76.9-101.6)	84.5 (73.0-98.0)	.441 c

^aLSI, limb symmetry index; RTS, return to sport.

^bIndependent samples t test.

^cMann-Whitney U test.

Discussion

The most important findings from this study were that almost half of the patients undergoing MLKI surgery were not able to regain full maximal quadriceps (54%) and hamstring (46%) muscle strength as compared with the uninvolved limb at a mean 7 years postoperatively. Furthermore, 28% to 44% of patients who completed the hop tests failed to achieve a normal LSI (≥90%), suggesting that functional recovery remains incomplete for many individuals for a prolonged period after surgery. Last, patients with bicruciate injuries demonstrated significantly lower quadriceps muscle strength and inferior performance on the 6-m timed hop test as compared with patients with single cruciate injuries, indicating greater long-term impairment in more complex injury patterns.

The present study demonstrated that even at a mean 7 years after injury, a significant proportion of patients had not regained full quadriceps and hamstring muscle strength (LSI ≥90%). These findings are consistent with previous literature indicating persistent muscle deficits after an MLKI.^7,10,11,16 Jenkins et al ¹⁶ reported that at 2 years postoperatively, quadriceps and hamstring strength recovered to 85% and 90% of the uninjured limb, respectively. Their prospective series of 50 MLKI cases primarily included 2-ligament injuries (76%), with fewer cases involving 3 ligaments (20%) or 4 (4%). Ebert et al ¹⁰ analyzed a cohort of 50 patients with MLKIs, of which 74% were Schenk KDI. At 24 months, the mean (SD) quadriceps LSI at 90 deg/s was 87% (10.4%), while hamstring torque LSI was better preserved at 97% (9.1%). The cohort of the present study, which was larger and followed for a longer period, supports and expands the findings of Jenkins et al and Ebert et al by showing that these strength deficits often persist well beyond 2 years postoperatively. Additionally, a systematic review ¹² of ACL reconstructions revealed that knee extensor muscle strength is meaningfully reduced (>10%) at 1 year postoperatively, with limited improvement up to and beyond 5 years after surgery, suggesting that quadriceps weakness may represent a broader challenge across different types of complex knee injuries. In the limited studies available, persistent strength deficits after MLKI reconstruction at or beyond 24 months have been reported.^7,11,22 Gigliotakaes et al ¹¹ noted persistent isokinetic knee extensor deficits after a 2-stage bicruciate procedure with a mean postoperative follow-up of 27 months, while Denti et al ⁷ identified isokinetic knee flexor and extensor strength deficits at 24-month follow-up in patients who underwent single-stage bicruciate reconstruction. Taken together, the findings from the present study and the available literature demonstrate that persistent strength impairments, especially for the quadriceps, are a common outcome after MLKI reconstruction.

From a clinical perspective, long-term quadriceps deficits are of particular concern, as reduced knee extensor strength has been associated with impaired function and altered joint loading. Considering the persistent muscle strength deficits observed after MLKI, it may be important to establish better long-term rehabilitation programs and critically evaluate the content of rehabilitation. Clinicians could consider incorporating exercise variation, challenging tasks, and motor learning principles to optimize rehabilitation while ensuring sufficient loading of the injured limb.^4,13,15,34 In addition to underloading, the intensity and volume of strength training might be too low to increase muscle strength and volume to satisfactory levels. It is not clear how the use of autografts versus allografts can influence muscle recovery after surgery for complex knee injuries; however, it was beyond the scope of this study.

More than 40% of patients in the present study failed to achieve the commonly accepted LSI threshold of 90% across single-leg hop tests, indicating that functional recovery remains incomplete for a substantial proportion of individuals. In addition, approximately 16% of the patients were unable to perform the hop tests at all, whereas 3 had undergone tibialis posterior tendon transfer for their foot drop. This reflects the wide variability in recovery in this population. Research on patients after ACL reconstruction has highlighted an increased risk of reinjury among those who do not meet the commonly used 90% LSI threshold, particularly concerning quadriceps peak torque and hop performance.^14,20 This adds clinical relevance to our findings and supports the need for rigorous, objective assessments before return to full activities. The present findings are consistent with those of Ebert et al, ¹⁰ who also reported that many patients failed to reach the accepted 90% LSI threshold. However, the authors did not indicate the proportion of patients who reached or were unable to reach the 90% LSI threshold, making direct comparison of recovery distribution difficult. Notably, Ebert et al highlighted that hop symmetry tends to recover earlier and more fully than quadriceps strength, emphasizing a so-called strength-function mismatch. This phenomenon suggests that patients may compensate during dynamic tasks such as hopping, jumping, and running, thereby masking persistent deficits in knee extensor muscle capacity. It may also indicate that achieving full (100%) muscle strength is not required to regain normal functional properties. Biomechanical analyses have shown altered joint kinematics and compensatory movement patterns during hop tasks in patients after an ACL reconstruction, which may increase the risk of secondary injury and limit performance.^8,19 While hop tests are valuable tools for assessing gross functional recovery, they should be interpreted alongside objective strength measurements and biomechanical analyses to characterize neuromuscular status more accurately.

The third main finding from this study is that patients with bicruciate injuries demonstrated significantly lower quadriceps strength for PT/BW and TW at 60 deg/s and inferior performance on the 6-m timed hop test as compared with patients with single cruciate injuries. This observation aligns with the growing body of evidence that injury severity strongly affects functional outcomes after an MLKI.^8,17,18,24 Ebert et al ¹⁰ reported similar findings with patients having KDII-KDIV injuries showing significantly lower LSI values on single- and triple-hop tests and greater quadriceps strength deficits throughout the first postoperative year. While some improvement in hop symmetry was noted by 24 months, performance remained lower than in less severe injuries. ¹⁰ Bicruciate injuries involve simultaneous disruption of the ACL and PCL, which not only compromises knee stability to a greater extent but also increases the complexity of surgical reconstruction, often leading to more extended periods of immobilization and delayed neuromuscular recovery. ²⁶ Moreover, these injuries are more frequently associated with concomitant nerve injuries and postoperative complications, which may further hinder or prolong rehabilitation. Nerve injuries may cause drop foot, often necessitating orthotic devices, gait retraining, and individually adapted exercises. Such factors extend the recovery process and may contribute to delayed muscle activation, persistent strength deficits, and functional limitations. In our study, among the patients with initial nerve deficits (n = 6), 3 were unable to complete the hop tests and 4 demonstrated notable strength reduction in the involved limb; 1 patient was represented in both categories.

The H/Q ratios in this study revealed relatively preserved muscular balance in the involved limb, with median ratios ranging from 0.49 to 0.51 for PT/BW and from 0.44 to 0.51 for TW across angular velocities. These values fall within the range considered physiologically normal (typically 0.4-0.6) and suggest that, despite overall strength deficits, the relative relationship between the flexor and extensor muscle groups was maintained. These findings are supported by Gigliotakaes et al, ¹¹ who reported an H/Q ratio of 0.59 at 60 deg/s and 0.65 at 180 deg/s. While a balanced H/Q ratio is generally viewed as protective against reinjury, excessively high ratios may reflect disproportionate quadriceps weakness rather than true hamstring dominance. ³³ This underscores the importance of presenting not only muscle balance but also absolute strength values during follow-up assessments. In the present study, quadriceps strength demonstrated weak but significant correlations with patient-reported outcomes, including International Knee Documentation Committee form, KOOS–Sport and Recreation, and KOOS–Quality of Life, suggesting that greater limb symmetry is associated with better perceived knee function. However, no significant association was observed between quadriceps strength and return to sport. These findings indicate that while muscle strength may contribute to subjective knee function, it alone does not determine return to sport in this population. Return to sport after MLKI is likely influenced by multiple factors, including psychological readiness, injury severity, and persistent functional limitations.

The wide dispersion of strength values observed in this cohort highlights the marked heterogeneity in long-term recovery after MLKI, with some patients achieving near-complete restoration of strength, while others exhibited substantial persistent deficits. Despite statistically significant paired side-to-side differences, the variability may partly be explained by the overlapping error bars seen in Figure 1.

Some limitations of this study should be acknowledged. First, there was a substantial loss to follow-up, with nearly one-third of eligible patients being nonresponders. However, there were no significant differences between responders and nonresponders in terms of age, body mass index, injury mechanism, or injury pattern. Also, there was a large spread in age (21-72 years), indicating quite different preinjury conditions for physical function. Furthermore, the cross-sectional design limits the ability to capture individual recovery patterns or changes over time. Importantly, this study lacked detailed information on the quality, frequency, and adherence to rehabilitation, all of which are critical factors influencing the recovery of strength and function. Greater insight into its nature would be valuable for better interpreting the results and guiding clinical practice. Most injuries in this cohort were sports related, with fewer resulting from high-energy trauma such as motor vehicle accidents. As high-energy MLKIs are generally associated with more extensive tissue damage and neurovascular injury, our findings may not fully generalize to populations with a higher proportion of such injuries, where long-term strength and functional outcomes could be worse.

Conclusion

Persistent deficits in quadriceps and hamstring muscle strength were observed 7 years after MLKI surgery, with a substantial proportion of patients failing to regain full functional symmetry in muscle strength and hop test performance. Patients with bicruciate injuries demonstrated significantly more pronounced deficits in muscle strength and hop test scores as compared with those with single cruciate injuries.