This study aimed to examine the effectiveness of an SLVB intervention program for the physical and psychological health outcomes of PWPD in Hong Kong. Following a 16-week intervention that involved participation in SLVB, compared with CG group, there were improvements in cardiovascular endurance, body composition (lower fat mass), and PA enjoyment in SLVB group. There were no differences in flexibility, upper body strength, and endurance between groups. Therefore, our main hypothesis was partially supported; the SLVB intervention was associated with significant improvements in physical (e.g., cardiovascular endurance and body composition) and psychological health (e.g., PA enjoyment) attributes in PWPD, compared to PWPD not exposed to the intervention.
Improvements in physical health
Similar to participants in traditional indoor volleyball  or SVB , participants in SLVB exhibited significant improvements in cardiovascular endurance and body composition at the end of the intervention. Improvements in cardiovascular endurance were expected due to the movements involved in playing volleyball in the sitting position, such as sliding on one’s buttocks and using (particularly upper) limbs .
The duration of the sessions in the SLVB programme exceeded the World Health Organization PA recommendations , in that adults should participate in at least 150 min of moderate-intensity aerobic activity weekly. This may be another explanation for the improvements in cardiovascular endurance observed among participants in the SLVB group. Increased cardiovascular endurance resulting from the intervention may have contributed to lower fat mass of the SLVB group. This result partially corroborates prior findings that exercises that improve cardiovascular endurance, improve fat distribution, and reduce overweight and obesity .
Lack of improvements in flexibility, upper body muscular strength, and muscular endurance. A previous study  confirmed the relationships between flexibility, muscular strength, muscular endurance, and performance of SVB skills. For example, serving and blocking were correlated with flexibility (i.e., shoulder stretch test), whereas defense, service, and grip strength were correlated. In addition, higher levels of muscular endurance predicted higher performance in serving, blocking, and overall performance of SVB players. Another study  also demonstrated that vertical reach in the seated position is one of the most crucial factors in the performance of SVB athletes. However, our findings contrast with these results. In the present study, inadequate blocking and spiking during the intervention likely contributed to the nonsignificant improvement in the flexibility of participants. Even though we had a training session for practicing both spiking and serving (underarm or overhead serving), most participants did not perform much spiking, and they often served using the underhand technique, which requires lower upper body muscular strength more than the overhead movement, during game sessions in our intervention. In addition, during game sessions in the intervention, underarm service was dominant among participants with a lower degree of skill. In addition, tests such as the grip strength test (forearm strength), might not fully reflect participant’s upper body muscle endurance improvement resulting from the intervention. Other SLVB movement-specific measurements should be considered in the future. Altogether, these factors explain the nonsignificant improvement in upper body muscular ability and flexibility.
Improvement in PA enjoyment
In this study, compared with the CG, the SLVB group reported greater PA enjoyment. These results are consistent with our previous results  and those of another study applying water volleyball, another adapted version of volleyball, to older adults . SLVB is a team sport in which players cooperate and work with their teammates. Players must collaborate and communicate with their team members in the processes of passing, setting, and spiking. Volleyball is generally played with high team spirits and may create a greater sense of enjoyment , along with improved competence, physical self-esteem, and the development of positive habits as was reported in previous studies [34,35,36]. In addition to the dynamics between participants themselves or the feelings of participants toward the activity, the coaches’ encouragement and feedback provided in the intervention might also contribute to participants’ relatedness, as suggested by self-determination theory , and consequently enhance the likelihood of PA enjoyment. All the above might explain the greater PA enjoyment of the SLVB group.
In sum, our above significant results may be because we used standardized fitness measurements, center- and group-based interventions, and professional guidance from intervention coaches. Notably, the adherence rate for the intervention program was high (71.81%). Approximately 80% of participants completed both pretest and posttest measures. Thus, our observed 20% dropout rate corresponded to our expected dropout rate. This expected dropout rate had been suggested by Hicks et al. , who studied long-term exercise training in people with spinal cord injury. This low dropout rate might be because our intervention entailed fun, self-efficacy, and social contacts, which encouraged participation, and this also helped overcome barriers such as the lack of opportunities for participation in sports and accessibility problems, as suggested in a previous study .
Limitations and recommendations for further research
Our study has some limitations. First, this was a study examining the effects of the SLVB intervention on the health of a small group of PWPD from one nongovernmental organisation. Also, given evidence that SLVB was beneficial to the physical and psychological health of general PWPD, evidence is warranted to test its impact in a larger group of PWPD and with specific type of PWPD. All these will enhance the generalizability of the intervention findings. Second, SLVB movement-specific measurements should be considered in future interventions; for example, measuring aerobic endurance while moving on the floor using hands. Third, because of resource constraints, no follow-up was conducted. Fourth, about 8 of 40 participants did not complete our posttest measures. Therefore, future studies should include follow-up assessment with a more effective plan to monitor adherence to the intervention and changes in physical and psychological measures through study designs such as the wait-list control trial design. Fifth, in addition to applying quantitative methods to examine the effects of SLVB on health among PWPD, researchers may need to adopt a mixed-methods approach to comprehensively examine both the training effects and participants’ experience in a future SLVB interventions program.