25 / 64
CARDIOVASCULAR JOURNAL OF AFRICA • Volume 31, No 6, November/December 2020
AFRICA
305
associations between moderate-to-vigorous PA or vigorous PA
and CIMT.
25
Teachers are considered to be in a high-stress profession where
sub-optimal facilities, lack of support, unsupportive parents,
teaching evaluation and time-management issues, changes in
curricula, organisational policies, heavy workload, overcrowded
classes, limited resources, high accountability, uncertainties over
job security, low salaries, fatigue and parental expectations all
contribute to the stress associated with the profession.
26-29
Due
to the workload and nature of the occupation, teachers spend
a lot of their time sedentary and little time at higher levels of
PA.
30
The high job demands together with a lack of PA may
lead to high blood pressure, heart disease, stroke, diabetes and
cancer.
27,28
Cardiovascular disease and physical inactivity are
among the significant causes of mortality and morbidity in both
the general population
31,32
and in teachers.
20
We aimed therefore
to investigate the relationship between objectively measured PA
over seven consecutive days and CIMT among teachers in South
Africa.
Methods
This research formed part of the Sympathetic Activity and
Ambulatory Blood Pressure in Africans (SABPA) prospective
cohort study, which commenced in 2008/2009 (phase 1) and
was followed up in 2011/2012 (phase 2). Phase 2 data were
collected similarly to the phase 1 baseline measurements.
33
Given
the objective of the study, a cross-sectional study design was
followed using the second phase of measurement.
Urban-dwelling South African school teachers residing in
the Dr Kenneth Kaunda education district (Potchefstroom
and Klerksdorp), North West Province, South Africa, were
recruited to participate in the SABPA study (
n
= 2 170). The
study excluded pregnant or lactating female teachers, users of
α
- and
β
-blockers, psychotropic substance abusers, blood donors
or people vaccinated in the last three months, or individuals
with tympanum temperature
≥
37.5°C. Preliminary screening
identified eligible participants (
n
= 409), all of whom were school
teachers (aged 25–65 years) of similar socio-economic standing.
Participants who wore the ActiHeart (GBO/67703, CamNtech
Ltd, Cambridgeshire, UK) for a full seven days or had less than
40 minutes of ‘lost’ time (
n
= 216) in phase 2 were included.
The SABPA study was approved by the Ethics Review
Board of the North-West University (Potchefstroom campus:
NWU-0036-07-S6) and adhered to the principles outlined in the
Declaration of Helsinki (2004). Permission to conduct this study
was obtained from the North West Department of Education,
as well as the South African Democratic Teachers Union. All
participants voluntarily signed an informed consent form before
any data were collected.
A 24-hour standardised diet was provided. Each participant’s
data were collected over eight days. During the first two days,
24-hour ambulatory blood pressure, information on lifestyle
risk factors and cardiovascular and biochemical measurements
were collected. For the following seven days, an ActiHeart
recorded the participant’s PA. The participants stayed over at
the Metabolic Unit research facility at North-West University
where they were introduced to the experimental set-up and were
assigned a private bedroom.
Before resuming their normal activities on day 1, the
Cardiotens ambulatory blood pressure (BP) monitor (Meditech,
Budapest, Hungary) was fitted to measure 24-hour BP. Normal
BP, categorised as systolic BP (SBP) and diastolic BP (DBP), was
130/80 mmHg.
34
On the second day of measurements, body composition and
CIMT were measured. Participants’ height (cm), body weight
(kg) and waist circumference (WC; cm) were measured by two
level-two kinanthropometrists in triplicate according to the
International Society for the Advancement of Kinanthropometry
(ISAK).
35
Waist-to-height ratio (WHtR) of the participants was
calculated as weight (kg)/height (m).
BMI was calculated as weight (kg)/height (m)
2
and expressed
in kg/m
2
.
36
BMI was classified according to the cut-off points of
the American College of Sports Medicine (ACSM)
36
as follows:
underweight = BMI < 18.5 kg/m
2
; normal weight = BMI
between 18.5 and 24.9 kg/m
2
; overweight = BMI between 25.0
and 29.9 kg/m
2
; and obesity = BMI ≥ 30 kg/m
2
. Intra- and inter-
observer variability were less than 5%.
PA of the participants was measured using a combined heart
rate and accelerometer (the ActiHeart) over seven consecutive
days. Participants were requested to continue with their daily
activities, continuously wearing the ActiHeart monitor while
awake or asleep. Individual step test calibration was not
performed due to the high cardiovascular risk profiles of
various participants
37
and time restrictions during clinical data
collection. Therefore, self-reported PA was used to programme
the ActiHeart for each participant.
38
The resting heart rate of the participants was obtained from
a resting 12-lead electrocardiogram (NORAV Medical Ltd PC
1200, software v5.030, Kiryat Bialik, Israel), performed by a
registered nurse, and was used to calculate the sleeping heart
rate required to be entered into the ActiHeart program when the
device was being fitted to each participant.
The seven-day recordings for each participant were visually
assessed to distinguish between time spent awake (awake time)
and time spent asleep. Heart rate, metabolic equivalent of task
(METs) and activity levels were used to distinguish between time
spent awake and asleep. When the heart rate gradually decreased
(throughout 15 or more epochs) in the evening to less than the
average heart rate in a selected awake-time sedentary sample
period, and the activity level was equal to zero, the participant
was considered to be sleeping. The end of sleeping time could
clearly be seen by an immediate increase in heart rate of more
than 10 to 20 beats per minute, as well as increased METs and
increased activity level.
The ActiHeart software was used to derive daily time spent in
various MET categories according to activity energy expenditure
(AEE). The derived daily time spent in multiple MET categories
was grouped according to daily awake time being sedentary (≤
1.5 METs) and time participating in light-intensity PA (1.5–3
METs).
39
AEE, total energy expenditure (TEE) and PA level
(PAL) were also determined by the ActiHeart using inbuilt
equations based on a branched model approach, calculated
based on the combination of heart rate and accelerometer. PAL
was calculated as TEE/resting energy expenditure (REE).
Ultimately, after the data were analysed, participants were
allocated to one of two PA groups for analysis purposes,
sedentary (≤ 1.5 METs) or light-intensity PA (1.5–3 METs),
depending on their total activity levels. Only one participant was
classified as moderate-to-high PA (> 3 METs) and, based on