Anterior Cruciate Ligament Bundles Elongation Patterns During Fast Running and Single-Leg Drop Landing: An In Vivo Biplane Radiography Study

Abstract

Background:

Understanding the anterior cruciate ligament (ACL) anteromedial bundle (AMB) and posterolateral bundle (PLB) elongation patterns during physiologic loading may inform graft-tensioning strategies in double-bundle ACL reconstructions. However, limited evidence exists regarding their in vivo elongation during high-impact activities.

Purpose:

To quantify the relative elongation of AMB and PLB during fast running and single-leg drop landing and to identify the knee flexion angle at peak elongation.

Study Design:

Descriptive laboratory study.

Methods:

Nineteen healthy athletes performed fast running and single-leg drop landing within a biplane radiography system. Tibiofemoral motion was tracked using a validated model-based tracking process that matched computed tomography–based subject-specific 3-dimensional bone models to synchronized biplane radiographs. ACL femoral and tibial attachment points were identified on magnetic resonance imaging and registered to the bone models. AMB and PLB lengths were calculated and normalized to lengths during supine computed tomography to determine relative elongation. Fast running kinematics were analyzed over the first 60% of stance (0% = initial contact, 100% = toe-off). Single-leg drop landing kinematics were analyzed from initial contact (0%) to maximum knee flexion (100%).

Results:

During fast running, AMB peak relative elongation was significantly greater than PLB (mean difference, 2.6%; 95% CI, 1.5-3.6; P < .001). During single-leg drop landing, PLB peak relative elongation was comparable with AMB (mean difference, –0.9%; 95% CI, –2.2% to 0.4%; P = .17). Peak relative elongation occurred at 29° (AMB) and 27° (PLB) of knee flexion during fast running (mean difference, 1.6°; 95% CI, 0.2°-3.0°; P = .029) and at 14° (AMB) and 8° (PLB) of knee flexion during single-leg drop landing (mean difference, 5.9°; 95% CI, 2.5°-9.4°; P < .001).

Conclusion:

AMB elongates more than PLB during fast running, whereas PLB elongates to a similar extent as AMB during single-leg drop landing. Peak elongations occur simultaneously during fast running, while PLB elongates earlier than AMB during single-leg drop landing.

Clinical Relevance:

These findings suggest that fixing AMB at 14° of knee flexion and PLB at a slightly lower angle may better reproduce native bundle function during high-impact activities.

Keywords

anterior cruciate ligament biplane radiography elongation high-impact activities

The anterior cruciate ligament (ACL) is a vital structure within the knee joint, responsible for maintaining stability by resisting anterior tibial translation and internal rotation of the tibia relative to the femur during daily and athletic activities. Anatomically, the ACL is well recognized to consist of 2 distinct bundles: the anteromedial bundle (AMB) and the posterolateral bundle (PLB).^17,25 These bundles function synergistically, with their specific biomechanical roles varying depending on the degree of knee flexion. Cadaveric studies have demonstrated that the AMB remains taut throughout knee extension to flexion, whereas the PLB is most elongated in knee extension and slackens by 5 to 6 mm during flexion.^1,13,15 It is further believed that the AMB is most important for resisting tibial anterior drawer, and the PLB has a role in controlling the internal-external torque response of the tibia or functions synergistically with the AMB against torque applied to the tibia.^1,23,27 These findings have laid the foundation for the development of the anatomic double-bundle ACL reconstruction technique, which aims to restore the native biomechanical properties of both bundles. While it has been reported that there is no difference in outcomes between single-bundle and double-bundle ACL reconstruction,^28,29,35 the biomechanical superiority of anatomic double-bundle ACL reconstruction, such as postoperative stability, has been reported in multiple studies.^{3,7,24,30,38,40} In ACL reconstruction, the knee flexion angle at the time of graft tensioning is a critical factor for ensuring proper graft function and achieving good knee joint stability.^6,19 However, there is no consensus on the optimal knee flexion angles at which each bundle should be fixed during surgery.

In vivo studies using dual fluoroscopy have further contributed to our understanding of ACL biomechanics, evaluating the elongation patterns of the AMB and PLB during dynamic movements such as weightbearing flexion and gait.^9,20,31,39 These studies consistently demonstrate that the bundles exhibit anisometric behavior, with task-dependent elongation patterns influenced by knee flexion and other tibiofemoral kinematics. Notably, Nagai et al³¹ reported significant differences between walking and running conditions in their investigation of relative AMB and PLB elongation patterns, highlighting the complexity of ACL biomechanics during higher-intensity exercise. Rehabilitation after ACL reconstruction typically progresses from low-intensity activities such as walking to more demanding tasks like running, eventually leading to return to sport. However, despite the clinical importance of understanding ACL behavior during advanced athletic tasks, there is limited evidence on the in vivo elongation patterns of the AMB and PLB during high-impact activities such as fast running and landing. Addressing this gap is crucial for optimizing both rehabilitation protocols and surgical techniques, ensuring a more comprehensive restoration of knee function.

Therefore, the purpose of this study was to measure peak relative elongation of the AMB and PLB and to identify knee kinematics at peak elongation during high-impact activities such as fast running and single-leg drop landing. It was hypothesized that there would be no difference in peak relative elongation and knee kinematics at peak elongation between the AMB and PLB.

Methods

Participants

This study was approved by the Institutional Review Board of the University of Pittsburgh (STUDY19080016). Nineteen participants provided informed written consent and were enrolled in this study. All participants were healthy collegiate athletes with no history of knee injury who were active in sports that require running, jumping, and/or cutting. Knee injury history was assessed through participant self-report and confirmed by reviewing magnetic resonance imaging (MRI) to exclude structural abnormalities. All participants were varsity-level or, if the university did not have a varsity team in their sport, top-level club athletes. Demographic data of all participants are summarized in Table 1.

Table 1

Demographic Data of Participants ^a

Characteristic	Total	Male	Female
Number	19	11	8
Age, y	20.1 ± 1.3	19.9 ± 1.2	20.4 ± 1.5
Height, cm	174.2 ± 10.1	182.0 ± 3.5	163.5 ± 5.3
Weight, kg	72.5 ± 11.0	77.4 ± 8.2	65.8 ± 10.6
BMI, kg/m²	24.0 ± 2.8	23.5 ± 2.1	24.8 ± 3.4
Sports, No.
Track and field		4	2
Ultimate Frisbee		3	2
Rugby		2
Softball			2
Cross-country		1
Tennis			1
Soccer		1
Lacrosse			1

Values are presented as the mean ± SD unless otherwise indicated. BMI, body mass index.

Collecting and Processing Dynamic In Vivo Kinematics Data

All data collection and processing were conducted in the University of Pittsburgh Biodynamics Laboratory. Each athlete performed fast-running and single-leg drop landing within a custom biplane radiography system as previously reported (Figure 1).^{14,31,33,34,36} Synchronized biplane radiographs were collected at 150 images per second (imaging parameters: 90 kV, 160 mA maximum, 1-ms pulse width) during each activity (Figure 2B). Ground-reaction forces were acquired using a dual-belt instrumented treadmill (Bertec), sampling at 900 Hz, and used to identify the stance phase in each activity based on a 50-N threshold. Fast running was performed at 5.0 m/s, and the single-leg drop landing was performed from a 20-cm platform while looking straight ahead with the arms extended and hands interlocked. Three trials were collected, processed, and analyzed for each side during fast running and the single-leg drop landing. Bone geometry was reconstructed from high-resolution computed tomography (CT) (GE Lightspeed 16; GE Medical Systems) (0.322 mm/pixel, 0.625 mm thickness, resliced to isometric voxels) (Figure 2C). Commercial software was used to identify bone tissue within each scan (Mimics; Materialize), and 3-dimensional models of each femur and tibia were created separately for the left and right knees. Anatomic coordinate systems were established for the right-side femur and tibia, as previously described,¹⁴ and mirror imaged onto the left side after coregistering the corresponding right and left bones for each participant (Figure 2D). Knee motion was determined for each trial using a previously validated volumetric model-based tracking process that matches a digitally reconstructed radiograph generated from the CT-based bone model to the biplane radiographs with a validated accuracy of 0.7 mm or better in translation and 0.9° or better in rotation (Figure 2E).² Tibiofemoral rotations were calculated using ordered rotations as described by Grood and Suntay,¹⁶ and translations were calculated as the vector from the center of the femoral condyles to the center of the tibial plateau and expressed in the tibial coordinate system (Figure 2F).¹⁴ Fast running kinematic data were analyzed over the first 60% of the stance phase (0% = initial contact, 100% = toe-off). Single-leg drop landing kinematic data were analyzed during the knee flexion eccentric phase, with 0% being the initial landing and 100% being maximum knee flexion. All kinematics data were normalized to the CT knee position. Knee flexion was calculated by subtracting this CT-based flexion angle from the dynamically acquired flexion angle to account for this initial posture. Kinematics data from each trial were interpolated to 1% instants of the activity and then averaged over all trials, creating an average waveform for each of 38 knees during each activity. Radiation exposure associated with biplane radiography was estimated at 0.27 mSv (PCXMC, STUK—Radiation and Nuclear Safety Authority), while radiation exposure from the CT scan averaged 1.27 mSv.

Figure 1.

Still photos of fast running and single-leg drop within the biplane radiography system. (A) Fast running was performed at 5.0 m/s on an instrumented treadmill. (B) Single-leg drop landing was performed from a 20-cm platform, while looking straight ahead with arms extended and hands interlocked.

Figure 2.

Data collection and processing using the biplane radiography system. (A) Participants performed fast running and single-leg drop landing within a biplane radiographic imaging system. (B) Synchronized biplane radiographs were collected at 150 Hz. (C) Subject-specific 3-dimensional (3D) bone models of the bilateral femur and tibia were created from a high-resolution computerized tomography scan. (D) Landmarks were placed on the 3D bone models to establish an anatomic coordinate system. (E) A validated volumetric matching process was used to match the digitally reconstructed radiographs to the distortion-corrected biplane radiographs. (F) Joint kinematics were calculated according to the anatomic coordinate systems.

Identification of the Centroid of the AMB and PLB

The knees were imaged by 3.0-T MRI using a T2-weighted DE3D sequence (0.29 × 0.29 × 0.30 mm/voxel) (Prisma Fit; Siemens). The femoral and tibial bones were segmented in the MRI images and registered to the CT-based bone models, as previously described.^31-34,36 The boundaries of the femoral and tibial ACL insertions were identified using high-resolution 3T MRI with the Mimics software (Materialize), as previously reported.⁴ Briefly, the boundaries of the whole femoral and tibial ACL insertions were manually identified using axial, coronal, and sagittal planes. For the identification process, the ligament-bone interface was manually marked by iteratively placing points on the insertion site boundary in each slice and adjusting their positions by comparing them to neighboring image slices. A previous validation of this technique indicated that the MRI-estimated centroids of the ACL insertion were biased on average 0.6 ± 1.6 mm proximally and 0.3 ± 1.9 mm posteriorly for the femur, as well as 0.3 ± 1.1 mm laterally and 0.5 ± 1.5 mm anteriorly for the tibia, compared with the “gold standard” centroids measured by the laser scanner.⁴

The separation of AMB and PLB insertions was conducted according to previously documented anatomic studies of ACL insertions.^{11,12,17,25,37} For the femoral ACL insertion, which has an oval shape, the division into AMB and PLB was made at the midpoint (50/50%) along the principal longitudinal axis of the entire ACL insertion using a custom MATLAB script (Matlab Version 2023a, The MathWorks Inc.). Similarly, the tibial ACL insertion was divided into AMB and PLB at the midpoint (50/50%), with the division line perpendicular to the projected mid-substance axis of the ACL.³¹ The reliability of this method for identifying tibial insertions of the AMB and PLB was verified using previously published data.³⁷ The centroids of the AMB and PLB areas were calculated and then mapped to CT models by performing a bone-to-bone registration from MRI to CT.

Calculation of the Relative Elongation of the ACL (AMB and PLB)

The centroid-to-centroid distance of the AMB and PLB was derived from the tracked movements of the femur and tibia during both fast running and single-leg drop landing. These distances were normalized relative to their lengths in full knee extension, as measured on CT, to calculate the ACL relative elongation (%), similar to the kinematics normalization. The relative elongation of the AMB and PLB was assessed at 1% intervals throughout the motion. To evaluate within-subject variability in relative ACL elongation (%), the standard deviation of the relative elongation at each percentage of the gait cycle was computed for each individual across multiple trials and then averaged across all participants. For each participant, the maximum relative elongations of the AMB and PLB during fast running and single-leg drop landing were determined, and the corresponding percentage of the movement and knee flexion angle at which these maxima occurred was recorded.

Data Analysis

A total of 57 running and 114 drop landing trials were included in this analysis. The knee kinematics included in this analysis comprised knee flexion, anterior-posterior (A-P) translation, and internal-external rotation. Generalized estimating equations were used to test for AMB versus PLB differences in (1) peak relative elongation and (2) tibiofemoral flexion angle at peak elongation while accounting for the correlation between left and right knees within each participant. All statistical analysis was completed using SPSS software v27.0 (SPSS, Inc). Significance level was set as P < .05. Based on 38 knees included in the analysis, α set at .05, and 80% power, the sample size was sufficient to detect a moderate to large effect size (Cohen’s f² = 0.28).

Results

Participants’ average age was 20.1 ± 1.3 years, and mean body mass index was 24.0 ± 2.8 kg/m². The relationship of the relative elongation, timing, and relative flexion angle of both activities is shown in Figure 3. In the present cohort, the average knee flexion angle during CT scan was 6.5°± 3.5°. Qualitatively, both bundles elongated as the knee flexed during fast running, and both the AMB and the PLB showed a pattern of peak elongation around 40% to 60% of stance at approximately 30° of knee flexion. On average, the PLB showed little elongation, with a relative elongation of around 0%, even at peak elongation. During the single-leg drop landing, the peak PLB elongation occurred during the first 20% of the landing time and at knee flexion angles of around 5°, while the peak AMB elongation occurred at approximately 40% of the landing time and at knee flexion angles of around 20°.

Figure 3.

The relationship of the relative elongation, timing, and relative flexion angle during fast running (A) and single-leg drop landing (B). Mean relative elongation of each bundle (AM, red; PL, blue) is shown on the left vertical axis, and relative tibiofemoral flexion angle (black) is shown on the right vertical axis during each movement. The 0% relative elongation is the resting anterior cruciate ligament bundle length during supine computed tomography. The shaded area represents 1 standard deviation. Arrows in corresponding colors denote the peak elongation points of the AM and PL bundles. AM, anteromedial; PL, posterolateral.

The relationship of the relative elongation, timing, and relative A-P translation of both activities is shown in Figure 4. Qualitatively, elongations of both bundles matched the A-P translation pattern for fast running. In contrast, during the single-leg drop landing, the AMB and PLB elongation decreased even in the presence of gradually increasing A-P translation.

Figure 4.

The relationship of the relative elongation, timing, and relative anterior-posterior translation during fast running (A) and single-leg drop landing (B). Mean relative elongation of each bundle (AM, red; PL, blue) is shown on the left vertical axis, and anterior-posterior translation (black) is shown on the right vertical axis during each movement. Positive values represent anterior tibial translation relative to the femur. The 0% relative elongation is the resting anterior cruciate ligament bundle length during supine computed tomography. The shaded area represents 1 standard deviation. Arrows in corresponding colors denote the peak elongation points of the AM and PL bundles. AM, anteromedial; PL, posterolateral.

The relationship of the relative elongation, timing, and relative internal-external rotation of both activities is shown in Figure 5. Qualitatively, during fast running, the AMB and PLB elongated as the tibia internally rotated. In contrast, during single-leg drop landing, the average PLB peak elongation occurred at around 6° of tibial internal rotation, while the average AMB peak elongation occurred at approximately 13° of tibial internal rotation. Both AMB and PLB shortened as the tibia rotated internally.

Figure 5.

The relationship of the relative elongation, timing, and relative internal-external rotation during fast running (A) and single-leg drop landing (B). Mean relative elongation of each bundle (AM, red; PL, blue) is shown on the left vertical axis, and internal-external rotation (black) is shown on the right vertical axis during each movement. Positive values for internal-external rotation represent internal tibial rotation relative to the femur, and negative values represent external rotation. The 0% relative elongation is the resting anterior cruciate ligament bundle length during supine computed tomography. The shaded area represents 1 standard deviation. Arrows in corresponding colors denote the peak elongation points of the AMB and PLB. AM, anteromedial; PL, posterolateral.

The quantitative summary of the relationship between peak elongation and tibiofemoral kinematics for each activity is shown in Table 2. During fast running, the peak AMB relative elongation (4.3%; 95% CI, 1.9 to 6.7) was significantly greater than the peak PLB relative elongation (1.7 %; 95% CI, –1.3 to 4.7) (mean difference, 2.6%; 95% CI, 1.5 to 3.6; P < .001). The peak AMB relative elongation occurred at 29° (95% CI, 27°-31°) of knee flexion, and the peak PLB relative elongation occurred at 27° (95% CI, 25°-30°) of knee flexion (mean difference, 1.6°; 95% CI, 0.2°-3.0°; P = .029). The peak AMB relative elongation occurred at 4.9 mm (95% CI, 4.4-5.5 mm) of A-P translation, and the peak PLB relative elongation occurred at 4.6 mm (95% CI, 3.9-5.3 mm) of A-P translation (mean difference, 0.4 mm; 95% CI, 0.1-0.7 mm; P = .008). The peak AMB relative elongation occurred at 14.7° (95% CI, 13.7° to 15.8°) of internal rotation, and the peak PLB relative elongation occurred at 13.9° (95% CI, 12.6° to 15.1°) of internal rotation (mean difference, 0.9; 95% CI, –0.01° to 1.7°; P = .053).

Table 2

Relationship Between Peak Elongation, Flexion Angle, Anterior-Posterior Translation, and Internal-External Rotation of Peak Elongation for Each Activity ^a

	Fast Running				Single-Leg Drop Landing
Characteristic	AMB	PLB	Mean Difference	P Value	AMB	PLB	Mean Difference	P Value
Peak elongation, %	4.3 (1.9 to 6.7)	1.7 (–1.3 to 4.7)	2.6 (1.5 to 3.6)	<.001	6.7 (4.7 to 8.7)	7.6 (5.2 to 10.1)	−0.9 (–2.2 to 0.4)	.17
Flexion angle of peak elongation, deg	29 (27 to 31)	27 (25 to 30)	1.6 (0.2 to 3.0)	.029	14 (11 to 18)	8 (5 to 12)	5.9 (2.5 to 9.4)	<.001
Anterior-posterior translation of peak elongation, mm	4.9 (4.4 to 5.5)	4.6 (3.9 to 5.3)	0.4 (0.1 to 0.7)	.008	4.0 (3.6 to 4.5)	3.1 (2.7 to 3.6)	0.9 (0.4 to 1.4)	<.001
Internal-external rotation of peak elongation, deg	14.7 (13.7 to 15.8)	13.9 (12.6 to 15.1)	0.9 (–0.01 to 1.7)	.053	10.9 (9.2 to 12.5)	8.2 (6.5 to 9.8)	2.7 (1.3 to 4.2)	<.001

Values are presented as mean (95% CI). AMB, anteromedial bundle; PLB, posterolateral bundle.

During single-leg drop landing, the peak PLB relative elongation (7.6%; 95% CI, 5.2% to 10.1%) was comparable with the peak AMB relative elongation (7.6%; 95% CI, 4.7% to 8.7%), with no statistically significant difference between them (mean difference, –0.9%; 95% CI, –2.2% to 0.4%; P = .17). The peak AMB relative elongation occurred at 14° (95% CI, 11°-18°) of knee flexion, and the peak PLB relative elongation occurred at 8° (95% CI, 5°-12°) of knee flexion (mean difference, 5.9°; 95% CI, 2.5°-9.4°; P < .001). The peak AMB relative elongation occurred at 4.0 mm (95% CI, 3.6-4.5 mm) of A-P translation, and the peak PLB relative elongation occurred at 3.1 mm (95% CI, 2.7-3.6 mm) of A-P translation (mean difference, 0.9; 95% CI, 0.4-1.4; P < .001). The peak AMB relative elongation occurred at 10.9° (95% CI, 9.2°-12.5°) of internal rotation, and the peak PLB relative elongation occurred at 8.2° (95% CI, 6.5°-9.8°) of internal rotation (mean difference, 2.7°; 95% CI, 1.3°-4.2°; P < .001).

Discussion

This study demonstrates that the relationship between ACL bundle elongation and knee kinematics is complex and activity-dependent. For example, the relative elongation of the AMB was significantly greater than the PLB in fast running, whereas the relative elongation of the PLB was comparable with AMB in single-leg drop landing. Furthermore, AMB and PLB reached peak elongation at approximately the same time (29° and 27° of knee flexion, respectively) during fast running. In contrast, the PLB reached its peak elongation at 8° of knee flexion, which preceded the AMB peak elongation that occurred at 14° during the single-leg drop landing. The differences in AMB and PLB elongation patterns observed in this study improve our understanding of in vivo ACL dynamics and could be used to develop more informed strategies in graft fixation in double-bundle ACL reconstruction.

To our knowledge, this study represents the first to evaluate the elongation of the AMB and PLB during high-impact activities, such as fast running and single-leg drop landing. The single-leg drop landing is an important activity used in clinical practice in the return-to-sport protocol after ACL reconstruction,¹⁸ and single-leg landing is a common event associated with ACL injury.^22,26 Previous studies have reported ACL elongation patterns through the gait cycle,^9,31,38 jogging,³¹ and lunge²⁰ motions. However, high-impact activities such as fast running and single-leg drop landing induce comparatively large external forces (ground-reaction force) and intersegmental forces (muscle) applied to the knee. In this study, the PLB demonstrated a greater peak elongation during single-leg drop landing than during fast running, while the AMB showed similar elongation magnitudes across both tasks. These findings collectively suggest that the single-leg landing induces greater relative elongation on each bundle compared with fast running.

The timing of peak elongation provides critical insight into the loading patterns experienced by the ACL during different dynamic tasks. Englander et al⁸ evaluated ACL strain in single-leg jumping using high-speed biplanar radiography and reported that peaks in ACL strain were observed just before takeoff and during mid-flight. The same group has reported that peak ACL elongation during gait occurs during the late swing phase, just before foot strike, followed by a rapid decrease in ACL elongation just after foot strike.^9,10 These results were counterintuitive because they indicated that during jumping and during gait, the ACL was most elongated in the air and during late swing, rather than after contact, when ground-reaction forces and muscle forces increase and when ACL injury most often occurs. In contrast, our data from single-leg landings indicate that the AMB and PLB were both elongated at landing, and the elongation increased slightly immediately after landing. The timing differences observed are biomechanically significant because noncontact ACL injuries often happen in the very early phase of landing. When comparing the AMB to PLB relative elongation patterns, Englander et al^9,10 reported that the AMB was more elongated than the PLB over the entire gait cycle. Nagai et al³¹ found that the maximum relative elongation of the AMB was greater during running than during walking, whereas the maximum relative elongation of the PLB was greater during walking than during running. This result was attributed to the knee being more extended during walking than during running. In the current study, during fast running, the PLB did not elongate beyond its resting length, as measured on CT images, while the AMB exhibited a maximum elongation of approximately 2%. In contrast, during single-leg drop landing, both the AMB and PLB exhibited a maximum elongation of 6% to 7%. The differences in relative elongation between activities suggest a discrepancy in the loading on the ACL fibers, and the single-leg drop landing was shown to increase the PLB elongation more, even at the same knee flexion angle. In clinical settings, the likelihood of ACL injury solely from fast running is low; however, ACL injuries are more common during single-leg landing and sudden changes in direction.⁵ Our finding that the PLB experiences its peak elongation early in the single-leg drop landing is consistent with this injury mechanism: the PLB is immediately recruited to restrain the anterior tibial movement and internal rotation when the foot strikes the ground. The earlier and rapid elongation of the PLB during landing observed in this study may therefore represent the ligament's first line of defense against injury-relevant forces. In contrast, the simultaneous peak elongations in running may lessen the injury risk. This temporal insight underscores the importance of the time window shortly after foot strike as a critical period for ACL bundle loading and potential injury.

Our findings carry important implications for anatomic ACL reconstruction, particularly for double-bundle graft techniques. The distinct elongation behavior of the AMB and PLB in vivo suggests that each bundle should be tensioned at a knee flexion angle that reflects its natural function. The results of this study suggest that tensioning the AMB at approximately 14° of knee flexion and the PLB at a slightly lower angle, with both grafts fixed in a slightly internally rotated tibial position, might allow for a more accurate reproduction of the natural function of the AMB and PLB during high-impact activities like single-leg drop landing.

This study has several limitations. First, the ACL length was normalized by the origin-to-insertion distance during CT in full extension; thus, only relative ACL elongation could be measured. Determining ACL slack length in vivo remains an unsolved challenge in biomechanics research, but normalizing the length to a static position is a common method for estimating ligament lengthening during dynamic activity.^{9,21,31,33,34} Furthermore, because it is difficult to completely distinguish between AMB and PLB insertion sites on MRI, the insertion sites were calculated and analyzed according to previous reports on the anatomic structure of ACL insertion sites.^31,37 Finally, it is important to recognize that kinematics during laboratory-based trials may differ from those that occur during athletic competition. These tests did not include cutting and pivoting activities that may be related to ACL injury, nor did they assess the effects of fatigue on ACL elongation.

Conclusion

ACL bundle elongation is activity-dependent: the relative elongation of the AMB was significantly greater than the PLB in fast running, whereas the relative elongation of the PLB was comparable with the AMB in single-leg drop landing. The peak elongation was simultaneous during fast running, while PLB elongated earlier than the AMB during single-leg drop landing. In addition, the maximum elongation of each bundle was greater during single-leg drop landing compared with fast running. These results provide a better understanding of how much and when the AMB and PLB are elongated during high-impact activities. Based on our findings, tensioning the AMB at approximately 14° of knee flexion and the PLB at a slightly lower flexion angle, with both grafts fixed in slight internal rotation, may allow for a more accurate reproduction of the native AMB and PLB functions.

Footnotes

Final revision submitted August 28, 2025; accepted October 8, 2025.

One or more of the authors has declared the following potential conflict of interest or source of funding: V.M. has received a grant from the National Institutes of Health and Department of Defense, has received consulting fees from Newclip and Smith & Nephew, holds stock or stock options with Ostesys, is a board member of the ACL Study Group, and has a patent (US Patent No. 9,949,684, issued on April 24, 2018, to the University of Pittsburgh). AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Ethical approval for this study was obtained from the University of Pittsburgh (STUDY19080016).

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