Measuring sub-maximal exercise capacity

Sub-maximal exercise tests can be either predictive or performance tests.(21) Predictive tests are designed to permit prediction of maximal cardiorespiratory fitness. Performance tests involve measuring the responses to a standardized activity; for example a stair climb or a timed performance of running or walking a specified distance.

Treadmill and cycle ergometers

A variety of methods for determining V̇O2max from sub-maximal exercise tests have been described.(22) Treadmill and cycle ergometer protocols can be modified to assess sub-maximal exercise capacity. The Modified Bruce treadmill protocol employs an additional initial phase with a lower workload than the initial stage of the maximal Bruce protocol and halves the increments in workload making the test feel smoother to the participant. The test is terminated when 85% of the predicted heart rate is reached. Predictive equations were subsequently developed using data from tests conducted in various groups (active and sedentary men, healthy adults and patients with cardiac disease). These equations use the exercise time achieved during the test which is a marker of the workload achieved, thus, exploiting the linear relationship between workload and oxygen uptake.(23, 24) Modification for the Bruce protocol allows it to be more suitable for less-able individuals;

however, the disadvantages associated with stability on the treadmill persist. Pearson’s r values for the correlation between predicted and measured V̇O2max were reported between 0.87-0.94 depending on the participant group tested.(25)

Ebbeling et al developed the Single-Stage Submaximal Treadmill Walking Test which involves treadmill walking at one speed and gradient.(26) The predictive equations developed from this test were established using the data from a sample of healthy adults aged 20-59 years old. Although the test itself is a useful means for assessing exercise

capacity in people who are less able or prone to fatigue, the value of predictive equations for use in older or disease populations is dubious.

The Astrand-Ryhming nomogram can be used to estimate V̇O2max from a submaximal Cycle Ergometer test (the Astrand-Ryhming cycle test).(27) It uses the measured heart rate and the workload achieved on the final stage for the estimation of V̇O2max.The nonogram has undergone several modifications to make it suitable for use in a wider variety of study populations than previous predictive equations permitted.(28, 29) This includes incorporation of an age-correction factor permitting use in older adults.(30) Predictive equations depend on the assumption of a linear relationship between heart rate and V̇O2. Although this assumption fails at high workloads and can result in an

underestimation of the predicted V̇O2max, it is a necessary assumption. Von Dobeln et al, describe an equation, based on the Astrand-Rhyming nonogram, which aimed to minimise the assumptions necessary to predict V̇O2max. They developed it using exercise data collected from men aged 30-70 years old, and included an age-correction factor permitting its use in older adults.(31) This equation was also developed for women.(30) Other

examples of sub-maximal cycle ergometer protocols are: the YMCA bike test and the ACSM bike test.

Self-paced walking tests

Walking is a ubiquitous form of activity, appropriate for the majority of older adults, and is the principal requirement for performing daily activities independently. Therefore, it

naturally provides a good basis for a submaximal exercise test. A single-stage, self-paced walking tests is commonly used. The 12 minute walk test (12-MWT) was first introduced by McGavin et al, in 1976 to assess exercise capacity in patients with pulmonary disease.(32) The test involves recording the distance that can be walked in 12 minutes. In this original protocol the test was carried out along a hospital corridor. 29 participants also undertook an incremental maximal cycle ergometer test including analysis of expired gases to

determine peak V̇O2. The authors report a significant positive correlation between distance travelled in the 2^nd of three 12-MWTs (carried out to assess reproducibility of the test) and the peak V̇O2 achieved (r=0.52; p<0.01).(32)

The 6 minute walk test (6MWT) was subsequently developed along the same principals as the 12MWT but was designed for patients with severe respiratory disease.(33) The 6MWT has since been applied to a range of groups of people with severe limitations in exercise capacity,(34-36) including older adults aged 65-89 years old.(37) Strengths of the 6 and 12 MWT are that they are easy to administer, inexpensive and involve walking which is an activity typical for everyday life; this may seem achievable by most participants to at least some extent. Limitations in self-paced walking tests include: requirement of a corridor or space that is big enough (and quiet in terms of thoroughfare) for the test to be carried out.

The corridor should be at least 30 meters long(38) in order to reduce turning points during the test which inevitably slow participants(39).

Step tests

Step tests offer a low cost and simple method for assessment of exercise capacity without need for expensive equipment. The Astrand-Rhyming step test was developed as a simple way of predicting V̇O2max from a submaximal exercise test that could be carried out in the home.(40) Other examples of submaximal free-standing step test protocols are:

McCardles step test and the Canadian Aerobic fitness test.The low cost of step tests makes them particularly appropriate for studies that enrol a high number of participants. A disadvantage of using a free-standing step test is that it also requires the participant to have reasonable balance and stability.