Enjoyment is both a predictor and outcome of physical activity participation [1–3]. Expected enjoyment from physical activities can increase exercise intentions  and the mere anticipation of positive emotions predicts physical activity adoption and maintenance . Moreover, stronger anticipation of negative emotions is associated with weaker physical activity intentions and behavior . Although enjoyment has been assessed in numerous studies, no measures of enjoyment have been appropriately validated for use with adult populations. Instead, "measurement equivalence"  is often assumed, a pervasive problem associated with many self-report instruments. Measurement equivalence refers to the assumption that a measure has the same meaning across different groups of people (i.e., group invariance), and that its items have the same meaning to individuals across time (i.e., longitudinal invariance). However, it is entirely plausible that questionnaire items hold different meaning to different groups, or that the meaning of items could change across measurement time-points. Each situation would threaten group and longitudinal invariance, two psychometrics properties that are essential in order for researchers and clinicians to draw meaningful interpretations of enjoyment scores.
Little is known about the development of physical activity enjoyment among older adults. Within the interactionist framework of social cognitive theory (SCT) [8, 9], self-efficacy beliefs and social factors interact to influence the self-monitoring of one's behavior, its determinants, and its effects. From the perspective of SCT, perceived enjoyment and social support should contribute to the self-regulation of exercise behavior . Additionally, researchers [11, 12] have suggested that experienced changes and satisfaction with those changes should result in more positive affective responses over time, which in turn should positively impact future exercise behavior. To date, however, older adults' affective responses to physical activity experiences have mainly been studied in terms of in-task relationships, such as their responses to graded-exercise testing conducted within a laboratory setting [12, 13]. However, the enjoyment older adults feel towards the domain of physical activity in general, and its antecedents and consequences, is relatively unexplored. Often, it is assumed that regular exercise is "intrinsically-motivated" but the benefit experienced from one's exercise efforts coupled with support from others may play a more important role in physical activity participation.
The objective of this study was to examine the validity and psychometric properties of the most commonly used measure of enjoyment, the Physical Activity Enjoyment Scale (PACES)  among a sample of older adults involved in a yearlong exercise program. A secondary purpose was to evaluate the construct validity of the scale with other theoretically-relevant constructs, including perceived social support, experienced exercise-related changes and behavior. The original 18-item PACES was developed by Kendzierski and DeCarlo  for a college-age population, and was intended to be uni-dimensional, but further testing in other populations revealed problems with its factor structure . Motl and colleagues  used a 16-item version, revised for adolescent girls, which has also been modified for use with younger children . An abbreviated 8-item version of the PACES has been used with adults of mixed ages [2, 18] and was found to be invariant across samples of adult runners and cyclists ; however, this sample  consisted of mostly young and middle-age adults, who have been shown to differ from older adults in their motives for physical activity  and perceived experiences of emotion [13, 21]. The full 18-item 1-factor structure of the PACES has only been evaluated in one study , and again, this study collapsed multiple age groups together, ranging in age from 25 to 75. Together, these findings call for a validation study of the PACES in a sample of older adults.
To date, no version of the PACES has been tested for longitudinal invariance. Without establishing longitudinal invariance, it is difficult to ascertain whether changes in the PACES, or lack thereof, may be attributable to true effects (e.g., intervention, developmental), or to the effects of an unstable, time-dependent measure. Interestingly, Rhodes and colleagues  have shown that many interventions designed to change affect, as measured by the PACES, have been ineffective. It is possible, however, that the psychometric properties of the PACES, and other affect scales, are unstable, which could lead researchers to draw false conclusions about any relationships with physical activity. Therefore, one should be cautious in making any interpretations regarding findings based on scales without establishing first that the scale is consistent across groups and time.
Some researchers have claimed that the original 18-item PACES contains questions pertaining to "antecedents and consequences" of the exercise experience , two aspects that might vary with time or could even conflict with each other. However, with an invariant measure of enjoyment, we would expect certain relationships between enjoyment and specific theoretically-based antecedents and consequences. Enjoyment has been positively associated with social support, as friends, family, and professionals can enhance physical activity experiences by providing instrumental, informational, emotional, and motivational support . Perceived social support has also been shown to predict exercise behavior indirectly through affect and self-efficacy [25, 26]. A meta-analysis  found a substantial effect of important others on exercise affect (ES = .63). Thus, as a means of evaluating convergent validity, we examined bivariate associations between our final PACES measure and social support, perceived change brought about by physical activity, and self-reported physical activity.
The purpose of this study was to systematically examine the psychometric properties of the PACES. Group invariance, longitudinal invariance, and convergent validity (with types of perceived social support, experienced exercise-related changes and behavior) were evaluated in a sample of older adults involved in a randomized controlled trial. Thus, we tested the feasibility of two, 1-factor models of PACES (i.e., 18-item and 8-item versions) currently being used in the literature. An alternative, theoretically-based shorter version of the scale was also constructed.