Acute Effects of Drop Jumps on Lower Limb Stiffness and Mechanical and Kinematic Parameters During High-Speed Treadmill Running

his study aimed to investigate the post-activation performance enhancement ef- fects of drop jumps (DJs) on lower limb stiffness [leg stiffness (K leg); vertical stiffness (Kvert)] and the related mechanical [maximal ground reaction force (Fmax), vertical displacement of the centre of mass (∆y), and lower limb length (∆L)] and kinematic parameters [step length (SL), step frequency (SF), flight time (FT), and contact time (CT)] during high-speed treadmill running. For this purpose, 18 male physical education students performed 10 s running bouts on a treadmill at a speed of 6.67 m·s −1 in an experimental condition (EC) or in a control condition (CC). During the EC, following a 5 min treadmill running warm-up at 2.22 m·s −1 , the participants were tested pre and 3 min post five DJs, while during the CC, the participants did not perform DJs. The study results revealed that EC significantly increased Fmax,∆y, SL, and FT, while SF decreased after performing drop jumps (mean dif- ferences: 0.026±0.007 kN,p= 0.003; 0.001±0.001 m,p= 0.025;0.034±0.013 m ,p= 0.019; 0.05±0.001 s,p= 0.005 and−0.057±0.023 Hz,p= 0.026, respectively). K legand Kvert showed no significant differences (p> 0.05) following the preconditioning exercise. The findings suggest that a plyometric stimulus of low volume and a short recovery period prior to high-speed treadmill running may be beneficial to acutely improve running mechanical and kinematic parameters

0.026±0.007 kN,p= 0.003; 0.001±0.001 m,p= 0.025;0.034±0.013 m ,p= 0.019; 0.05±0.001 s,p= 0.005 and−0.057±0.023 Hz,p= 0.026, respectively). K legand Kvert showed no significant differences (p> 0.05) following the preconditioning exercise. The findings suggest that a plyometric stimulus of low volume and a short recovery period prior to high-speed treadmill running may be beneficial to acutely improve running mechanical and kinematic parameters without affecting lower limb stiffness. Keywords:acute effect; running mechanics; running kinematics; stiffness 1. Introduction A warm-up prior to a competitive exercise bout is a widely accepted strategy in train- ing sessions or athletic competitions which enhances subsequent performance. This may be attributed to post-activation performance enhancement (PAPE) as well as to temperature- related mechanisms and neuromuscular adaptations to the stretch reflex, Golgi tendon organ, and muscles’ elastic energy [1,2]. The mechanism underpinning PAPE involves various physiological and neuromuscular processes, including the phosphorylation of contractile proteins’ activation of fast-twitch muscle fibres, an increase in muscle tem- perature, and muscle and muscle fibre water content. Briefly, a preconditioning activity can enhance the contractile properties of muscle fibres which leads to producing higher amounts of force and power. Additionally, the rise in muscle temperature and the in- creased water content within muscle fibres following the preconditioning activity can influence cross-bridge cycling rates, thereby augmenting force generation and improving subsequent performance [1]. Warm-ups that consist of maximal or near maximal isometric contractions [3,4] , heavy load exercises [5–7], and plyometric exercises [8–10] have often Appl. Sci.2025,15, 242 https://doi.org/10.3390/app15010242

Appl. Sci.2025,15, 242 2 of 11 been used as a preload stimulus to elicit PAPE. Priming plyometric exercises during warm- up may be beneficial to performance in explosive athletic events such as running, jumping, and throwing [11–13]. In addition, the positive effects of plyometric exercises when used as a precondition activity have been examined regarding running performance and also regarding critical kinematic parameters such as contact time (CT), flight time (FT), step length (SL), and step frequency (SF) [9,14,15]. However, a limited number of studies have been conducted regarding the acute changes in running kinematics after a PAPE protocol. Additionally, from a mechanical point of view, the elastic properties of lower limbs are considered to be a crucial factor that affects running performance. The elastic properties of lower limbs during the stance phase of the running gait are successfully described by leg and vertical stiffness (K legand Kvert, respectively). Stiffness reflects the ability to store and return potential elastic energy during cyclic locomotion such as running and hopping [16]. It is, however, necessary to obtain an optimal level of stiffness to enhance the hopping and jumping performances [17]. Research about the effects of PAPE on stiffness is limited. Previous studies reported an increase in Kvertduring a vertical jump after three repetitions at 90% of a one Repetition Maximum (1RM) back squat (BS) exercise [18]. Furthermore, in a previous study, a series of parallel BSs (5, 4, and 3 repetitions at 30%, 50%, and 70% of the 1RM, respectively) followed by a 3 min rest period did not produce any change in Kvert and countermovement jump (CMJ) [19]. On the other hand, an increase in K leg(20.4%), in maximal running speed (2.9%), and in running economy (6.0%) have been reported after performing a potentiating exercise (6×10 s strides) with a weighted vest [20]. The drop jump (DJ) is one of the plyometric exercises which is commonly used as an effective approach to promote PAPE [21]. Briefly, previous studies reported that following five repetitions of DJs, from a height of 0.4 m, just before a throwing action, can improve throwing performance in

performing a potentiating exercise (6×10 s strides) with a weighted vest [20]. The drop jump (DJ) is one of the plyometric exercises which is commonly used as an effective approach to promote PAPE [21]. Briefly, previous studies reported that following five repetitions of DJs, from a height of 0.4 m, just before a throwing action, can improve throwing performance in athletes with a high percentage of fast-twitch muscle fibres [12]. Additionally, it has been highlighted that the inclusion of five DJs with a height related to the individual’s best reactive strength index, 3 min before a 1000 m trial, induces an increase in performance in elite male endurance runners [22]. Similarly, it has been indicated that there is an increase in the take-off vertical velocity after a precondition activity of five DJs 2 min before jumping attempts during a long jump competition [21]. Finally, an improvement in CMJ performance 2 min after five DJs in volleyball players [8] has been reported. However, to the authors’ knowledge, the inclusion of DJs as a preload stimulus to elicit PAPE effects on lower limb stiffness together with related mechanic and kinematic characteristics during treadmill running at a high speed have not yet been investigated. Therefore, the aim of this study was to examine the acute effects of five DJ repetitions on K leg, Kvert, and the related mechanical [maximal ground reaction force (Fmax), change in leg length (∆L), and vertical displacement of the centre of mass (∆y)] and kinematic parameters (CT, FT, SF, and SL) during treadmill running at a speed of 6.67 m·s −1 . It was hypothesised that K legand Kvertand the associated running mechanical and kinematic parameters during treadmill running would be improved after the potentiating intervention. 2. Materials and Methods 2.1. Experimental Design To test the hypotheses of the present study, a within-participant experimental design [an experimental condition (EC) and a control condition (CC)] was used to examine the acute effect of DJs on spring mass variables (K leg, Kvert, Fmax,∆y, and∆L) and running kine- matics (CT, FT, SF, and SL) during high-speed treadmill running. Pre- and postcondition assessments

Methods 2.1. Experimental Design To test the hypotheses of the present study, a within-participant experimental design [an experimental condition (EC) and a control condition (CC)] was used to examine the acute effect of DJs on spring mass variables (K leg, Kvert, Fmax,∆y, and∆L) and running kine- matics (CT, FT, SF, and SL) during high-speed treadmill running. Pre- and postcondition assessments were used to investigate the PAPE effects of the EC and CC on the dependent variables. For the evaluation of the dependent variables, participants ran on a motorised treadmill at a speed of 6.67 m·s −1 .

Appl. Sci.2025,15, 242 3 of 11 2.2. Participants Eighteen physical education male students [(mean±SD) age: 22.11±2.22 years, body mass: 70.33±5.69 kg, and stature: 1.78±0.04 m] with sufficient experience in treadmill running provided written informed consent to voluntarily participate in this study, which was approved by the Ethics Committee of the School of Physical Education and Sport Science of Athens, Greece, and was conducted in accordance and agreement with the Declaration of Helsinki. All participants were without physical limitations or musculoskeletal injuries that could affect the validity of the testing. 2.3. Procedures The participants visited the lab four times. All sessions included a 5 min warm-up on a motorised treadmill (Technogym Runrace 1200, Gambettola, Italy) at a speed of2.22 m·s −1 . During the last 30 s, the speed was steadily increased until a speed of 5.55 m·s −1 was obtained. Thereafter, the speed was steadily reduced until the treadmill’s belt came to a halt. In the first session, participants were familiarised with the testing procedures. Specifically, in order to be familiarised with high-speed treadmill running, participants performed 7×10 s running bouts at 6.67 m·s −1 , separated by a 5 min recovery. The second session included the collection of participants’ anthropometric characteristics (body mass, stature, and leg length) and familiarisation with the DJ technique. In particular, participants were instructed to step off the block to the floor and land with both feet, avoiding any initial upward propulsion. Additionally, they were instructed to jump up as high as possible after landing, while keeping their hands on their hips. During the third and fourth sessions, participants were randomly assigned to execute one of the two testing protocols, the EC and CC, which were separated by at least 48 h. During the EC session, 5 min after the standardised warm-up, participants performed the pre-test (they ran on a treadmill at 6.67 m·s −1 for 10 s using their preferred SL and SF). After a 10 min resting period, they executed 5 consecutive DJs from a height of 0.4 m (~30 s total time duration). After 3 min of recovery, the participants performed the

session, 5 min after the standardised warm-up, participants performed the pre-test (they ran on a treadmill at 6.67 m·s −1 for 10 s using their preferred SL and SF). After a 10 min resting period, they executed 5 consecutive DJs from a height of 0.4 m (~30 s total time duration). After 3 min of recovery, the participants performed the post-test running trial (similar to pre-test) (see Figure). During the CC session, participants performed the EC session protocol without the 5 DJs (see Figure).Appl. Sci. 2025, 15, x FOR PEER REVIEW 3 of 11 assessments were used to investigate the PAPE effects of the EC and CC on the dependent variables. For the evaluation of the dependent variables, participants ran on a motorised treadmill at a speed of 6.67 m·s −1 . 2.2. Participants Eighteen physical education male students [(mean ± SD) age: 22.11 ± 2.22 years, body mass: 70.33 ± 5.69 kg, and stature: 1.78 ± 0.04 m] with sufficient experience in treadmill running provided written informed consent to voluntarily participate in this study, which was approved by the Ethics Committee of the School of Physical Education and Sport Science of Athens, Greece, and was conducted in accordance and agreement with the Dec- laration of Helsinki. All participants were without physical limitations or musculoskeletal injuries that could affect the validity of the testing. 2.3. Procedures The participants visited the lab four times. All sessions included a 5 min warm-up on a motorised treadmill (Technogym Runrace 1200, Gambettola, Italy) at a speed of 2.22 m·s −1 . During the last 30 s, the speed was steadily increased until a speed of 5.55 m·s −1 was obtained. Thereafter, the speed was steadily reduced until the treadmill’s belt came to a halt. In the first session, participants were familiarised with the testing procedures. Spe- cifically, in order to be familiarised with high-speed treadmill running, participants per- formed 7 × 10 s running bouts at 6.67 m·s −1 , separated by a 5 min recovery. The second session included the collection of participants’ anthropometric characteristics (body mass, stature, and leg length) and familiarisation

In the first session, participants were familiarised with the testing procedures. Spe- cifically, in order to be familiarised with high-speed treadmill running, participants per- formed 7 × 10 s running bouts at 6.67 m·s −1 , separated by a 5 min recovery. The second session included the collection of participants’ anthropometric characteristics (body mass, stature, and leg length) and familiarisation with the DJ technique. In particular, partici- pants were instructed to step off the block to the floor and land with both feet, avoiding any initial upward propulsion. Additionally, they were instructed to jump up as high as possible after landing, while keeping their hands on their hips. During the third and fourth sessions, participants were randomly assigned to execute one of the two testing protocols, the EC and CC, which were separated by at least 48 h. During the EC session, 5 min after the standardised warm-up, participants performed the pre-test (they ran on a treadmill at 6.67 m·s −1 for 10 s using their preferred SL and SF). After a 10 min resting period, they executed 5 consecutive DJs from a height of 0.4 m (~30 s total time duration). After 3 min of recovery, the participants performed the post-test running trial (similar to pre-test) (see Figure 1). During the CC session, participants performed the EC session protocol without the 5 DJs (see Figure 1). Figure 1. Schematic representation of the experimental design for the PAPE and control condi- tions. 2.4. Data Collection All the pre- and post-tests were recorded by a high-speed camera (Casio Exilim EX- F1, Tokyo, Japan) sampling at 1200 Hz. The high-speed camera was fixed on a tripod at a Figure 1.Schematic representation of the experimental design for the PAPE and control conditions. 2.4. Data Collection All the pre- and post-tests were recorded by a high-speed camera (Casio Exilim EX-F1, Tokyo, Japan) sampling at 1200 Hz. The high-speed camera was fixed on a tripod at a height of 0.4 m and placed 1 m away from the treadmill, perpendicular to the running direction. The shutter speed was set at 1200 at 1/1250 s, and

Data Collection All the pre- and post-tests were recorded by a high-speed camera (Casio Exilim EX-F1, Tokyo, Japan) sampling at 1200 Hz. The high-speed camera was fixed on a tripod at a height of 0.4 m and placed 1 m away from the treadmill, perpendicular to the running direction. The shutter speed was set at 1200 at 1/1250 s, and the zoom lens was adjusted to ensure a limited area of/at the moment of touchdown (field of view 0.4×0.3 m). Participants were asked to wear the same sports shoes and to refrain from intense exercise during the testing period.

Appl. Sci.2025,15, 242 4 of 11 2.5. Data Analysis The measurements of lower limb stiffness were constructed according to the “sine wave” method proposed by Morin et al. [23]. Specifically, the stiffness of a deformable sys- tem is quantitatively described as the ratio of the applied force to the ensuing deformation. In the case of a purely linear spring, the correlation between the applied compressive force and the subsequent deformation (∆L) is characterised by a linear relationship. Thus, the stiffness of the system, denoted as (k) (expressed in N m −1 ), is represented by the slope of this linear correlation [23]. This method allows for the estimation of K legand Kvertin running by using only a few simple mechanical parameters (CT and FT, body mass (m), anteroposterior velocity (v), and lower limb length (L) from the great trochanter and the ground). According to the sine wave method, the force curve over time during contact can be fitted by means of a simple sine function. This method produced a lower bias (0.12–6%) compared to the reference values from a force plate [23]. Coleman et al. [24] evaluated several mathematical models and found that the sine wave method presented values closest to those presented by the gold standard model (5% mean difference;ICC = 0.901). Further- more, the measurements of K legand Kvertobtained during treadmill running by using the sine wave method are symmetrical [25] and highly reliable for both inter-day and intra-day (ICCs: 0.873–0.982) designs [26,27]. The analysis of all video-recorded steps was performed by the Quintic Biomechanics v31 (Consultancy Ltd., Birmingham, UK) software. The CT and FT were obtained according to regular assessments [27], and a total of 10 consecutive steps of each leg were analysed. The 10 steps’ mean value for CT and FT were used for the calculation of K legand Kvert using the following computations: Kvert= Fmax ∆y (1) Fmax=mg π 2 θ FT CT +1 ι (2) ∆y=− FmaxCT 2 mπ 2 +g CT 2 8 (3) K leg= Fmax ∆L (4) ∆L=L− s L 2 − θ vCT 2 ι 2 +∆y (5) The correction

value for CT and FT were used for the calculation of K legand Kvert using the following computations: Kvert= Fmax ∆y (1) Fmax=mg π 2 θ FT CT +1 ι (2) ∆y=− FmaxCT 2 mπ 2 +g CT 2 8 (3) K leg= Fmax ∆L (4) ∆L=L− s L 2 − θ vCT 2 ι 2 +∆y (5) The correction factor (1.0496 K) was performed before the analysis of K legand Kvert values in order to improve the accuracy of the method according to the suggestion of Coleman et al. [24]. 2.6. Statistical Analysis Data are presented as means±standard deviation. The normality of the distribution of the data was checked by the Shapiro–Wilk test. A two-way (time×condition) repeated measures ANOVA was performed to investigate the impact of time (pre – post) and condition (EC and CC) on the dependent variables. The sphericity assumption was checked by Mauchly’s test of sphericity, and the degrees of freedom were corrected using the Greenhouse–Geisser correction when necessary. Bonferroni’s post hoc comparisons were performed when applicable, to examine interaction effects as well as the main effects of time and condition on the dependent variables. The magnitude between pre and post intervention effects was assessed according to Cohen’s d effect size (ES) [28]. The criteria to interpret the magnitude of the ES were as follows: trivial (d < 0.20), small (d = 0.20),

Appl. Sci.2025,15, 242 5 of 11 moderate (d = 0.60), and large (d = 1.20). All statistical analyses were performed in SPSS v27 (IBM Corp., Armonk, NY, USA), and statistical significance was set at an alpha level of 0.05. 3. Results All the dependent variables were normally distributed. Means and standard de- viations were calculated in each trial for the measured variables. Moreover, intraclass correlation coefficients ranging from 0.93 to 0.97 were found for all the analysed variables measured between values of the familiarisation and testing days (premeasurements for the EC and CC). These results show the reliability and constancy of the running pattern. The descriptive data and post–pre changes’ ESs are shown in Table. Table 1.Mean values, standard deviation, and effect size for running kinematics and spring–mass variables between pre and post measurements for both conditions. Variable CC EC Pre Post ES Pre Post ES CT (s) 0.166 ±0.010 0.166 ±0.011 0.14 0.167 ±0.011 0.167 ±0.010 0.03 FT (s) 0.126 ±0.021 0.126 ±0.020 0.02 0.126 ±0.022 0.130 ±0.021 * 0.99 SF (Hz) 3.442 ±0.218 3.436 ±0.206 0.09 3.429 ±0.228 3.379 ±0.212 * 0.72 SL (m) 1.945 ±0.128 1.948 ±0.120 0.07 1.953 ±0.133 1.981 ±0.127 * 0.74 Fmax(kN) 1.906 ±0.190 1.904 ±0.183 0.06 1.907 ±0.189 1.932 ±0.186 * 0.82 ∆y (m) 0.042 ±0.005 0.042 ±0.005 0.02 0.042 ±0.005 0.043 ±0.005 * 0.71 ∆L (m) 0.222 ±0.025 0.224 ±0.026 0.21 0.224 ±0.026 0.226 ±0.026 0.15 Kvert(kN/m) 45.983 ±4.832 45.823±5.150 0.14 45.724 ±5.332 45.140±5.179 0.33 K leg(kN/m) 8.693 ±1.407 8.658 ±1.454 0.14 8.639 ±1.451 8.705 ±1.476 0.15 CC: control condition, EC: experimental condition, ES: effect size, CT: contact time, FT: flight time, SF: step frequency, SL: step length, Fmax: maximal ground reaction force,∆y: vertical displacement of the centre of mass, ∆L: change in leg length, Kvert: vertical stiffness, and K leg: leg stiffness, * significant different from Pre (p< 0.05). In the results of the analysis of variance, a significant interaction effect (condition x time) was present for the FT (F = 8.14,p= 0.011,η 2= 0.32), SL (F = 4.76,p= 0.043,η 2 = 0.22), Fmax(F = 6.67,p= 0.019,η 2= 0.28), and∆y (F

The study examines the effects of drop jumps on running mechanics.