Walking as the Type of Physical Activity for Measuring Cardio-Respiratory Fitness - Research Paper Example

Introduction For over 20 years, cardiac rehabilitation has proved to be effective in reducing deaths in cardiac patients up to 20-25% (Oldridge 1988). Cardio-respiratory fitness has been used in many patients with the efforts of proving that it can reduce cardiac-related deaths in cardiac patients. Myers et al (2002) define cardio-respiratory fitness (CRF) as the “maximum ability of the heart and lungs to deliver oxygen and the ability of the muscles to extract it”. It can also be defined as the result of integrated efforts between the cardiopulmonary and skeletal systems (Myers et al 2002). Therefore, cardio-respiratory fitness is regulated by VO2 (ml/kg/min), a representative of an individual’s oxygen uptake which is expressed as VO2 max. This maximal oxygen uptake is determined by cardiac output and the difference in arteriovenous oxygen when the body is physically exhausted. It is usually expressed as: VO2 max = ( HR x SV ) x a-VO2 In a healthy person, the above expression represents his physiological limit after maximum efforts have been applied something which cardiac patients cannot achieve since they are physically unable to attain such efforts and therefore it is impossible to calculate the VO2 max of such patients. However, it is possible to calculate both the relative VO2 (the rate of oxygen consumption over a period of time) and VO2 peak (the highest rate of oxygen consumption over a period of time) of cardiac patients. VO2 can be measured either directly or estimated but direct measurement is expensive and requires professionals for operating the equipment. Despite these two challenges, direct measurement is the most effective method of calculating VO2 since it has been found that the estimate method in most cases gives overvalued results in cardiac patients due to their reduced oxygen intake kinetics due to their disease. According to Myers et al (1991), the above is usually reflective of some exercise testing protocols. The oxygen uptake by an individual is directly proportional to their energy consumption which is relative to their body mass and time and is presented as metabolic equivalents (METs). A definition of metabolic equivalents by Jetté et al (1990) is that it is “the quantity of oxygen consumed by the body from inspired air under basal conditions and is equal, on average, to 3.5 ml·kg-1·min-1”. For instance, 6 METs is equivalent to VO2 of 21 ml/kg/min-1. This alternative method of presenting oxygen consumption can be easily interpreted by the general population whereas for health professionals it is vital when prescribing the best exercise for the cardiac population. According to Arena et al (2007), there are several factors that determine the VO2 of an individual. For cardiac patients, their VO2 is determined by factors such as disease, several medications and ageing with ageing having a direct effect on fitness that according to Fleg et al (2005) causes a 3-6% decline in VO2 of individuals in the age bracket of 20-30 years and after the age of 70 years the decline is 20% every ten years. Research has shown that exercising especially in the cardiac population reduces cardiac related deaths with a study by Vanhees et al (1995) tabling the results that every 1% increase in VO2 peak leads to a 2% reduction in mortality. In addition, Dorn et al (2001) found out that a 1 MET gain led to a 10% reduction in deaths among male myocardial infarction patients while on the other hand Kavanagh et al (2003) discovered that every 1 ml·kg−1·min−1 gain in VO2 peak resulted in a 10% decrease in cardiac mortality. From the above results, it is evident that exercise increases an individual’s cardio-respiratory fitness and hence it is vital that for reliable and valid results to be obtained, accurate methods of measuring should be used. After cardiac rehabilitation, cardio-respiratory fitness can be measured using laboratory tests such as Treadmill test and cycle-aerometer though they tend to be complicated, costly and take a lot of time. However, apart from the expensive laboratory tests, there are several other exercise tests that can be used since they are simpler, cheaper and more practical. Examples of such alternative practical tests include six minutes walk test and incremental shuttle walking test with the latter being the most popular walking test of cardio-respiratory fitness in the UK. Incremental shuttle walking test (ISWT) Development As Singh et al (1992) indicates, though the incremental shuttle walking test that had twelve levels was originally meant for patients with respiratory complications, it was later used for CVD patients and for creating the modified shuttle walking test that has 15 levels, the Modified Shuttle Walking Test (MSWT) (Bradley et al 2000). This test is used in coronary heart disease patients, patients who have had cardiovascular surgery or pacemaker insertion and patients with HF (Tobin and Thow 1999) so as to assess the progress of their cardio-respiratory fitness after a CR programme. Furthermore, the above test is more economical both in time and finances as it requires such easy to use equipment like an audio player, a CD, A HR monitor, two cones and a 10 metres walkway. Additionally, ISWT is a safe test for patients since unlike the treadmill test it does not cause much physiological stress for the patient as it increases the speed of walking incrementally (Jolly et al 2008, Singh et al 1994). In an ISWT, individuals are required to walk forward and backwards as they turn around two cones and increase their speed as they go on under the control of an audio signal. When the patient is not able to reach the cone at the required time, the test is stopped. It can also be stopped when the patient wishes to do so. The results of an ISWT are affected by factors such as the percentage increase in the distance walked; use of regression equations for such populations as CABG patients as required by research (Fowler et al 2005); and use of ACSM formulas to analyse the percentage increase in METs per level (ACSM 2009). The American College of Sports Medicine has given guidelines that are usually used for calculating the levels of ISWT whereby each level has been assigned a value for METs based on VO2 max (ACSM 2006). These guidelines however, are based on walking on a flat course as opposed to the ISWT levels that are include a turn at both ends of every shuttle. In addition, these IWST MET values as used in cardiac rehabilitation are inaccurate threefold since they are based on healthy individuals without considering the effect of disease during exercise. Furthermore, there is acceleration and deceleration of the body due to turning around the cones which resultantly increases energy expenditure (Martin et al 1993). Moreover, ISWT levels are only a minute long hence are not long enough as credibility requires so as to be recognized as steady-state exercise. The ISWT protocol indicates that METs are based on the functioning of both the aerobic and anaerobic systems during exercise. Walking Economy Walking is an important form of exercise that improves the quality of life through preventing some diseases such as cardiac diseases. The energy used in walking depends on the speed and walking technique. It is also determined by factors such as the “movement of the body on a vertical plane with the acceleration and deceleration of each stride, the internal activities that balance the body and keep it in position and the work done by the cardio-respiratory system so as to allow for the metabolic and physiological demands of movement to be met” (DiPrampero 1986). In other words, walking economy is affected by several physiological, anatomical and biomechanical factors. According to Martin et al (1983), there is a close relationship between aerobic demand and walking economy whereby increased muscular activity leads to increased aerobic demand both in the working muscles and the cardio-pulmonary system. Therefore, according to Burdett et al (1983), the key determinants of walking velocity are aerobic demand and power. Bobbert (1960) and DiPrampero (1960) found out that the energy cost of walking is most economical at 1.0-1.4 m.s-1. Beneke and Meyer (1997) have observed that through gait improvement and increase in velocity, patients below the economical value will be more energy economical and can in fact realize up to1.25 J.kg-1.m-1. The problem: As mentioned above, ISWT is the most popular test used for cardio-respiratory fitness in cardiac patients in the UK but anecdotal evidence has shown that the initial walking speed of 0.5 m·s-1 is uncomfortable. Originally, ISWT was designed for COPD patients with airway obstruction and have lower cardio-respiratory fitness as compared to cardiac patients (Singh et al 1992). Airway obstruction causes breathlessness that in turn affects cardio-respiratory fitness. Tobin and Thow (1999) criticised the ISWT initial walking speed as they found out that it was too slow for CABG patients. They therefore suggested that the speed in earlier levels be more flexible. There is a complicated relationship between metabolic energy and physical energy during slow walking which explains how slow walking is extremely inefficient (Whittle 2007) as it causes one to spend a greater energy than their workload. Bunc and Heller (1990) have tabled evidence on Whittle’s explanation by reporting that people who are less physically fit such as those in cardiac rehabilitation have a lower mechanical efficiency than those who are more fit. This means that the cardiac population expends more energy at lower speeds than the more fit population and therefore finds the rest of the test to be tougher. Whittle (2007) indicates that the ‘free’ walking speed or in other words the comfortable speed ranges between 0.91-1.63 m/s for females and 0.96-1.68 m/s for males all in the age bracket of 50-64 years. On the other hand, for those aged 65-80 years the range is 0.80-1.52 m/s and 0.81-1.61 m/s for females and males respectively. Since majority of cardiac patients fall in the age bracket of 50-80 years, the first two levels of the ISWT test where the walking speed does not go beyond 0.67 m/s is therefore not necessary. Therefore, level 3 which has a speed of 0.84 m/s is the most appropriate beginning point for patients. The walking speed in the original standard ISWT designed for COPD patients was 1.52 m/s which is considerably higher that than that in the first two levels of the standard ISWT which is 0.5 and 0.67 respectively (Singh 1992). However, in reality, it is not possible that majority of people walk at such low speeds even if they have cardiac complications. Method: Participants Participants were allocated on availability and a total of 33 individuals were selected with 17 being males and the rest were females. All participants were Caucasian and clients of the Imperial centre, a cardiac rehabilitation centre situated at Imperial University for patients attending phase four. In addition, all participants had attended cardiac rehabilitation for a minimum of 24 weeks, one session per week, with each session lasting an hour on the approximate. The descriptive data recorded was the mean age (years) of 58.36 ± 7.8, mean height (cm) of 156.64 ± 5.2 and body mass (kg) of 78.6 ± 9.79. Protocol The whole of this study was done at Imperial University in a sports and exercise laboratory that was well ventilated. Before commencement of the study, all participants were required to complete a consent form and a PARQ (Appendix 12). After the participants were through with filling out the documents, the practitioner went through to check the answers provided after which he declared the participants fit for testing. Subsequently, the participants’ age, gender, height, body mass, heart rate and blood pressure were recorded. The heart rate max for each participant was recorded so as to provide the termination criteria. Participants were then fitted with a valid oxygen analyser as per the guidelines by McLaughlin et al (2001). The analyser was used in cardio-pulmonary exercise testing to measure physical activity expenditure so as to obtain values such as VO2. To measure participants’ heart rate, a Polar heart rate monitor was used. The guidelines used were in accordance with SIGN (Guideline 9). In order to use the Borg scale appropriately, the participants were taken through the use of the RPE during the test. Large RPE charts were placed on the ends of the shuttle so as to make them always visible to the participants so that they could give their rating when required to do so. At first, an audio cassette was used to give instructions about the test and later the practitioner gave a summary of the instructions as well as the termination criteria. The termination criteria was based on the following conditions; development of angina symptoms such as tightening of the chest and dyspnoea; reaching a heart rate max of 85%; inability to keep up with the set pace (being more than 0.5 metres away from marker when the beep sounds three times); request by the participant; and request by the practitioner if the participant’s physical state was not good. The participants were then allowed to ask any questions before the commencement of the test protocol. The test started by playing the audio cassette. A single beep was used to determine the pace at which each participant was travelling at and all participants were required to be in-sync with the beep on arrival at each marker on every shuttle before turning and travelling back to the marker in the opposite end. The markers were placed 9 metres apart whereas the remaining metre was covered in the distance travelled when turning around each marker whereby each turn was approximately 0.5 metres. In the first level, the practitioner walked around the markers with the participants at a speed of 0.5 m/s so as to achieve a suitable pace. At the end of every level, the participants were asked to record their RPE rating using the charts provided. The participants were also informed that at the start of a new level the speed would be increased and a triple beep would be used to alert them. The speed at which the participants walked at for each level is listed below. The test was terminated when one of the termination criteria was reached. On termination, the following information was recorded; total distance covered expressed as the total shuttles covered by 10 metres, the participants’ peak heart rate and the termination criteria. The practitioner requested the participants to complete the shuttle they were undertaking and then go for another so as to be able to obtain the immediate post exercise measures. Finally, the participants were requested to sit down so that post heart rate and blood pressure could be recorded. Subsequent tests were performed 14 days apart under two conditions with the first condition following the protocol for the standard ISWT as performed above whereas the second condition involved the same procedure only that the participants started at a higher level (level three). The second condition allowed participants to start at level three at a speed of 0.84 m/s instead of the first level’s speed of 0.50 m/s as per the standard ISWT. The removing of the first two levels was done due to results obtained from a previous pilot study that will be discussed later. Statistical Analysis Paired tests were done using SPSS statistics Windows software to compare the between-condition differences in primary and secondary measures. Discussion Singh et al (1992) designed the first standard ISWT which was used on COPD patients and produced a walked mean distance of 366 m which is far below the ranges used in this study. Cardiac rehabilitation (CR) patients who participated in this research walked a mean distance of 746 m in the first protocol which was the standard ISWT whereas in the second protocol which can be referred to as the abbreviated ISWT, they covered a mean distance of 744 m. before discussing the results of this study, it is important to fist compare the results with those obtained from other similar researches using the ISWT. The physical fitness of the CR patients is paramount before proceeding with the comparisons. The statistics of the sample population were as follows; the patients were of low CR/medium risk and had attended a 24 weeks programme of cardiac rehabilitation and during the study were on exercise for a minimum of two hours per week. Additionally, the 24 weeks CR programme had two sessions a week with each session lasting an hour; therefore the CR had lasted 48 hours in total. Due to the extensive exercise done coupled with physical benefits, the practitioner expected the results of the sample to be higher than those in the general cardiac rehabilitation population. A research by Robinson et al (2009) on post rehabilitation MI and CABG patients with a mean age of 59.4±9.2 years is a good comparative sample with this research. Robinson et al CR programme was of 12 weeks with one session per week and each session taking one hour amounting to12 hours of CR in total. In this 12 weeks programme patients walked a mean distance of 766 m as compared to the 822 m and 834 m covered in this research. The 13% difference in distance walked is due to the difference in hours of CR attended by the two groups which results into differences in fitness levels between the two groups. Another comparative study is that done by Arnott (1997) that recorded the mean distance walked of 554 m which gives a 41% difference with that of this research. Arnott’s research was performed on post CR CABG patients for a period of 10 weeks but the amount of sessions and hours per session were not stated. This sample was also mainly composed of males of the same age as those used in this research. A further comparison can be done on a sample by Fowler et al (2005) whose participants walked a mean distance of 469 m which gives a 42 % difference. In addition, the patients were of the mean age of 61.2 ± 8.5 years with majority being male and were all 6-8 weeks post surgery. Furthermore, Woolf-May and Ferret (2008) studied post MI male patients of the mean age of 63.5±6.5 years who walked a mean distance of 430 m that gives a 47 % difference. The above results were so because the patients were less fit as they were on pre-phase four as compared to those used in this study. Due to the several variables affecting the cardio-respiratory fitness of each sample, it is difficult to compare distances covered by all samples. Despite having similar age and gender ratio, there were great differences in the distances covered by each sample which were caused by differences in physical fitness, comorbidities, previous amount of CR attended as well their current point of CR. From the examples used where the range is 430-834 m, the difference in distance covered is 53%. However, of all samples, the one with the highest mean distance walked by CR patients is the one used in this research which can be attributed to the intensive CR attended. In literature, distance covered is reported in metres instead of the more accurate method of converting it to VO2 or METs using a regression equation. The reason for this is to make it easy for practitioners and clients to interpret the results as well as make it easy for comparisons between different samples. Results should therefore be reported as a rate as opposed to amount where for instance distance represents the amount of space between two points whereas a rate is the total time it takes one to travel between these two points. Speed It s a rate and reflects the work output. Just like distance which is measured as an SI unit, speed should also be measured as so which should be in metres per second (m/s) instead of miles per hour (mph) so as to make it easy for both practitioners and clients to understand. For the standard and abbreviated ISWT the mean maximum speed was 1.5 m/s and 1.9 m/s respectively whereby the 19% increase in the abbreviated ISWT was achieved even though there were no significant changes in physiological measures such as VO2 peak, heart rate and RER. The above difference is evidence that abbreviated ISWT increases efficiency of CR patients making them more economical such that they are able to work out more without consuming additional oxygen as compared to standard ISWT. Despite the CR patients walking the same distance in both the standard and abbreviated ISWT, the abbreviated ISWT protocol reflected a higher efficiency and cardio-respiratory fitness due to walking at higher speeds as compared to the standard ISWT where speed is slow at the first two levels. The results were so because during the abbreviated ISWT, the participants completed the exercise while at very high average speeds which were maintained all along. The mean maximum speed can be calculated using the mean distance travelled whereby the distance can only be achieved at a certain speed during the ISWT. Therefore speed comparisons of this study and others are identical where that of Robinson et al (2009) 2.03 m/s, Arnott (1997) 1.86 m/s, Fowler et al (2005) 1.69 m/s, Woolf-May and Ferret (2008) 1.69 m/s as compared to that of both the standard and abbreviated ISWT which was 1.5 m/s and 1.9 m/s respectively. This means that patients can still achieve similar maximum speeds despite covering different distances which further reflects the functional capabilities of cardiac patients though they may not be short-lived. VO2 VO2 and METs are used interchangeably with 1 MET being equivalent to VO2 rate of 3.5 ml/kg/min-1. Orendurff et al (1972) indicated that VO2 can be calculated best at 1.3 m/s where gait is more efficient since at slower and faster speeds gait becomes less efficient. A parabolic curve with values ranging from 0.6 m/s-2.0 m/s can be used to represent the VO2 values. Mechanical equations can be used to calculate efficiency at faster but at slower speeds it there is no clear method of calculating such efficiency. In both tests, the mean VO2 was similar and as presented in figure 15 there is a slight decrease between levels 10-12 of the abbreviated ISWT. This decrease can be attributed to a lower VO2 by one participant. When measuring VO2 in cardio-respiratory fitness, there are disparities on whether the value should be expressed as a maximum (VO2 max) or as a peak value (VO2 peak). VO2 peak is the highest recorded VO2 per 15 seconds whereas VO2 max is the highest possible oxygen intake. However, this maximal effort is not applicable in ISWT since it is a sub-maximal test and cannot be maintained by cardiac patients for long. The direct measurement of VO2 in this study proved to be effective unlike estimated measures that give overestimated values. However, it is rarely used in the clinical setting due to the high costs associated with it. Oxygen consumption during exercise Oxygen consumption is usually measured by analysing the inhaled and exhaled gases in the lungs which represents the ability of the muscles to use oxygen that has been delivered to them by the cardio-pulmonary system during an activity. Physiological adaptations such as rise in ventilation, haemoglobin concentration, blood volume, cardiac output, blood flow in the periphery and aerobic metabolism in the mitochondria determine the ability of a muscle to adapt to the demands of an increased workload (Hughson and Tschakovsky 1999). According to McArdle et al (2007), a steady rate of oxygen consumption is achieved after 3-4 minutes which up to this point continues to rise exponentially to reach the oxygen deficit which is the specific amount of oxygen that would have been used if the aerobic system was able to respond to this intensity immediately. At this point, the short-term anaerobic systems come in aid of the aerobic system and ensure that blood lactate does not form and if it forms it is immediately oxidised or resynthesised in the kidneys (Gladden 1989). Since the stages in the ISWT take a1 minute duration, it is impossible for the aerobic system to adapt quickly enough so as to produce a steady oxygen consumption. As a consequence, the oxygen deficit increases meaning that the anaerobic systems will be in constant demand and that according to Karlsson et al (1972), both ATP/PCr depletion and lactate concentration increase over time. There is a linear increase in lactate concentration whereas the ATP/PCr depletion is linear up to ~4L of oxygen after which it flattens. Evidence of the above response has been seen in elite athletes (Edwards et al 1999), cardiac rehabilitation patients (Koike et al 1994) and in people of various ages (DeLorey et al 2004). In both tests, VO2 peak was identical due to the acceleration at the start of every stage, the turning effect at the end of each shuttle and the relatively short 1-minute stages that did not allow for complete aerobic adaptation to the walking velocity. Though the concept of anaerobic activities has been overlooked in the past, future study will address the issue. Limitations It is evident that this study had a small number of participants which consequently led to generalization of the results though there is no other study that has suggested an ideal sample size. In addition, the study sample used projected high fitness levels that created a hindrance in making comparisons with other samples since there were large differences in fitness. Another limitation is that it is almost impossible to obtain a standard measure which is synonymous with a population sample since there are many variables that affect oxygen consumption. Therefore, it is difficult to come up with an equation based on patients’ best walking speed (Whittle 2007). An important issue that should be addressed by future studies is whether the ISWT should be assessed using the patients’ maximum speed or their amount of oxygen consumed. Conclusion The ISWT used above uses walking as the type of physical activity for measuring cardio-respiratory fitness. This activity was chosen since people are familiar with it and that it is part of their everyday life unlike the treadmills and cycle ergometers that unpopular among the patients. VO2 values obtained from both the standard and abbreviated ISWT were identical and participants obtained a higher working rate. This therefore suggests the need for a regression equation that will be a measuring tool for CR patients’ work capacity. The patients’ work capacity is affected by many factors such as age making it difficult to achieve these goals. In addition, it is important that further study is done on the biochemical factors in an ISWT more so on the functioning at slow walking speeds and turning points. Further, results showed that patients achieved higher maximum speed after excluding the first two stages of the ISWT; hence the abbreviated/shortened ISWT can be a very effective alternative to the standard ISWT. The two minutes saved when calculated across all clinics and hospitals using the test are found to save a lot of time which consequently saves on money used in such tests. Read More