Study Notes on Biomechanical Alterations in the Unweight Phase of the Single-Leg Countermovement Jump After ACL Reconstruction

Authors: Roberto Ricupito, Marco Bravi, Fabio Santacaterina, Giandomenico Campardo, Riccardo Guarise, Rosalba Castellucci, Ismail Bouzekraoui Alaoui, Florian Forelli
Received: 10 July 2025
Revised: 28 July 2025
Accepted: 29 July 2025
Published: 30 July 2025
Citation: Ricupito et al. (2025). Biomechanical Alterations in the Unweight Phase of the Single-Leg Countermovement Jump After ACL Reconstruction. J. Funct. Morphol. Kinesiol., 10, 296. https://doi.org/10.3390/jfmk10030296
Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. Open access under CC BY license.

Anterior cruciate ligament reconstruction (ACLr) often results in limb asymmetries and variable athlete return-to-performance rates.
The Single-Leg Countermovement Jump (SLCMJ) is commonly employed to evaluate postoperative knee function.
Limited research has focused specifically on deficits during the unweighting phase of the jump.

The study included 53 recreational athletes (11 females, 42 males), each 6 to 9 months post-ACLr.
Utilized a dual force plate system (1000 Hz) to measure jump metrics: jump height, negative peak velocity, minimum force, and center of mass (COM) displacement.
Employed paired t-tests and Wilcoxon tests for comparative analysis between ACLr limb and the contralateral limb.

Findings indicated that the ACLr limb exhibited:
  - Lower negative peak velocity (−0.80 ± 0.40 m/s vs. −0.94 ± 0.40 m/s, p < 0.001).
  - Higher minimum force (36.75 ± 17.88 kg vs. 32.05 ± 17.25 kg, p < 0.001).
  - Reduced COM displacement (−17.62 ± 6.25 cm vs. −19.73 ± 5.34 cm, p = 0.014).
Eccentric phase duration remained statistically unchanged between limbs.

Altered neuromuscular control in early SLCMJ phases post-ACLr emphasizes rehabilitation focus on eccentric strength and restoration of symmetry.

ACL injuries are prevalent across various sports with return-to-competition rates ranging from 55% to 81%.
Re-injury risks fluctuate between 1.5% and 37.5%.
Injuries predominantly occur through deceleration, particularly during landings or directional changes, influenced by both cognitive and physical perturbations.
Functionality on deceleration is essential for injury prevention; this ties to the eccentric function of the neuromusculoskeletal system to manage kinetic energy.
An increase in loading of the non-reconstructed limb may lead to contralateral ACL injuries.
Deficits in eccentric strength or muscle inhibition can hinder jumping mechanics and jumping performance.
As the SLCMJ is a key assessment tool emphasizing knee function, it reflects the neuromuscular strategy during vertical jumps and is associated with athletic performance metrics.

A multicenter retrospective study was conducted, spanning December 2023 to January 2024 across physiotherapy clinics in Rome, Italy, approved by Ramsay Healthcare's Ethical Committee.
Sample size estimation through Monte Carlo simulations due to insufficient existing normative data on SLCMJ metrics.

Inclusion:
- Adults (> 18 years) post primary ACLr (6-9 months) with specific grafts (patellar tendon, hamstring tendon, etc.) and Tegner Activity Scale scores ranging 5 to 10.
Exclusion:
- Allograft reconstructions, revision procedures, or patients with additional knee surgeries.

Specific unweight phase is characterized by a decline in vertical force which initiates the jump through relaxation of agonist muscles.
The unweighting phase initiates when vertical force dips below a threshold derived from 5 times the standard deviation of body weight measured in the weighing phase.
A standardized warm-up procedure precedes SLCMJ trials, ensuring a two-jump trial measurable by dual force plates.

Jump Height (VJH):
- Calculated via the impulse-momentum relationship. Formula:
$VJH = \frac{Take\ Off\ Velocity^2}{2 \times 9.81 m/s^2}$
Negative Peak Velocity:
- Derived from acceleration integration during the unweighting phase. Formula:
$v_{peak-negative} = \int_{t_0}^{t_1} a(t)dt$
Minimum Force and COM Displacement:
  - Minimum force reflects the lowest vertical ground reaction force during the jump. Formula:
   $F_{min} = \min(F_z(t)), t \in [t_0, t_1]$
  - COM displacement is tracked as:
   $\Delta y_{COM} = \int_{t_0}^{t_1} v(t)dt$

Descriptive analyses to characterize the population, employing t-tests for normally distributed variables and Wilcoxon tests for non-parametric data.
ANOVA and the Kruskal–Wallis test were used for categorical analysis across varied metrics.
Significance threshold set at p < 0.05.

The study included 52 patients (41 males, 11 females) post-ACLr, revealing significant differences across various metrics between ACLr and healthy limbs such as jump height, peak negative velocity, minimum force, and COM displacement, while no significant differences appeared in eccentric phase duration.

The study identifies persistent asymmetries in vertical jump mechanics indicating compensatory strategies that might affect performance.
Key findings demonstrate alteration in unweighting strategies, emphasizing the importance of continuous assessment post-ACLr.

The study reveals significant biomechanical alterations in athletes after ACL reconstruction, highlighting the necessity for targeted rehabilitation strategies aimed at restoring symmetry and efficacy in movement patterns.