Participants and procedure
Over a 6-month period, patients with schizophrenia of the University Psychiatric Centre of Kortenberg and the Brussels Nighthospital Belgium were invited to participate. Psychiatric diagnosis based on the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV) criteria  was established by experienced psychiatrists responsible for the patients’ treatment. Patients were included if (a) acute symptoms were (at least partially) remitted, and (b) patients were stable on antipsychotic medication, i.e. using the same dosage for at least four weeks prior to inclusion. Patients were excluded if they had a DSM-IV diagnosis of substance dependence. Somatic exclusion criteria included evidence of significant cardiovascular, locomotor and endocrine disorders which, according to the American College of Sports Medicine , might prevent safe participation in the sub-maximal exercise tests. All participants received a physical examination and baseline electrocardiogram before testing.
Healthy control subjects were recruited among the personnel (i.e. health care workers, researchers, students) of the participating centres. All control subjects were volunteers who received a general physical examination in the previous year and reported to be free of significant psychiatric, cardiovascular, neuromuscular and endocrine disorders that might hinder safe participation. By selection, gender distribution and mean values for age and BMI did not differ significantly between healthy controls and schizophrenia patients. This matching was performed by an independent statistician blinded for the physical activity and physical fitness outcomes. All participants filled out a physical activity questionnaire and performed afterwards a muscular fitness and walk test. Participants were requested to refrain from eating, drinking coffee or smoking during a two-hour period prior to the tests. Patients were also screened for psychiatric symptoms and extrapyramidal side-effects of antipsychotic medication and received a fasting metabolic laboratory screening.
The study procedure was approved by the Scientific Committee of the University Psychiatric Centre of the Catholic University of Leuven, Belgium. All participants gave their written informed consent.
Demographic patients’ data (including illness duration) were obtained from medical records while age for the control participants was self-reported. All participants were asked whether they currently smoked. Those participants who responded affirmatively to this question were asked how many cigarettes they smoked per day.
The six-minute walk test (6MWT) was performed according to the American Thoracic Society guidelines  in an indoor corridor with a minimum of external stimuli. Two cones, 25 m apart, indicated the length of the walkway. Participants were instructed to walk back and forth around the cones during six minutes, without running or jogging. Resting was allowed if necessary, but walking was to be resumed as soon as the participants were able to do so. The protocol stated that the testing was to be interrupted if threatening symptoms appeared. The total distance walked in 6 minutes was recorded to the nearest decimetre. Standardised encouragements were provided at recommended intervals. Supervision and measurement of the 6MWT was performed by one of four trained members (three physical therapists, one research nurse). With an intra class correlation of 0.96 [95% confidence interval (CI):0.94-0.98)], the 6MWT has been shown to be a reliable test to assess the exercise capacity in patients with schizophrenia .
Health related muscular fitness
The health related muscular fitness was measured by a standing broad jump test , using a tape measure on a foam mat. Participants were asked to stand behind a line drawn perpendicular to the tape measure and jump forward as far as possible using arm swing and knee bending before jumping. The distance jumped was recorded from the take-off line to the farthest point backward of the participant. Higher scores indicate a better muscular fitness. The standing broad jump test previously showed an excellent reproducibility in patients with schizophrenia with an intra class correlation of 0.98 [95% confidence interval (CI):0.96-0.99)] .
Physical activity participation
The International Physical Activity Questionnaire (IPAQ)  uses a structured format that asks participants to recall activities for each of the last seven preceding days in morning, afternoon, and evening time periods. Data from the IPAQ were summarised according to walking, moderate (activities that take moderate physical effort and make you breathe somewhat harder than normal such as carrying light loads, bicycling at a regular pace, or easy swimming), and vigorous activities (e.g., activities that take hard physical effort and make you breathe much harder than normal such as heavy lifting, digging, aerobics, or fast bicycling) per week. On the basis of what activities participants self-reported, the interviewer also clarified the perceived intensity of that specific activity. A continuous indicator was calculated as a sum of weekly metabolic equivalent (MET)-minutes per week of walking, moderate- and vigorous-intensity exercise. The MET energy expenditure was estimated by weighting the reported minutes per week within each activity category by a MET energy expenditure estimate assigned to each category of activity. MET-levels were obtained from Ainsworth et al.  The weighted MET-minutes per week were calculated as duration × frequency per week × MET-intensity, which were then summed across activity domains to produce a weighted estimate of total physical activity from all reported activities per week. Previous research indicated that the IPAQ can be considered as a reliable surveillance tool to assess levels of PA in patients with schizophrenia .
Metabolic and anthropometric measurements
Body weight was measured in light clothing to the nearest 0.1 kg using a SECA beam balance scale, and height to the nearest 0.1 cm using a wall-mounted stadiometer. In patients with schizophrenia a 2-hour 75-g glucose load oral glucose tolerance test (OGTT) was performed according to previously proposed guidelines [23, 24]. Patients were initiated on an overnight fast and were monitored during the OGTT. Waist circumference (WC) was measured to nearest 1 cm at the level of the umbilicus and at the end of expiration with the subject upright Blood pressure was recorded twice in the sitting position after a five minutes rest with an Omron M6 (HEM-7001-E) (Omron® Healthcare Europe). The average of both measures was taken. Patients received a full fasting laboratory screening. The presence of metabolic syndrome was assessed using the International Diabetes Federation criteria [25, 26]. According to these criteria, for a patient to be defined as having the MetS they must have: central obesity (defined as waist circumference ≥94 cm for Europid men and ≥80 cm for Europid women, with ethnicity specific values for other groups) plus any two of the following four factors: (1) raised TG level: ≥150 mg/dL (1.7 mmol/L), or specific treatment for this lipid abnormality, (2) reduced HDL cholesterol: <40 mg/dL (1.03 mmol/L*) in males and <50 mg/dL (1.29 mmol/L*) in females, or specific treatment for this lipid abnormality, (3) raised blood pressure: systolic BP≥130 or diastolic BP≥85 mm Hg, or treatment of previously diagnosed hypertension, (4) raised fasting plasma glucose (FPG) ≥ 100 mg/dL (5.6 mmol/L), or previously diagnosed type 2 diabetes.
Psychiatric and extra-pyramidal symptoms
Psychiatric and extra-pyramidal symptoms (EPS) were assessed by an independent and well trained nurse using the Psychosis Evaluation tool for Common use by Caregivers (PECC) . The semi-structured PECC-interview evaluates 20 symptom items on a 7-point scale. Symptoms are grouped in 5 factors: positive, negative, depressive, cognitive and excitatory symptoms. The scores for each factor range from 4 to 28. The total score ranges from 20 to 140. EPS scores range from 4 to 16. Higher scores indicate more severe symptoms/side-effects. Validation results suggest that the PECC can be successfully used for the evaluation of these symptoms in schizophrenia .
Current antipsychotic medication use was recorded for each patient and converted into a daily equivalent dosage of chlorpromazine using Gardner's table .
Unpaired t-tests with post hoc Bonferroni correction for continuous variables (p=0.005) and Fisher exact tests for categorical variables (gender) (p<0.01) were used to examine differences in characteristics between patients and healthy controls. Additionally, we calculated effect sizes (Cohen's d) to compare health related muscular fitness, walking capacity and physical activity participation between patients with schizophrenia and healthy controls. To identify independent predictors of the walking capacity and health related muscular fitness, a multiple stepwise regression analysis was carried out. Significance level was set here at p=0.01. Statistica 9 (Statsoft Inc, Tulsa, OK, 2009) was employed for the data analyses.