Cardiorespiratory And Muscle Oxygenation Responses During Work In Healthy And Low Back Pain Subjects

Abstract

Differences may exist between healthy (H) and low back pain (LBP) subjects both centrally and peripherally as a result of the deconditioning associated with low back pain. The examination of cardiorespiratory and muscle oxygenation was undertaken to test these two groups during lifting and lower activity. PURPOSES To compare H and LBP subjects on: (1) ventilatory threshold (VT) during a repetitive incremental lifting and lowering, (2) the relationship between VT and the psychophysically determined maximum acceptable lifting weight (MAW), and (3) muscle blood volume (Mbv) and oxygenation (Mox) responses in the MAW test. METHODS Informed consent was obtained from H (N=12) and LBP (N=12) subjects (Mean±SD): age (39.3±6.73 yr), height (1.72±0.09 m), and body mass (81.8±15.3 kg); age (37.7±2.3 yr), height (1.74±0.09 m), body mass (80.0±14.53 kg), respectively. The volunteers completed a 20-min psychophysical test (10 lifts/min) to determine the MAW for an 8-hour workday. Open circuit spirometry and near infrared spectroscopy were used to measure the cardiorespiratory, and Mbv and Mox responses. Mbv and Mox variables were calculated as ‘delta’ = min value during MAW - resting value; ‘range’ = max during recovery - min during MAW, and 1/2 recovery time was 50% max recovery. VT was identified non-invasively by the ventilatory equivalents. A 2-way ANOVA and an ANCOVA were used to for analysis of VT and MAW, and NIRS variables, respectively. RESULTS VO2 at VT was significantly (p ≤ 0.05) lower in the LBP compared to the H subjects. In the H subjects, VO2 at MAW was significantly lower during the psychophysical test, while in the LBP subjects MAW, HR and VE were significantly greater at VO2 at MAW. Significant correlations were observed for the absolute VO2 at VT and VO2 at MAW in the healthy (r = 0.76) and LBP subjects (r = 0.89). There were no significant differences (p>.05) between groups for the mean values of the Mox and Mbv variables. Significant correlations were noted for both groups in the cardiorespiratory responses during the psychophysical MAW and VT. At the MAW, there was a significant correlation between absolute VO2 and: Mbv-delta(r = 0.79) and Mbv-range (r = 0.70) in the LBP subjects. CONCLUSIONS The cardiorespiratory stress was significantly greater in the LBP subjects, most likely due to reduced central circulation, as no significant differences between the groups were observed in erector spinae Mox and Mbv. Localized muscle blood volume is implicated in the reduced VO2 during a psychophysical test in LBP subjects.