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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 29, No 1, January/February 2018
AFRICA
33
diameter (LVED) [Mindray DP-6600, Shenzhen, China
(using a 2.5–3.5 MHz cardiac transducer)]. Resting and stress
electrocardiograms (ECG) were recorded using a Schiller AT
6 ECG machine (Schiller AG, Switzerland) using standardised
treadmill protocols.
9
Vertical jump height (VJH) was measured
as the maximal jump height from a two-legged standing starting
position. The best result from three trials was recorded.
10
The
sit-and-reach test was used to measure truncal flexibility (cm).
9
Statistical analysis
Statistica 13.2 (StatSoft, Tulsa, USA) was used for all analyses.
The Student’s
t
-test was used to compare anthropometric,
cardiac and fitness profiles between male and female participants
(
p
<
0.05). Complete data were not available for all measures
as more advanced body composition measuring tools,
echocardiography and instruments used for performance were
available at only selected regions. The mean of the sample was
used for participants who had missing data.
To determine whether the sample size was suitable, a null
hypothesis of
<
5% was rejected. Assuming a standardised
difference of 0.58 with 90% power for LVED, the minimum
number of participants required in each group is 78 with the
alpha value set at 5%.
11
The actual sample size was above the
minimum required.
Bivariate analyses were conducted to determine the
correlation between variables using Pearson’s correlation by
gender. Multivariable linear regression analyses were conducted
to determine whether independent variables were associated with
the explosive power outcome (VJH) and the cardiac morphology
outcomes (ejection fraction percentage, IVS thickness, and
LVED. Based on the outcome of the bivariate analysis, the
following independent variables were included in the initial
multiple regression model for the fitness outcome: age, gender,
systolic BP, diastolic BP, body fat, lean muscle mass, IVS
thickness, ejection fraction and LVED.
The following independent variables were included in the
initial multivariable linear regression models for the three cardiac
outcomes: age, gender, systolic BP, diastolic BP, body fat, muscle
mass, resting heart rate, IVS thickness, ejection fraction, LVED
and VJH. Multico-linearity was checked using variance inflation
factor (VIF) analysis. All independent variables had VIFs
<
2.2,
indicating no co-linearity.
Results
The participants were from Gauteng (
n
=
35), Kwa-Zulu Natal
(
n
=
27), Mpumalanga (
n
=
36), Western Cape (
n
=
15), Eastern
Cape (
n
=
19), Free State (
n
=
19) and Limpopo (
n
=
16). The
mean age of the study population was 14.8
±
1.3 years, with a
mean BMI of 20.6
±
2.4 kg/m
2
. The female (
n
=
82) footballers
were younger compared with the males (
n
=
85), and presented
with significantly higher body fat measures and lower lean mass
(Table 1).
Resting BP, IVS thickness and LVED were significantly
higher in males compared with females; however end-diastolic
volumes were similar. Flexibility did not differ between the males
and females, however VJH was significantly higher in males
compared with females (Table 2).
For those who had cardiac auscultation conducted (79 males
and 20 females), none of the females presented with abnormal
auscultation, while five of the males had functional systolic
ejection murmurs (6.3%) and one had a tricuspid regurgitation
murmur (3/6) (1.3%). No resting or stress ECGs showed any
pathological abnormalities.
Using bivariate analysis, there was a significant positive
correlation between VJH and age, and lean mass, while body fat,
diastolic BP and trunk flexibility were negatively correlated in
males. In females, body fat and resting heart rate were negatively
correlated with VJH, while lean mass was positively correlated.
Table 2. Pearson’s correlations for VJH, ejection fraction,
IVS thickness and LVED presented by gender
Variables
VJH Ejection fraction IVS thickness
LVED
Males Females Males Females Males Females Males Females
Age
0.4
*
0.1
0.2 0.01 –0.2 –0.1 –0.2 –0.1
BMI
0.2 –0.1 –0.2 0.1
0.2 –0.1
0.2
*
–0.1
Body fat
–0.3
*
–0.4
*
–0.5
*
0.1
0.4
*
–0.2 –0.1 –0.2
Lean mass 0.7
*
0.7
*
0.1 –0.1 –0.1 –0.3
*
0.1 –0.3
*
Ejection
fraction
0.2 –0.1
–0.4 0.5
*
–0.02 0.5
*
IVS
thickness
–0.1 –0.2 –0.4
*
0.5
*
0.1 0.9
*
LVED 0.1 –0.2 –0.02 0.5
*
0.1 0.9
*
Resting
heart rate
–0.2 –0.3
*
0.3
*
0.1 –0.4
*
–0.3
*
–0.1 –0.3
*
Systolic BP –0.1 0.2
0.1 –0.004 0.1 –0.02 0.03 –0.04
Diastolic
BP
–0.3
*
0.2 –0.1 –0.1
0.04 –0.5
*
–0.1 –0.6
*
VJH
0.2 –0.1 –0.1 –0.2
0.1 –0.2
Trunk
flexibility
–0.3
*
0.02 0.1 –0.04 –0.2
*
0.1
0.1 0.2
Data presented as
r
coefficient for males (
n
=
85) and females (
n
=
82);
*
p
<
0.05.
BMI: body mass index; BP: blood pressure; LVED: left ventricular end-diastolic
diameter; IVS: interventricular septal thickness; VJH: vertical jump height.
Table 1. Characteristics of adolescent South African
footballers by gender
Variables
Total sample
(
n
=
167)
Males
(
n
=
85)
Females
(
n
=
82)
Percentage
difference
†
Proportion (%)
100
50.9
49.1
1.8
Age (years)
14.8
±
1.3
15.5
±
1.1
14.1
±
1.1
*
9.5
Height (m)
1.61
±
0.1
1.66
±
0.1
1.56
±
0.1
*
6.2
Weight (kg)
54
±
8.5
58.3
±
7.2
49.5
±
7.3
*
16.3
BMI (kg/m
2
) by age
range
12–13 years (
n
)
19.9
±
2.3 (26) 20.1
±
1 (4)
19.8
±
2.5 (22)
1.5
14–15 years (
n
)
20.7
±
2.1 (92) 20.9
±
1.4 (35) 20.5
±
2.5 (57)
1.9
16–17 years (
n
)
21.1
±
2.2 (49) 21.1
±
1.9 (46) 20.7
±
0.2 (3)
1.9
Body fat (kg)
14.4
±
4.5
12.1
±
3.6
16.8
±
4.1
*
32.5
Lean mass (kg)
51.1
±
4.9
53.5
±
3.9
48.7
±
4.7
*
9.4
Ejection fraction (%)
64
±
7.2
64.8
±
7
63.1
±
7.3
2.7
IVS thickness (mm)
9.2
±
1.7
9.9
±
1.5
8.4
±
1.6
*
16.4
LVED
44.8
±
7.5
48.7
±
3.8
40.7
±
8.1
*
17.9
Resting heart rate
(bpm)
68.9
±
9.8
65.8
±
10.6 72.2
±
7.6
*
9.3
Peak heart rate (bpm) 174.8
±
9
176.7
±
8.1 172.8
±
4.8
*
2.2
Systolic BP (mmHg) 113.8
±
10
117.2
±
10.9 110.3
±
7.5
*
6.1
Diastolic BP (mmHg) 71.2
±
8.2
72.3
±
8.1
69.3
±
8
*
4.2
VJH (cm)
37.2
±
10.3 43.1
±
8.9
31.2
±
8
*
32
Trunk flexibility (cm) 40.3
±
4.9
40.3
±
3.7
40.4
±
5.9
0.3
Data presented as mean
±
SD;
*
p
<
0.05 versus males;
†
Percentage difference (male
minus female/male).
BMI: body mass index; BP: blood pressure; IVS: interventricular septal thickness;
VJH: vertical jump height; LVED: left ventricular end-diastolic diameter.