# Quadriceps muscle strength of adolescents recovers faster than those of adults after anterior cruciate ligament reconstruction using hamstring graft

Marc Dauty, Loïc Geffroy, Antoine Chalopin, Pierre Menu, Jérôme Grondin, Thomas Hirardot, Vincent Crenn, Alban Fouasson-Chailloux

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## ► To cite this version:

Marc Dauty, Loïc Geffroy, Antoine Chalopin, Pierre Menu, Jérôme Grondin, et al.. Quadriceps muscle strength of adolescents recovers faster than those of adults after anterior cruciate ligament reconstruction using hamstring graft. Journal of ISAKOS, 2025, 12, pp.100901. 10.1016/j.jisako.2025.100901. hal-05328063

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Submitted on 23 Oct 2025

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Original Research

# Quadriceps muscle strength of adolescents recovers faster than those of adults after anterior cruciate ligament reconstruction using hamstring graft

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Marc Dauty<sup>a, b, c</sup>, Loïc Geffroy<sup>a, d</sup>, Antoine Chalopin<sup>e</sup>, Pierre Menu<sup>a, b, c</sup>, Jérôme Grondin<sup>a, c</sup>, Thomas Hirardot<sup>a, c</sup>, Vincent Crenn<sup>f</sup>, Alban Fouasson-Chailloux<sup>a, b, c, *</sup>

ARTICLE INFO

Keywords:

Anterior cruciate ligament reconstruction

Adolescents

Muscle inhibition

Sport

ABSTRACT

**Objectives:** Knee muscle strength recovery after anterior cruciate ligament reconstruction is usually evaluated using isokinetic limb symmetry index to help take the decision to allow return to the previous sport. However, the role of age on knee strength recovery is not well known. So, we aimed to compare the isokinetic knee muscle strength recovery according to age at 4 and 7 months after anterior cruciate ligament reconstruction using hamstring graft.

**Methods:** From a historical cohort (2018-2023) of patients who completed isokinetic monitoring at 4 and 7 months after anterior cruciate ligament reconstruction using hamstring graft, two groups were compared according to age: ≤19 and > 20 years old. The quadriceps and hamstring limb symmetry indexes were studied as the main parameters. A multivariate analysis was realized to assess the weight of potential inter-subject explanatory parameters on knee strength evolution.

**Results:** Two hundred fifty-three adolescents were compared with 247 adults. The adolescent quadriceps symmetry index at 60°/s was significantly higher than that of adults (79% vs 72% and 88% vs 82% at 4 and 7 months of follow-up, respectively). The hamstring symmetry index was not statistically significantly different. No inter-subject explanatory parameters were found after the multivariate analysis.

**Conclusion:** Knee strength recovers faster in adolescents than in adults after anterior cruciate ligament reconstruction using hamstring graft. This result should be carefully considered because adolescents could be exposed to return to sport too early, while the risk of graft rupture is still high, if strength cutoff is only considered.

**Level evidence:** III.

What are the new findings?

* Quadriceps muscle strength of young athletes (≤19 years old) recovers faster than those of adults after anterior cruciate ligament reconstruction using hamstring graft.

* The return to sport could be quicker in young athletes depending on isokinetic parameters.

* Other parameters must be taken into consideration in young athletes due to the higher risk of anterior cruciate ligament and graft rupture compared with adults.

<footnote><sup>a</sup> Nantes Université, Médecine Physique et Rédadaptation Locomotrice et Respiratoire, Médecine du sport, CHU Nantes, 44093 Nantes, France</footnote>

<footnote><sup>b</sup> Inserm, UMR 1229, RMeS, Regenerative Medicine and Skeleton, ONIRIS, Nantes Université, 44042 Nantes, France</footnote>

<footnote><sup>c</sup> Institut Régional de Médecine du Sport (IRMS) des Pays de Loire, 44093 Nantes, France</footnote>

<footnote><sup>d</sup> Santé Atlantique, Chirurgie Orthopédie Pédiatrique, Avenue Claude Bernard, 44800 Saint-Herblain, France</footnote>

<footnote><sup>e</sup> Nantes Université, Chirurgie Infantile, CHU Nantes, 44093 Nantes, France</footnote>

<footnote><sup>f</sup> Nantes Université, Clinique Chirurgicale Orthopédique et Traumatologique, CHU Nantes, 44093 Nantes, France</footnote>

<footnote>* Corresponding author. Locomoteur et respiratoire, Hôpital Saint Jacques, 85 rue Saint Jacques, cedex 01, 44035 Nantes, France. Tel.: +33 2 40 84 62 11. E-mail address: alban.fouassonchailloux@chu-nantes.fr (A. Fouasson-Chailloux).</footnote>
## INTRODUCTION

The incidence of anterior cruciate ligament (ACL) tear is estimated at 68.6/1,00,000 person-years in the US population and is higher in males than in females [1]. Age-specific patterns differ according to sex, with a peak of incidence between 19 and 25 years in males and a peak of incidence between 14 and 18 years in females [1]. After ACL injury, more and more patients choose surgical reconstruction to return to sport [1]. The rate of ACL reconstruction (ACLR) is estimated to be ranged from 54.0 to 77.4 per 1,00,000 person-years in Australia with a peak in males aged from 20 to 24 years old [2]. In adolescents, the rates of ACLR per 1,00,000 person-years increased from 61.4 in 2002 to 74.6 in 2014 [3]. The peak of incidence was observed in females aged 10 to 19 years [2-4], and females and males aged from 13 to 17 years had the greatest absolute increase of ACLR rates (females: 1.8% per year; males: 0.96% per year) [4]. In adults, 65% return to sport, while in adolescents, the rate is higher at 90%, but with a higher risk of graft rupture [5-7]. The decision-making process to return to sport is based on recommendations secondary to clinical experience or review reports [8]. A higher quadriceps strength is associated with the return to sport [8,9]. In adolescents, the quadriceps and hamstring strength limb symmetry index (LSI) ≥90% and the single-hop test LSI ≥90%, were associated with the return to sport and to maintain at the same level during the year following the return to sport [10,11]. Because knee strength is used as one of the criteria to return-to-sport activities, different factors affecting the quadriceps strength recovery were studied [12-15]. The postoperative quadriceps strength was significantly and conversely associated with age at six months post-ACLR using hamstring graft in adults [13,15], and the younger patients could achieve better quadriceps strength recovery [14]. A risk of muscle strength recovery delay was reported in older patients [12]. So, the aim of this study was to confirm or not this tendency by comparing the quadriceps and hamstring limb symmetry index recovery at 4 and 7 months between adolescents and adults after ACLR using hamstring graft.

## METHODS

A cohort of 1,096 patients with primary ACLR followed from 2018 to 2023 was considered in this retrospective study. Each participant, or parents when <18 years old, gave written consent. The ethics committee, Groupe Nantais d'Ethique dans le Domaine de la Santé (GNEDS) approved the protocol under the registration number: 25-16-02-161. The study was conducted in accordance with the Helsinki Declaration [16].

### Population

The patients' follow-up was carried out in the sports medicine department of a University Hospital. Inclusion criteria were the following: i) primary ACLR using hamstring graft without bone axial correction or posterior or collateral ligament surgery association, ii) age between 14 and 40 years old, iii) at least two evaluations by isokinetic assessment during the follow-up, before 180 and 330 days post-ACLR. Exclusion criteria were the following: i) ACLR using other grafts, ii) knee infection, arthrofibrosis, anterior or posterior knee pain and swelling at 4 months post-ACLR [17-19], iii) contralateral knee surgery or trauma. After screening, a total of 500 individuals out of the 1,096 originals, were split into two groups, according to age, ≤19 (n = 253) and ≥20 years old (n = 247). This cutoff was chosen according to the World Health Organization criteria [20].

### Procedures

ACLR were performed with standardized arthroscopic procedure by 10 different surgeons with more than 10 years of experience. The

hamstring autograft was harvested through a longitudinal incision over the pes anserinus insertion. To construct the tendon graft, the tendon was folded to obtain a diameter measuring between 7 and 9 mm. Only one tendon (semitendinosus) or 2 tendons (semitendinosus and gracilis) were used depending on the graft diameter [21]. Adolescents were operated trans-epiphyseally according to the similar procedure performed in adults when the skeletal system was considered mature. Otherwise, an all-epiphyseal technique was used to preserve the growth plates.

### Study parameters

At 4 months post-ACLR, age, anthropometric data, delay between ACL injury and reconstruction surgery were collected. The sport level before the ACL tear was established using the Tegner activity scale [22]. The 9 and 10 scores defined high-level practice of a pivotal-contact sport practiced at a national, international, or professional level. The seven to eight scores defined the competitive sport practice of the aforementioned sports at a regional level, or/and the practice of other sports requiring non-contact pivots. The score of five to six corresponded to the recreational practice of ski or racket sport, or recreational jogging on even ground at least 2 to 5 times per week or competitive cycling. The score of three and four corresponded to moderately heavy or light labor.

The Lysholm score was assessed at 4 and 7 months post-ACLR to measure the functional impact in terms of disabilities and activity limitations [23]. A global score of less than 65 points out of 100 was considered low, while a score of more than 84 points was considered high. The clinical difference is established at 10 points [24].

### Isokinetic parameters

The patients performed a knee isokinetic assessment at 4 months post-ACLR (before 180 days) and a second assessment at 7 months (before 330 days) post-ACLR. After a 10-min warm-up on a cycle ergometer, the subject was installed in a sitting position on a HUMAC® isokinetic dynamometer (Medimex, Sainte-Foy-lès-Lyon, France). The knee joint range of motion was from 0° of extension to 100° of flexion. The peak torque was corrected by gravity. The isokinetic assessment consisted of three concentric repetitions at an angular velocity of 60°/s, followed by a 30-s recovery period and then, five repetitions at 180°/s [25].

The knee LSI for the quadriceps or the hamstrings was calculated as follows: (peak torque of the operated knee reported to the peak torque of the non-operated knee) × 100 [26]. LSI was chosen as the main parameter to compare the two groups with neutralized differences related to patient weight. The reliability of the quadriceps and hamstring LSI was considered good to excellent [25,27].

The one-leg hop test was measured at 7 months post-ACLR using a tape meter with a precision of 1 cm [28]. Each player had to jump as far as possible on one leg, beginning with the non-operated side. The hop LSI was calculated with the same method as the isokinetic LSI.

### Physiotherapy sessions

Early rehabilitation was performed to treat knee swelling and pain [29]. The number of physiotherapy sessions was established at 4 post-operative months by interviewing the patients. The nature of the sessions had not been assessed, even if all the patients had an initial prescription for an accelerated rehabilitation program [30].

### Return-to-sport activities

Return to cycling was allowed when knee range of motion reached at least 0-120°. No return to other sports was allowed before the first isokinetic test at 4 months post-ACLR [31]. The return-to-sport activities was collected between the first and the second isokinetic test and
simplified in three categories: no return-to-sport activities, return to cycling, and return-to-sport activities requiring running.

## Statistical analysis

Data analysis was performed with the SPSS 29.0® software (IBM, Armonk, USA). Quantitative parameters were provided as means and standard deviations, while qualitative parameters were expressed as absolute values and percentages. The Kolmogorov-Smirnov test was used to test the normality dispersion of the studied parameters. At baseline, the comparison of quantitative variables between the two groups (adolescents ≤19 and adults ≥20 years) was realized using a student t test after a Levene's test for the variances comparison. A $\chi^2$ test was performed for qualitative variables. The time effect between the two isokinetic tests was measured by comparing the adolescent group and the adult one using an analysis of variance for repeated measures (2 times × 2 groups). Effect sizes were assessed by partial eta squared $\eta^2$ (quantitative variables) defined as trivial, small, moderate, and large for values $\eta^2 < 0.01$, $>0.01$, $>0.06$, $>0.14$, respectively, or by Cohen's d (qualitative variables) defined as small, moderate, and large for values $\le 0.2$, $\le 0.5$, and $\ge 0.8$ [32]. A generalized linear multivariate model analysis had been considered to assess the weight of potential inter-subject explanatory parameters on the evolution of the knee strength LSI (sex, meniscal repair, Tegner activity scale before ACLR in accordance with three classes: 5–6; 7–8 and 9–10). The delays between ACL injury and ACLR, between ACLR and the first and the second isokinetic test were not taken into consideration due to the lack of correlation between these delays and the knee strength ($r < 0.300$ after Pearson correlations). The alpha level of statistical significance was set at $p < 0.05$. The sample size estimation was 177 participants per group to detect a 5% difference in Quadriceps LSI with a standard deviation of 15%, given an a priori level of $\alpha = 0.05$ and an acceptable $1-\beta = 0.80$.

## RESULTS

A total of 500 patients, mainly men (62.2%), (age: $20.9 \pm 5.7$ years old [14–40]; weight: $68.7 \pm 13.2$ kg [35–145]; height: $173 \pm 8$ cm [150–196]) were followed. A pivot contact sport was practiced in 80.8% of the cases before ACLR (soccer: 42.4%, basketball: 24.4%, handball: 14.0%). The most frequent sport level was 7 Tegner activity scale (35.8%). The 14.6% of the population competed at an elite level, i.e. Tegner score at 9–10. Meniscal surgery was associated with ACLR in 32.4% of the cases.

Two distinctive groups were identified according to age (253 adolescents $\le 19$ and 247 adults $\ge 20$). At baseline, the two groups were

different for sex, anthropometric parameters, sport activities, and sport level (Tables 1 and 2). The adolescent group was more frequently operated on the menisci (Table 1). The delays between ACLR and the first isokinetic testing and between ACLR and the second isokinetic testing for the adolescent group was longer (5 and 8 days, respectively) compared with those of the adult group (Table 2).

At 4 months, the number of physiotherapy sessions was not statistically significantly different between the two groups.

Return to activities between 4 and 7 months post-ACLR was not different (Table 3). At 7 months post-ACLR, the delay between the two isokinetic measurements was short of 5 days for the adolescent group (Table 2).

The isokinetic quadriceps LSI were statistically significantly higher for the adolescent group compared with the adult group at 4 and 7 months post-ACLR [QLSI160: $F(2,498) = 32$; $p < 0.001$; $\eta^2$:0.064; QLSI180: $F(2,498) = 23$; $p < 0.001$; $\eta^2$:0.046], whereas the isokinetic hamstrings LSI were not different (Table 3). The Lysholm scores at 4 and 7 months [$F(2,498) = 8.6$; $p = 0.003$; $\eta^2$:0.18] and the one-leg hop test at 7 months were statistically higher in the adolescent group (Table 3).

No time effect was present for the quadriceps LSI at 60 and $180^\circ$/s, [$F(1.498) = 0.39$; $p = 0.67$; $\eta^2$: 0.002] and [$F(1.498) = 0.28$; $p = 0.75$; $\eta^2$: 0.001], respectively (Fig. 1), and for the hamstring LSI at $60^\circ$/s and $180^\circ$/s, [$F(1.498) = 1.37$; $p = 0.25$; $\eta^2$: 0.006] and [$F(1.498) = 0.2$; $p = 0.79$; $\eta^2$: 0.001], respectively (Fig. 2). No inter-subject explanatory parameters (sex, meniscus surgery, sport level before ACLR) were found after performing multivariate analysis to explain knee strength LSI evolution. Inter-subject main effects of sex, meniscus surgery and sport level before ACLR were shown in **supplemental materials A and B**.

## DISCUSSION

Adolescents who underwent ACLR using hamstring autograft had statistically higher quadriceps LSI, higher Lysholm scores at 4 and 7 months post-surgery and higher single-hop test LSI at 7 months post-surgery than adults. Otherwise, the hamstring LSIs were not statistically different according to the age groups and so was the kinetic evolution of the knee LSI (time effect).

At baseline, more patients in the adolescent group were female, had undergone meniscus repair and had a lower pre-ACLR sport level compared with the adult group. However, these three different parameters did not statistically influence the recovery of quadriceps and hamstrings strength. The sex distribution of our two groups is representative of populations reported in the literature [1,4].

<table><caption>**Table 1**
Comparison of qualitative parameters between adolescents and adults.</caption><thead><tr><th></th><th>Adolescents n = 253</th><th>Adults n = 247</th><th>p</th><th>η<sup>2</sup></th></tr></thead><tbody><tr><td>Sex:</td><td></td><td></td><td>&lt;0.0001</td><td>0.035</td></tr><tr><td>Male</td><td>134 (52.9%)</td><td>177 (71.7%)</td><td></td><td></td></tr><tr><td>Female</td><td>119 (47.1%)</td><td>70 (28.3%)</td><td></td><td></td></tr><tr><td>Meniscal surgery:</td><td></td><td></td><td>0.01</td><td>0.021</td></tr><tr><td>No</td><td>158 (62.5%)<sup>a</sup></td><td>180 (72.9%)<sup>a</sup></td><td></td><td></td></tr><tr><td>Meniscal repair</td><td>95 (37.5%)<sup>a</sup></td><td>67 (27.1%)<sup>a</sup></td><td></td><td></td></tr><tr><td>• Medial meniscus</td><td>41</td><td>32</td><td></td><td></td></tr><tr><td>• Lateral meniscus</td><td>26</td><td>26</td><td></td><td></td></tr><tr><td>• Medial and lateral menisci</td><td>28</td><td>9</td><td></td><td></td></tr><tr><td>Sports before ACLR:</td><td></td><td></td><td>0.02</td><td>0.013</td></tr><tr><td>Soccer</td><td>90 (35.6%)<sup>a</sup></td><td>122 (49.4%)<sup>a</sup></td><td></td><td></td></tr><tr><td>Basketball</td><td>70 (27.7%)</td><td>52 (21.1%)</td><td></td><td></td></tr><tr><td>Handball</td><td>37 (14.6%)</td><td>33 (13.3%)</td><td></td><td></td></tr><tr><td>Others</td><td>56 (22.1%)</td><td>40 (16.2%)</td><td></td><td></td></tr><tr><td>Tegner activity scale before ACLR</td><td></td><td></td><td>&lt;0.0001</td><td>0.033</td></tr><tr><td>• 5-6</td><td>91 (36%)<sup>a</sup></td><td>57 (23.1%)<sup>a</sup></td><td></td><td></td></tr><tr><td>• 6-8</td><td>136 (53.8%)</td><td>143 (57.9%)</td><td></td><td></td></tr><tr><td>• 9-10</td><td>26 (10.3%)<sup>a</sup></td><td>47 (19%)<sup>a</sup></td><td></td><td></td></tr></tbody></table>

Abbreviations: ACLR = anterior cruciate ligament reconstruction.

<footnote><sup>a</sup> Significant difference between adolescents and adults ($\chi^2$ test and Bonferroni test).</footnote>
<table><caption>**Table 2**
Comparison of quantitative parameters between adolescents and adults.</caption><thead><tr><th></th><th>Adolescents n = 253</th><th>Adults n = 247</th><th>p</th><th>Cohen's d</th></tr></thead><tbody><tr><td>Age (year)</td><td>16.2 ± 1.6</td><td>25.6 ± 4.3</td><td>&lt;0.0001</td><td>3.32</td></tr><tr><td>Weight (kg)</td><td>62.9 ± 10.4</td><td>74.6 ± 13.2</td><td>&lt;0.0001</td><td>11.90</td></tr><tr><td>Height (cm)</td><td>171 ± 8</td><td>176 ± 8</td><td>&lt;0.0001</td><td>8.46</td></tr><tr><td>BMI (kg/cm²)</td><td>21.3 ± 2.3</td><td>23.9 ± 2.8</td><td>&lt;0.0001</td><td>0.25</td></tr><tr><td>Delay between ACL injury and reconstruction (days)</td><td>183 ± 218</td><td>197 ± 318</td><td>0.57</td><td>0.27</td></tr><tr><td>Delay between ACLR and the first isokinetic test (days)</td><td>130 ± 21</td><td>125 ± 22</td><td>0.009</td><td>2.16</td></tr><tr><td>Delay between ACLR and the second isokinetic test (days)</td><td>219 ± 41</td><td>211 ± 38</td><td>0.02</td><td>3.99</td></tr><tr><td>Delay between the first and the second isokinetic test (days)</td><td>98 ± 31</td><td>103 ± 28</td><td>0.06</td><td>1.96</td></tr><tr><td>Physiotherapy sessions (n) from ACLR to 4 months</td><td>37 ± 12</td><td>39 ± 15</td><td>0.11</td><td>1.42</td></tr></tbody></table>

Abbreviations: BMI = body mass index; ACLR = anterior cruciate ligament reconstruction.

From our results, sex had not statistically influenced strength (LSI) recovery after ACLR. Yet, quadriceps and hamstring strength after ACLR are reported as factors influenced by sex in children and adolescents when these parameters are normalized to the body weight [33–35]. A decrease of 12–16% in knee strength was shown in females compared with males on the involved knee but also on the uninvolved one. Thus, because this decrease in knee strength affects both sides, it may explain why LSIs are comparable between the two sexes [35]. When the knee strength is measured isometrically with a handheld dynamometer at 6 months post-ACLR, female adolescents demonstrated a lower quadriceps LSI than males (5.7%) while hamstring LSI is similar [36]. This difference with our results might be explained by the isometrical method. In adults, the female sex was reported as a predictive parameter for the quadriceps strength recovery, but in favor of a higher quadriceps LSI [37]. Several studies have shown that sex is not associated with the quadriceps strength [13,38,39].

The lack of consequence of meniscal repair on the recovery of knee muscle strength has also already been described after ACLR in adults and in adolescents at 3, 6 and 9 months post-ACLR using hamstring graft [13, 35, 37, 40]. This result could be explained by the rehabilitation protocol, which protects the operated knee from pain and swelling, whether there

has been a meniscal repair or not. Limiting standing and walking (with crutches) was recommended during the first post-surgery weeks to reduce knee swelling and thus allowed recovery of knee range of motion more easily without causing too much compression on a possible meniscal repair [40].

The pre-injury Tegner activity scale has not influenced the knee strength after ACLR. Indeed, no association has also been highlighted in the literature after multivariate analysis in adults or in adolescents [13–15,33,37,38].

Age is a key factor in quadriceps strength recovery after ACLR using a hamstring graft. The adolescents presented a greater quadriceps LSI at 4 and 7 months post-ACLR compared with the adults but the kinetic recovery of the muscle knee inhibition was similar. In adults, correlations between age and quadriceps LSI at 60°/s had already been described, at 3 months (r = 0.250) post-ACLR, with “a risk of muscle recovery delay when ACLR was performed in older patients” [12]. Krych et al. [39] confirmed the importance of age by showing that patients aged 24 years old, more frequently presented a quadriceps LSI >85% than those aged 27 years old. After multivariate analysis, a negative effect was observed for patients over 30 years old. It may be explained by the presence of minimal articular cartilage damage [39]. Then, different authors looked

<table><caption>**Table 3**
Isokinetic strength LSI, Lysholm score, hop test, Tegner activity scale, and RTA comparison between adolescents and adults.</caption><thead><tr><th>Four months post-ACLR</th><th>Adolescents n = 253</th><th>Adults n = 247</th><th>p</th><th>Cohen's d</th></tr></thead><tbody><tr><td>LSI Q60 (%)</td><td>79.3 ± 12.4</td><td>72.1 ± 14</td><td>&lt;0.0001</td><td>0.13</td></tr><tr><td>LSI H60 (%)</td><td>85.1 ± 12.4</td><td>84.4 ± 13.2</td><td>0.54</td><td>0.12</td></tr><tr><td>LSI Q180 (%)</td><td>84.5 ± 11</td><td>78.3 ± 12.6</td><td>&lt;0.0001</td><td>0.12</td></tr><tr><td>LSI H180 (%)</td><td>85.5 ± 15</td><td>85.9 ± 13.9</td><td>0.75</td><td>0.14</td></tr><tr><td>Lysholm score</td><td>98 ± 4</td><td>96 ± 7</td><td>0.001</td><td>0.15</td></tr><tr><td>Tegner activity scale</td><td></td><td></td><td>0.01</td><td>0.024<sup>#</sup></td></tr><tr><td>• 3</td><td>15 (5.9%)*</td><td>28 (11.4%)*</td><td></td><td></td></tr><tr><td>• 4</td><td>233 (92.1%)*</td><td>203 (82.1%)*</td><td></td><td></td></tr><tr><td>• 5</td><td>5 (2%)*</td><td>14 (5.7%)*</td><td></td><td></td></tr><tr><td>• 6</td><td>0</td><td>2 (0.8%)</td><td></td><td></td></tr><tr><td>Seven months post-ACLR</td><td></td><td></td><td></td><td></td></tr><tr><td>LSI Q60 (%)</td><td>88.5 ± 10</td><td>82.3 ± 12.7</td><td>&lt;0.0001</td><td>0.21</td></tr><tr><td>LSI H60 (%)</td><td>87.6 ± 10.9</td><td>88.6 ± 11.6</td><td>0.32</td><td>0.11</td></tr><tr><td>LSI Q180 (%)</td><td>91.2 ± 9.2</td><td>85.9 ± 11.2</td><td>&lt;0.0001</td><td>0.10</td></tr><tr><td>LSI H180 (%)</td><td>88.7 ± 13.9</td><td>88.6 ± 13.6</td><td>0.93</td><td>0.13</td></tr><tr><td>Lysholm score</td><td>99 ± 3</td><td>98 ± 5</td><td>0.007</td><td>4.44</td></tr><tr><td>Hop test LSI</td><td>94.3 ± 5.4</td><td>91.5 ± 6.5</td><td>&lt;0.0001</td><td>0.05</td></tr><tr><td>RTA between 4 and 7 months</td><td></td><td></td><td>0.08</td><td>0.010<sup>#</sup></td></tr><tr><td>No</td><td>11 (4.4%)</td><td>11 (4.3%)</td><td></td><td></td></tr><tr><td>Bicycling</td><td>36 (14.2%)</td><td>54 (21.7%)</td><td></td><td></td></tr><tr><td>Sport with running</td><td>206 (81.4%)</td><td>183 (74%)</td><td></td><td></td></tr><tr><td>Tegner activity scale</td><td></td><td></td><td>0.015</td><td>0.033<sup>#</sup></td></tr><tr><td>• 3</td><td>0*</td><td>10 (4%)*</td><td></td><td></td></tr><tr><td>• 4</td><td>84 (33.2%)</td><td>84 (34%)</td><td></td><td></td></tr><tr><td>• 5</td><td>140 (55.3%)*</td><td>115 (46.6%)*</td><td></td><td></td></tr><tr><td>• 6</td><td>24 (9.5%)</td><td>30 (12.2%)</td><td></td><td></td></tr><tr><td>• 7</td><td>5 (2%)</td><td>7 (2.8%)</td><td></td><td></td></tr><tr><td>• 8</td><td>0</td><td>1 (0.4%)</td><td></td><td></td></tr></tbody></table>

Abbreviations: χ² NR = χ² test not carried out; LSI = limb symmetry index; Q = quadriceps; H = hamstring; 60 = angular velocity at 60°/s; ACLR = anterior cruciate ligament reconstruction; RTA = return to activities. *Significant difference between adolescents and adults (Bonferroni test after χ² test); # = effect sizes by eta square (η²).
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Quadriceps LSI (estimated marginal mean in %) recoveries from 4 (time 1) to 7 months post-ACLR (Time 2) at 60 (A) and 180°/s (A) is comparable between adolescents and adults after ponderation by sex, association or not with meniscal repair, and Tegner activity scale before ACLR. ACLR = anterior cruciate ligament reconstruction.</figcaption></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 2.** Hamstring LSI (estimated marginal mean in %) recoveries from 4 (time 1) to 7 months post-ACLR (time 2) at 60 (A) and 180°/s 5(B) is comparable between adolescents and adults after ponderation by sex, association or not with meniscal repair, and Tegner activity scale before ACLR. ACLR = anterior cruciate ligament reconstruction; LSI = limb symmetry index.</figcaption></figure>

<figure><img src="image_8.png" /></figure>

for factors associated with a quadriceps LSI at 60°/s > 80–85%, and each time, age was a significant factor after multivariate analysis [13,15,37, 38]. The odds of poor strength recovery (quadriceps LSI< 80%) at 1 year post-ACLR using a hamstring graft was 5 times higher for patients aged >40 years versus those < 20 years [38]. Indeed, when the age group of patients < 20 years was taken as reference, the different groups over 31 years old had significant odds of poor strength recovery (3 times higher) [38]. When studies include adolescents and adults, young age is always associated with a good quadriceps strength recovery (LSI>85%) [14, 15]. In adolescents, significant differences in strength recovery expressed in absolute values were found according to early (11–14 years), middle (15–17 years), and late adolescence (18–20 years) [33, 34]. After calculation of the LSI with the published data, the means of quadriceps LSI at 60°/s at 7 months post-ACLR using a hamstring graft was 90.5% in the 11–14 years group, and 88.2% in the 15–17 years group [34].

The higher quadriceps LSI in adolescents at 4 and 7 months post-ACLR compared with those of adults raises two main points. Firstly, why adolescents have better quadriceps strength than adults at 4 and 7 months after ACLR. The role of the skeletal maturity has been suggested [34], but the link between skeletal maturity and strength recovery after surgery is not known. In middle adolescents, puberty status can explain the increase of muscle strength, particularly in men, which can explain

the difference, sometimes shown between men and women on the uninvolved knee and after reporting absolute values to the body weight [41]. Yet, we do not know if it is due to puberty or a hormonal action. In adults, the answer seems simpler because degenerative cartilage lesions are suspected of being the cause of low knee strength [39]. However, how can we explain that the kinetics of the knee strength recovery was not different between adults and adolescents (Fig. 1). The second point is the use of quadriceps LSI to help in the decision-making process to return to sport. Indeed, many adolescents recover good knee muscle strength between 4 and 7 months post-ACRL according to cut-offs >85% [10,15,38,42]. The use of these cut-offs is questionable to decide to return to vigorous sports because a slower return to sport is protective for avoiding a second ACL injury [43]. The same question exists concerning the hop tests, with LSI values > 90% at 7 months post-ACLR.

## LIMITATIONS

Firstly, the postoperative rehabilitation protocol might have differed between patients after ACLR. Indeed, all the patients benefited from the same prescription of physiotherapy, but they performed outpatient rehabilitation, so it was not possible to know if the physiotherapists strictly followed the prescription. However, the quantity of physiotherapy sessions and the return-to-activity post-ACLR were collected,
and no difference between the two groups was found. Secondly, the adolescent group did not differentiate subgroups according to the skeletal maturity or the surgical technique [44]. Thirdly, the quadriceps LSI before ACLR was not assessed while this parameter was identified as predictive for a good LSI after ACLR [13–15,37,38]. Yet, the strength deficit is particularly due to the arthrogenic muscle inhibition secondary to the pain and the effusion induced by the surgery [45].

## CONCLUSION

Adolescents who underwent ACLR using hamstring autograft had a statistically higher quadriceps LSI, statistically higher Lysholm scores at 4 and 7 months post-surgery, and statistically higher single-hop test LSI at 7 months post-surgery compared with adults. This recovery allows adolescents to quickly reach strength cut-offs usually used for returning to sport. These cut-offs should be reviewed in adolescents due to the risk of graft rupture, and a multimodal functional testing is necessary to allow the return to sport.

## Ethics committee approval of the study protocol

Groupe Nantais d'Ethique dans le Domaine de la Santé-GNEDS 2025.

## Authors contributions statement

MD, AC and AFC were responsible for the conception and design of the study and data collection; MD, VC, TH and AFC were involved in the processing and statistical analysis of data; AFC, MD, LG and PM were involved in the drafting of the manuscript; and all authors contributed to the interpretation of the data for the work and revising it critically for important intellectual content. All the authors finally approved the manuscript. AFC takes responsibility for the integrity of the work as a whole. All authors have read and agreed to the published version of the manuscript.

## Data sharing statement

All data relevant to the study are included in the article or are available as supplementary files.

## Funding

No funding was received from external sources.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jisako.2025.100901.

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