Relationship between Muscle-Tendon Stiffness and Drop Jump Performance in Young Male Basketball Players during Developmental Stages

Background: The relationship between stiffness and drop jump performance in athletes in various stages of development has yet to be fully investigated. The first aim of this study was to investigate the association between the stiffness of the patellar and quadriceps tendon (PT, QT), gastrocnemius–Achilles tendon unit (GAT), and rectus femoris (RF) using drop jump (DJ) performance in young basketball players. The second aim was to investigate possible variations in the stiffness levels of those tissues in different developmental stages. Methods: The stiffness levels of the GAT, PT, QT, and RF were measured in both limbs in 73 male basketball players aged 12 to 18 years. The reactive strength index (RSI), contact time (CT) and jump height (JH) during 30 and 40 cm DJs were also measured. Results: Pearson correlation coefficients showed a significant association between DJ performance and PT, QT, GAT, and RF dynamic stiffness. Moreover, the youngest subjects were found to have lower stiffness values than the older ones. Conclusions: Tissue stiffness can affect athletic performance by modifying the stretch-shortening cycle in young basketball players. Stiffness of muscles and tendons increases during the maturation process. Further investigations could shed light on the effect of training on the stiffness of muscles and tendons.


Introduction
Currently, more than 450 million people in the world play basketball, making it among the five most popular sports on the planet [1]. Jumping ability has been shown to be a predictor of functional capacity [2][3][4] and the risk of falling [5], as well as a strong predictor of basketball performance both in elite and recreational players [6,7]. When humans jump, the tendon structures are assumed to be the major source of the elastic component; hence, the elasticity of tendon structures is a leading factor contributing to the amount of stored energy [8]. In sports with stretch-shortening cycle (SSC) movements, such as jumps in basketball, a stiffer muscle-tendon unit may be required for the storage and release of elastic energy [8,9]. Given the young age at which athletes begin playing basketball, this structure undergoes a development process that runs through puberty and probably contributes to the performance of high-level athletes. This is one of the reasons why the most common overuse injuries in this sport involve the knees (patellofemoral pain or "jumper's knee") and ankles (Achilles tendon pain) [10][11][12]. In addition, athletic training often aims to achieve higher levels of lower limb stiffness, as it is associated with better repeated sprint ability and horizontal jumping performance in basketball players [13][14][15].
(1−β) = 0.85, a total of 72 subjects must be enrolled. Given this outcome, seventy-three healthy young males, aged 12 to 18 years, took part in the study. In order to analyze variations in stiffness across developmental stages, the subjects were divided into three age groups (≤13, 14-15 and ≥16 years). To identify the age at peak height velocity (APHV), the maturity offset (MO) of the three groups was estimated using the alternative model for boys by Moore et al. (2015) [35]. The subjects' age, weight, height, BMI and MO are displayed in Table 1. All of the participants started playing mini basketball on a feeder team of an Italian first division team at the age of 6 to 8 years. At the time of the data collection, they were practicing basketball 3 to 5 times per week for an average of 7.2 h per week. The participants had no history of surgery or any other orthopedic injuries in the previous year and were not following any particular therapy. All participants signed an informed consent form, and in the case of underage subjects, the consent form was signed by their parents. The study was approved by the Human Research Ethical Committee of the Urbino University (No. 31_2020) and was performed in accordance with the guidelines of the Declaration of Helsinki. All tests were performed in a controlled setting (in the same gym, with the same operators, and with the same instruments) during the basketball season.

Procedures
The assessment of stiffness was performed after a warm-up consisting of 5 min of pedaling at moderate intensity, followed by 5 min of squat jump exercises and dynamic stretching. The dynamic stiffness of the gastrocnemius-Achilles tendon unit (GAT), the Patellar Tendon (PT), the Quadriceps Tendon (QT) and the Rectus Femoris (RF) was measured on the left and right sides using the MyotonPro device. For GAT assessment, participants were in a prone position on a massage table with their knees extended and feet hanging barefoot and unsupported from the edge of the table. The measurement points were at 8,9,10,11,12,16, and 20 cm from the plantar aspect of the heel, as shown in Figure 1 [33]. For the PT assessment, participants were seated with their knees flexed at a 90 • angle and completely relaxed. The measurement point was at the midpoint between the distal edge of the patella and the tibial tuberosity. For the RF and QT assessments, participants were lying supine with their feet on the massage table. The RF was assessed at the midpoint between the anterior superior iliac spine and the superior edge of the patella, whereas the QT was assessed at the distal 1/3 point between the RF assessment point and the upper edge of the patella.
Drop jump performance was assessed immediately after the stiffness measurements were taken using an optoelectronic system called Optojump (Microgate s.r.l., Bolzano, Italy). This device consists of a transmitting and receiving bar, each having 96 LEDs. Continuous communication takes place between the LEDs on the transmitting and receiving bars, and the mechanism determines the duration of any communication interruptions between them. This allows for 1/1000-s precise measurement of flight and contact times throughout the execution of a sequence of jumps. Participants performed three DJ30s and three DJ40s with their hands placed on their hips, with 30 s of passive rest between jumps. The subjects were instructed to keep their leg muscles as stiff as possible, trying to minimize joint flexions during landing. The main goal of the DJ was to minimize landing contact time while trying to reach the maximum jump height. Data on contact time (CT30 and CT40), jump height (JH30 and JH40) and the RSI (RSI30 and RSI40) of each drop jump were then calculated and the best DJ30 and DJ40 (defined as the highest jump) were selected for subsequent analysis. Specifically, the RSI is an index of reactive strength derived from the ratio of jump height to ground contact time. It makes it possible to assess the subject's ability to change rapidly from eccentric to concentric muscle contraction as well as his or her explosiveness during the drop jump [36]. Drop jump performance was assessed immediately after the stiffness measurements were taken using an optoelectronic system called Optojump (Microgate s.r.l., Bolzano, Italy). This device consists of a transmitting and receiving bar, each having 96 LEDs. Continuous communication takes place between the LEDs on the transmitting and receiving bars, and the mechanism determines the duration of any communication interruptions between them. This allows for 1/1000-s precise measurement of flight and contact times throughout the execution of a sequence of jumps. Participants performed three DJ30s and three DJ40s with their hands placed on their hips, with 30 s of passive rest between jumps. The subjects were instructed to keep their leg muscles as stiff as possible, trying to minimize joint flexions during landing. The main goal of the DJ was to minimize landing contact time while trying to reach the maximum jump height. Data on contact time (CT30 and CT40), jump height (JH30 and JH40) and the RSI (RSI30 and RSI40) of each drop jump were then calculated and the best DJ30 and DJ40 (defined as the highest jump) were selected for subsequent analysis. Specifically, the RSI is an index of reactive strength derived from the ratio of jump height to ground contact time. It makes it possible to assess the subject's ability to change rapidly from eccentric to concentric muscle contraction as well as his or her explosiveness during the drop jump [36].

Statistical Analysis
All data were checked for normality using the Shapiro-Wilk test and for equality of variances using Levene's test. A Pearson's correlation (rP) analysis was then used to calculate the relationship between drop jump performance variables and muscle or tendon stiffness values. The alpha level was initially set to 0.05, but was then corrected to 0.0236 using the Benjamini-Hochberg procedure to decrease the false discovery rate (FDR < 5%). The effect sizes of rP were established as described by Hopkins and defined as trivial (r = Figure 1. Gastrocnemius-Achilles tendon unit (GAT) stiffness measurement at 8,9,10,11,12,16, and 20 cm from the plantar aspect of the heel. Note that the participant is in a prone position with the knees extended and feet hanging barefoot and unsupported from the edge of the table.

Statistical Analysis
All data were checked for normality using the Shapiro-Wilk test and for equality of variances using Levene's test. A Pearson's correlation (rP) analysis was then used to calculate the relationship between drop jump performance variables and muscle or tendon stiffness values. The alpha level was initially set to 0.05, but was then corrected to 0.0236 using the Benjamini-Hochberg procedure to decrease the false discovery rate (FDR < 5%). The effect sizes of rP were established as described by Hopkins and defined as trivial (r = 0-0.1), small (r = 0.1-0.3), moderate (r = 0.3-0.5), large (r = 0.5-0.7), very large (r = 0.7-0.9) and nearly perfect (r = 0.9-1.0) [37]. GAT data were analyzed by means of a two-way ANOVA for independent measures (age and measurement point as independent factors). Jump performance parameters, PT, RF and QT data were analyzed using a one-way ANOVA with age as an independent factor. To identify which groups differed from each other, a post-hoc analysis was performed using the Bonferroni procedure. All data were analyzed using Microsoft Excel 2016 and JASP (Version 0.16.4) (University of Amsterdam, Amsterdam, The Netherlands).

Drop Jump Performance Parameters in Different Age Groups
Drop jump performance parameters of all participants and age groups are shown in Table 2. There were differences in RSI and JH data between the ≥16 age group and the Int. J. Environ. Res. Public Health 2022, 19, 17017 5 of 12 younger groups, but no differences were detected between ≤13 and 14-15 age groups for any variable. CT remained constant among all age groups.

Correlation between Drop Jump Performance Parameters and Stiffness
When considering the whole group of participants, a Pearson's correlation analysis showed that RSI and JH were related to the stiffness of PT, RF and GAT at 8, 9, 10 and 12 cm in both the right and left limbs. In particular, DJ40 showed a larger effect on performance parameters than DJ30, as shown in Tables 3 and 4. Figure 2 depicts the correlations between RSI40 and JH40 with RF and PT stiffness. On the other hand, no effects or negligible effects were detected for contact time, GAT at 16 and 20 cm, while QT stiffness showed a significant correlation with JH only in the left limb at both DJ30 and DJ40.  Table 4. Correlations between DJ40 performance and stiffness parameters (mean ± SD).

Tendon and Muscle Stiffness in Different Age Groups
The analysis of GAT stiffness among the age groups showed a significant increase from 12 to 18 years for both left and right limbs, with all age groups found to be significantly different from each other (p < 0.001), as shown in Figure 3. A post hoc analysis showed that GAT stiffness decreased from 8 to 20 cm in all age groups, with significant differences among all measurement points when the distance was 2 cm or larger (p < 0.001), except for 8-10 cm (p = 0.012) and 9-11 cm (p = 0.067).
Similarly, for both left and right limbs, RF, PT and QT stiffness showed a significant increase from 12 to 18 years (Figure 4). In particular, for RF, the muscle stiffness of the ≤13 group was lower than that found in the 14-15 (p < 0.001) and ≥16 groups (p < 0.001), while no difference was found between the 14-15 and ≥16 age groups (p = 0.069). For PT, the tendon stiffness of the ≤13 group was lower than that of the 14-15 (p = 0.028) and ≥16 groups (p = 0.018), while no difference was found between the 14-15 and ≥16 age groups (p = 1.000). The same result was found for QT, with the tendon stiffness of the ≤13 group found to be lower than that of the 14-15 (p = 0.009) and ≥16 groups (p = 0.002), while no difference was found between the 14-15 and ≥16 age groups (p = 1.000).
showed that GAT stiffness decreased from 8 to 20 cm in all age groups, with significant differences among all measurement points when the distance was 2 cm or larger (p < 0.001), except for 8-10 cm (p = 0.012) and 9-11 cm (p = 0.067).
Similarly, for both left and right limbs, RF, PT and QT stiffness showed a significant increase from 12 to 18 years (Figure 4). In particular, for RF, the muscle stiffness of the ≤13 group was lower than that found in the 14-15 (p < 0.001) and ≥16 groups (p < 0.001), while no difference was found between the 14-15 and ≥16 age groups (p = 0.069). For PT, the tendon stiffness of the ≤13 group was lower than that of the 14-15 (p = 0.028) and ≥16 groups (p = 0.018), while no difference was found between the 14-15 and ≥16 age groups (p = 1.000). The same result was found for QT, with the tendon stiffness of the ≤13 group found to be lower than that of the 14-15 (p = 0.009) and ≥16 groups (p = 0.002), while no difference was found between the 14-15 and ≥16 age groups (p = 1.000).
No differences were found between the left and right limbs for any stiffness parameter or GAT measurement points.

Pearson's Correlation Analysis between Stiffness and DJ Performance Parameters
The first aim of this study was to investigate the potential relationships between lower extremity tendon and muscle stiffness with the performance parameters of DJ30 and DJ40. First, as expected, jump height and RSI were found to improve from ages 12 to 18 years, while contact time did not change for DJ30 and DJ40, except for a decreasing trend in the ≥16 age group (see Table 2). After a Pearson's correlation analysis, we found that the stiffness of GAT (except for 16 and 20 cm), RF, PT and QT showed a small to No differences were found between the left and right limbs for any stiffness parameter or GAT measurement points.

Pearson's Correlation Analysis between Stiffness and DJ Performance Parameters
The first aim of this study was to investigate the potential relationships between lower extremity tendon and muscle stiffness with the performance parameters of DJ30 and DJ40. First, as expected, jump height and RSI were found to improve from ages 12 to 18 years, while contact time did not change for DJ30 and DJ40, except for a decreasing trend in the ≥16 age group (see Table 2). After a Pearson's correlation analysis, we found that the stiffness of GAT (except for 16 and 20 cm), RF, PT and QT showed a small to moderate correlation with RSI and JH (Figure 2), and the effect size of all correlations increased with box height (DJ30 to DJ40). Nevertheless, we found no correlation between stiffness and CT in young basketball players. These findings suggest that drop jump performance may be influenced by the GAT, RF, PT and QT stiffness at rest.
These results are in line with a recent investigation by Ando, et al. (2021) [21], which showed that medial gastrocnemius stiffness (measured during a 15 cm drop jump as the shear modulus through ultrasound shear wave elastography) correlated with the same performance parameters (RSI and JH) but not with the contact time. In addition, our results are in line with other studies [38,39] in which the authors demonstrated that drop jumps performed with different modes and instructions show different responses. In particular, the authors showed that when participants were asked to reach the maximum jump height or to make their best effort, without further instruction, they obtained the best JH, with no correlation with contact time. On the other hand, when participants were asked to keep their limbs as stiff as possible, they obtained lower JH and there was a significant correlation with contact time. In both cases, however, the authors found a strong correlation with RSI. Furthermore, the authors found that the jumps with the greatest height were strongly correlated with ground reaction force at the start of the propulsive phase. Likewise, Walsh et al. (2004) [40] also reported that the greatest power output occurred in moderate stiffness drop jumps. Therefore, it is not surprising that contact time was not related to jump height, as the time required to transfer high reaction forces varies with the athlete's rate of force development capacity. In our study, the instructions given to the participants were to maintain the minimum contact time by seeking the greatest possible height, and for data analysis, we collected the highest DJ30 and DJ40. Indeed, this may be why we found no correlation between stiffness levels and contact time.
Regarding the RF, PT and QT stiffness, this is the first study to investigate and find a significant correlation with RSI and JH ( Figure 2). Not surprisingly, the rectus femoris muscle, quadriceps and patellar tendon were shown to be directly involved in the execution of the drop jump, in particular during the landing phase [41,42]. The relationship between RF, PT and QT stiffness and DJ performance parameters is linked with risk of injury, in particular patellar tendinopathies. In fact, it is known that the ability of the patellar and quadriceps tendons to lengthen during the jump can be compromised by tendinopathy, which reduces the range of movement during DJ landings [43]. Many studies [44][45][46] state that smaller flexion ranges of the lower body joints (stiff landing), larger moment/torque at the knee, and higher knee angular velocities are risk factors for patellar tendinopathy, as well as indicators of present or past quadriceps or patellar injury. Hence, the positive relationship shown in the present study between QT and PT stiffness (assessed with MyotonPro) and DJ performance could be useful in further investigations for the prevention of jumper's knee. It is worth noting that, surprisingly, for the QT stiffness, we only found a correlation for the left limb with DJ30 and DJ40. We speculate that the different ways of measuring QT with the gold standards SWUE (30 • flexed knee) and MyotonPro (extended knee) [47] may have reduced the validity of this measurement. Therefore, we believe that specific studies are needed to define the correct knee flexion angle or the measurement site before continuing to assess QT stiffness by means of myotonometry.
Finally, it is worth mentioning the study by Konrad et al. (2022) [19], who found no correlation between Achilles tendon and patellar stiffness and DJ40 performance parameters (RSI, JH, and CT) using the MyotonPro. One possible explanation for this discrepancy could be (in addition to the smaller number of subjects) that their participants were physically active adults lacking experience in jumping skills, whereas our participants were young basketball players who were familiar with the DJ and good jumping technique. Moreover, it is worth noting that the extensive training experience (about 5 to 10 years) of our participants could have affected their stiffness levels, but we cannot discriminate between the effects of training and maturation alone because of the lack of an untrained control group.

Stiffness of GAT, QT, PT and RF through Developmental Stages
The second aim of this study was to examine how tissue stiffness changes through various developmental stages. Our results show that, in splitting the participants into three age groups, all GAT stiffness values increase from ≤13 to ≥16 years; also, as expected and also shown by Morgan et al. (2018) [48], in all groups the highest values were from 8 to 12 cm, after which they decrease to 20 cm and the differences become smaller with age (see Figure 3). These results suggest that the stiffest component of the Achilles tendon is in its distal portion, and that this portion grows significantly from 12 to 18 years in young basketball players. Mean values of RF, PT and QT also increase significantly from ≤13 to ≥16 years, but unlike GAT, they slow down from 14-15 to ≥16 years. However, unlike tendon stiffness, RF values also increased quasi significantly (p = 0.069) from 14-15 to ≥16 years, indicating that the growth of muscle stiffness probably continues more consistently than PT and QT over the developmental stages examined in the present study (see Figure 4). In accordance with our results, several studies have reported increases in muscle cross sectional area (CSA) during maturation, with some suggesting that the greatest changes occur in boys around the age of 13-15 years [49,50]. A roughly two -fold rise in patellar tendon CSA from childhood to adulthood has also been noted in earlier research [51], which suggests an increase in tendon stiffness given the relationship between tendon CSA and stiffness [52]. Both patellar [22,51] and Achilles tendon stiffness [53] have also shown approximately two-fold increases in magnitude with age. Additionally, these studies found no or very little difference between older children (>14 years) and adults, supporting the notion that adult values can be attained shortly after the time of peak height velocity [51,54,55].
A limitation of this study is that MO was assessed using the alternative model for boys by Moore et al. (2015) [35], which uses height instead of sitting height. However, our results show that the three age groups matched the biological and chronological ages, providing an overall picture of the maturation process. As a practical application, coaches and practitioners could use the JH and RSI of a drop jump to roughly monitor the increase of stiffness level during the maturation process and accordingly adjust the training load. A direct measurement of stiffness is still advised, especially during the sensitive phases of growth such as APHV.

Conclusions
This study showed for the first time that muscle and tendon stiffness measured with MyotonPro is related to drop jump performance in young male basketball players. The stiffness of GAT, RF, PT and QT increase with age during adolescence, but the effects of training are still unclear. Further investigation is warranted to clarify the effects of training on muscle and tendon stiffness and the implications of stiffness for injury risk and incidence in young athletes. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: Not applicable.