Reliability and Validity of the Single Breath Count and Breath Holding Time Dysfunctional Breathing Variables in Patients with Non-Specific Chronic Neck Pain

Background/Objectives: The single breath count (SBC) and breath holding time (BHT) are increasingly used as dysfunctional breathing (DB) outcome measures; yet, their properties derive from populations in which they act as surrogates of vital capacity. We examined their within-session repeatability (intra-session reliability) and convergent–divergent construct validity in non-specific chronic neck pain (NSCNP). Methods: Cross-sectional psychometric analysis was performed on the baseline dataset of a randomized controlled trial (n = 90; 54 women; age 41.25 ± 10.93 years). Three consecutive trials per test were recorded. Reliability was quantified with two-way random-effects, absolute-agreement ICCs (ICC2,1) and the minimal detectable change (MDC95%); validity was tested against 27 hypotheses specified before analysis but after trial registration (ClinicalTrials.gov NCT05229393) and protocol publication across biochemical, biomechanical, psychophysiological, musculoskeletal, psychological and demographic measures. Results: Within-session repeatability was excellent on the retained trials (ICC2,1 0.969 for BHT, 0.974 for SBC; MDC95% 4.65 s and 5.18 counts). Both correlated with chest wall expansion, more strongly for the BHT (r = 0.346 and 0.483) than the SBC (r = 0.268 and 0.253); neither correlated with end-tidal CO2. The BHT was unrelated to pain, disability, anxiety and kinesiophobia, as hypothesized, whereas the SBC correlated inversely with the Neck Disability Index beyond the divergent threshold (r = −0.359), persisting after adjustment for sex, body mass index and age and for height. Under strict adjudication, 15 of 25 adjudicable hypotheses (60.0%) were confirmed (BHT 9/12; SBC 5/12), below the 75% criterion, the shortfall concentrated in the SBC. Against the Nijmegen threshold of ≥19, the SBC discriminated poorly (AUC 0.626) and the BHT not at all (0.588). Conclusions: Both are reliable within a session, but construct validity is partial and asymmetric: in this cohort, the BHT showed a predominantly biomechanical pattern of construct associations of moderate magnitude (12–23% shared variance), whereas the SBC shares more variance with disability and sex than with any respiratory index. Both are better reported as multidimensional measures within a broader battery than as domain-specific outcomes or stand-alone screening tools.