# Abstract

 The aim of this study was to compare changes in young soccer player's fitness after traditional bilateral vs. unilateral combined plyometric and strength training. Male athletes were randomly divided in two groups; both received the same training, including strength training for knee extensors and flexors, in addition to horizontal plyometric training drills. The only difference between groups was the mode of drills technique: unilateral (UG; *n* = 9; age, 17.3 ± 1.1 years) vs. bilateral (TG; *n* = 9; age, 17.6 ± 0.5 years). One repetition maximum bilateral strength of knee muscle extensors (1RM_KE) and flexors (1RM_KF), change of direction ability (COD), horizontal and vertical jump ability with one (unilateral) and two (bilateral) legs, and limb symmetry index were measured before and after an 8-week in-season intervention period. Some regular soccer drills were replaced by combination of plyometric and strength training drills. Magnitude-based inference statistics were used for between-group and within-group comparisons. Beneficial effects ( *p* < 0.05) in 1RM_KE, COD, and several test of jumping performance were found in both groups in comparison to pre-test values. The limb symmetry index was not affected in either group. The beneficial changes in 1RM_KE (8.1%; *p* = 0.074) and 1RM_KF (6.7%; *p* = 0.004), COD (3.1%; *p* = 0.149), and bilateral jump performance (from 2.7% \[ *p* = 0.535\] to 10.5% \[ *p* = 0.002\]) were *possible* to *most likely* beneficial in the TG than in the UG. However, unilateral jump performance measures achieved *likely* to *most likely* beneficial changes in the UG compared to the TG (from 4.5% \[ *p* = 0.090\] to 8.6% \[ *p* = 0.018\]). The improvements in jumping ability were specific to the type of jump performed, with greater improvements in unilateral jump performance in the UG and bilateral jump performance in the TG. Therefore, bilateral strength and plyometric training should be complemented with unilateral drills, in order to maximize adaptations. # Results 

## Within-group changes Beneficial changes in 1RM_KE (TG: 8 out of 9 individual improvements; UG: 4 out of 9 individual improvements), COD (TG: 9/9; UG: 6/9), CMJ (TG: 8/9; UG: 6/9), CMJd (TG: 5/9; UG: 8/9), CMJnd (TG: 4/9; UG: 7/9), SJnd (TG: 3/9; UG: 6/9), and HC3Jnd (TG: 4/9; UG: 7/9) were found in both groups in comparison to pre-test values. In the TG, beneficial gains were reported 1RM_KF (6 out of 9 improvements), SJ (9/9) and HCMJ (9/9). In the UG SJd (8/9), H3Jd (8/9), H3Jnd (8/9), and HC3Jd (8/9) were likely to very likely improved (Table [2](#) ). The limb symmetry index was not affected in either group. Furthermore, significant differences in each group were reported in Table [2](#) . ::::table-wrap

::: caption
Changes in athletic performance following bilateral combined plyometric
and strength training ( *n* = 9) or unilateral combined plyometric and
strength training ( *n* = 9). Data are mean ± SD.
:::

```
             **Bilateral ( *n* = 9)**   **Unilateral ( *n* = 9)**                                                                                                                                                                          
```

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```
             **Pre-test**               **Post-test**               **% (CL90%)**      **ES (CL90%)**        **Chances**   **Outcome**           **Pre-test**   **Post-test**      **% (CL90%)**       **ES (CL90%)**        **Chances**   **Outcome**
```

1RM_KE (kg) 207.9 ± 30.2 237.2 ± 51.0 ^*^ 12.9 (7.4; 18.6) 0.77 (0.46; 1.09) 100/0/0% Most Likely 179.8 ± 9.1 182.4 ± 8.4 ^*^ 1.5 (0.4; 2.6) 0.27 (0.07; 0.47) 73/27/0% Possibly
1RM_KF (kg) 93.8 ± 11.2 99.8 ± 10.2 ^*^ 6.6 (2.2; 11.2) 0.48 (0.16; 0.80) 93/7/0% Likely 86.5 ± 2.9 86.8 ± 2.9 0.3 (−0.3; 1.0) 0.09 (−0.08; 0.25) 12/87/1% Likely trivial
COD (s) 5.67 ± 0.29 5.46 ± 0.20 ^*^ 3.7 (1.3; 6.0) 0.66 (0.23; 1.09) 96/4/0% Very Likely 5.80 ± 0.29 5.67 ± 0.16 2.2 (−0.7; 5.1) 0.41 (−0.13; 0.96) 76/21/4% Likely
CMJ (m) 0.37 ± 0.05 0.40 ± 0.05 ^*^ 8.6 (4.2; 13.1) 0.56 (0.28; 0.84) 98/2/0% Very Likely 0.38 ± 0.04 0.40 ± 0.05 5.2 (−0.8; 11.6) 0.51 (−0.08; 1.11) 82/15/3% Likely
CMJd (m) 0.22 ± 0.03 0.22 ± 0.03 2.8 (0.1; 5.6) 0.18 (0.01; 0.35) 41/59/0% Possibly 0.23 ± 0.03 0.26 ± 0.03 ^*^ 10.0 (6.9; 13.1) 0.85 (0.59; 1.10) 100/0/0% Most Likely
CMJnd (m) 0.20 ± 0.03 0.20 ± 0.02 2.7 (−0.5; 6.0) 0.19 (−0.03; 0.42) 47/52/1% Possibly 0.22 ± 0.02 0.24 ± 0.03 ^*^ 6.4 (2.1; 10.9) 0.50 (0.17; 0.83) 93/6/0% Likely
LSI_CMJ (%) 90.8 ± 4.6 90.7 ± 4.8 −0.1 (−4.0; 3.8) −0.02 (−0.71; 0.66) 28/40/32% Unclear 91.9 ± 6.3 91.1 ± 6.8 −0.8 (−3.2; 1.6) −0.11 (−0.42; 0.20) 5/65/30% Possibly harmful
SJ (m) 0.35 ± 0.05 0.37 ± 0.04 ^*^ 6.5 (2.6; 10.6) 0.36 (0.15; 0.58) 90/10/0% Likely 0.36 ± 0.05 0.34 ± 0.04 −6.0 (−12.8; 1.2) −0.45 (−0.99; 0.09) 3/18/79% Likely harmful
SJd (m) 0.21 ± 0.03 0.21 ± 0.03 1.1 (−1.4; 3.6) 0.08 (−0.11; 0.26) 12/87/1% Likely trivial 0.22 ± 0.02 0.24 ± 0.02 ^*^ 8.1 (3.1; 13.2) 0.64 (0.25; 1.02) 97/3/0% Very Likely
SJnd (m) 0.18 ± 0.02 0.19 ± 0.02 3.1 (−0.1; 6.5) 0.26 (−0.01; 0.53) 66/33/1% Possibly 0.22 ± 0.03 0.23 ± 0.03 ^*^ 4.8 (1.3; 8.5) 0.35 (0.09; 0.60) 84/15/0% Likely
LSI_SJ (%) 87.7 ± 7.9 88.6 ± 9.2 1.0 (−1.3; 3.3) 0.09 (−0.13; 0.32) 20/78/2% Likely trivial 91.5 ± 8.4 89.8 ± 3.1 −1.5 (−6.3; 3.6) −0.14 (−0.61; 0.33) 11/48/41% Possibly harmful
H3Jd (m) 6.49 ± 0.30 6.53 ± 0.28 0.5 (−0.2; 1.2) 0.10 (−0.04; 0.24) 11/89/0% Likely trivial 6.19 ± 0.32 6.62 ± 0.31 ^*^ 7.1 (3.7; 10.5) 1.21 (0.64; 1.78) 99/0/0% Very Likely
H3Jnd (m) 6.29 ± 0.30 6.29 ± 0.31 0.0 (−0.4; 0.3) −0.01 (−0.07; 0.05) 0/100/0% Most Likely trivial 5.97 ± 0.33 6.35 ± 0.34 ^*^ 6.4 (2.7; 10.3) 0.99 (0.42; 1.55) 98/1/0% Very Likely
LSI_H3J (%) 96.8 ± 4.2 96.1 ± 3.9 −0.7 (−1.4; 0.0) −0.14 (−0.28; 0.00) 0/78/22% Likely trivial 96.5 ± 2.8 95.2 ± 3.3 −1.4 (−3.8; 1.1) −0.43 (−1.20; 0.34) 8/21/71% Possibly harmful
HC3Jd (m) 5.62 ± 0.40 5.70 ± 0.39 1.5 (−1.9; 4.9) 0.17 (−0.23; 0.57) 45/49/6% Unclear 5.66 ± 0.42 5.98 ± 0.38 ^*^ 5.8 (1.7; 10.0) 0.65 (0.20; 1.10) 95/5/0% Likely
HC3Jnd (m) 5.49 ± 0.47 5.71 ± 0.71 4.0 (−0.8; 9.1) 0.39 (−0.08; 0.87) 76/21/2% Likely 5.28 ± 0.56 5.70 ± 0.45 ^*^ 8.2 (2.1; 14.6) 0.64 (0.17; 1.11) 94/6/0% Likely
LSI_HC3J (%) 95.6 ± 1.6 95.7 ± 2.4 0.1 (−1.7; 1.9) 0.04 (−0.95; 1.03) 39/28/33% Unclear 93.3 ± 5.9 95.3 ± 3.8 2.2 (−2.5; 7.2) 0.30 (−0.34; 0.94) 61/30/9% Unclear
HCMJ (m) 2.24 ± 0.08 2.34 ± 0.07 ^*^ 4.5 (2.5; 6.5) 1.17 (0.67; 1.67) 100/0/0% Most Likely 2.20 ± 0.15 2.21 ± 0.15 0.7 (0.1; 1.3) 0.09 (0.01; 0.17) 2/98/0% Very unlikely
H3CMJ (m) 6.56 ± 0.58 6.65 ± 0.56 ^*^ 1.3 (0.5; 2.2) 0.13 (0.05; 0.21) 8/92/0% Unlikely 6.67 ± 0.58 6.84 ± 0.52 2.7 (−2.1; 7.6) 0.28 (−0.23; 0.79) 62/33/6% Unclear

*1RM_KE, one-repetition maximum in knee extensors; 1RM_KF,
one-repetition maximum in knee flexors; COD, modified agility test; CMJ,
bilateral countermovement jump; CMJd and CMJnd, unilateral
countermovement jump with dominant and non-dominant leg; LSI_CMJ, limb
symmetry index in unilateral countermovement jump; SJ, bilateral squat
jump; SJd and SJnd, unilateral squat jump with dominant and non-dominant
leg; LSI_SJ, limb symmetry index in unilateral squat jump H3Jd and
H3Jnd, horizontal triple jump with dominant and non-dominant leg;
LSI_H3J, limb symmetry index in unilateral horizontal triple jump; HC3Jd
and HC3Jnd, crossover horizontal triple jump with dominant and
non-dominant leg; LSI_HC3J, limb symmetry index in unilateral crossover
triple jump; HCMJ, bilateral horizontal jump with arm swing; H3CMJ,
triple bilateral horizontal jump with arm swing. Significant differences
(p < 0.05) between pre-test to post-test were indicated with ^*^\*.:::: 

## Between-group changes Results from between-group analyses are illustrated in Figure [2](#) . The beneficial changes in 1RM_KE (8.1% [CL90%: 4.2; 11.8]; *QC* = 99/0/0%; *p* = 0.074) and 1RM_KF (6.7% [CL90%: 2.8; 10.4]; *QC* = 98/1/0%; *p* = 0.004), COD (3.1% [CL90%: 0.5; 5.7]; *QC* = 94/5/1%, *p* = 0.149), CMJ (2.7% [CL90%: −4.8; 9.7]; *QC* = 61/23/16%; *p* = 0.535), SJ (10.5% [CL90%: 3.5; 17.0]; *QC* = 98/1/1%; *p* = 0.013) and HCMJ (4.4% [CL90%: 2.7; 6.0]; *QC* = 100/0/0%; *p* = 0.002) were possibly to most likely beneficial in the TG than in the UG. However, CMJd (8.4% [CL90%: 4.7; 12.1]; *QC* = 100/0/0%; *p* = 0.006), CMJnd (5.9% [CL90%: 0.0; 12.1]; *QC* = 91/6/3%; *p* = 0.081), SJd (8.6% [CL90%: 3.6; 13.8]; *QC* = 99/1/0%, *p* = 0.018), H3Jd (4.8% [CL90%: 1.4; 8.2]; *QC* = 98/1/1%, *p* = 0.023), H3Jnd (5.0% [CL90%: 1.2; 8.9]; *QC* = 98/2/0%; *p* = 0.034) and HC3Jd (4.5% [CL90%: 0.1; 9.1]; *QC* = 89/9/2%; *p* = 0.090) achieved likely to most likely beneficial changes in the UG compared to the TG. <figure> <p><img src="" /></p> <figcaption>Efficiency of the unilateral (UNI) compared to bilateral (BIL) combined plyometric and strength training to improve one-repetition maximum in knee extensors (1RM_KE), one-repetition maximum in knee flexors (1RM_KF), a modified agility test (COD), bilateral countermovement jump (CMJ), unilateral countermovement jump with dominant and non-dominant leg (CMJd and CMJnd), limb symmetry index in unilateral countermovement jump (LSI_CMJ), bilateral squat jump (SJ), unilateral squat jump with dominant and non-dominant leg (SJd and SJnd), limb symmetry index in unilateral squat jump (LSI_SJ), horizontal triple jump with dominant and non-dominant leg (H3Jd and H3Jnd), limb symmetry index in unilateral horizontal triple jump (LSI_H3J), crossover horizontal triple jump with dominant and non-dominant leg (HC3Jd and HC3Jnd), limb symmetry index in unilateral crossover triple jump (LSI_HC3J), bilateral horizontal jump with arm swing (HCMJ), and triple bilateral horizontal jump with arm swing (H3CMJ) (bars indicate uncertainty in the true mean changes with 90% confidence limits). Trivial areas were the smallest worthwhile change (SWC) (see Methods).</figcaption> </figure>


Extracted trial result:
Outcome: Change of direction improvement (continuous)
Intervention: Bilateral training
Comparator: Unilateral training
Intervention group size: 9
Intervention mean: 5.46
Intervention standard deviation: 0.2
Comparator group size: 9
Comparator mean: 5.67
Comparator standard deviation: 0.16
Notes: Found in table raw values