Dissertation (Ph.D.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references (p.89-95).

<p> A novel method has been developed to determine the quantitative relationship between the percentage ofla fibres stimulated synchronously, and the percentage of human flexor carpi radialis (FCR) motoneurons (MNs) discharged reflexly. The method assumes a normal distribution of Ia fibre thresholds to electrical stimulation. Among the 11 healthy subjects tested during relaxation, there were considerable differences in the reflex excitability of the FCR MNs to quantitative Ia fibre inputs. The Ia fibre input-FCR MN output curves were either initially steeply-rising, initially slowly-rising, or initially and latterly steeply-rising. When the results were averaged, however, the curve for the 11 subjects in the relaxed state appeared to be fairly linear throughout the entire range of the Ia fibre inputs, and a mean of82% of the Ia fibres discharged approximately 20% of theMNs. </p> <p> Regardless of the variability in the shape of individual input-output curves during relaxation, potentiation of the FCR MN output was observed during weak wrist flexion in 10 of the 11 subjects over the full range of the Ia fibre inputs. In contrast, a depression of the MN output was exhibited in all 8 subjects who weakly contracted the extensors over the full range of the Ia fibre inputs. The changes in the Ia input-MN output relationship in going from rest to voluntary contractions of wrist muscles are thought to reflect modulation by presynpatic inhibition of the Ia terminals. With very large Ia inputs during wrist extension, however, there is a steep rise in the input-output curve, which could indicate a decrease in presynaptic inhibition of Ia terminals in the FCR muscle. The modulation ofthe input-output relationship observed in the present study is consistent with the task-dependent differences of reflex excitability observed by Stein et al. (1988). </p> / Thesis / Master of Applied Science (MASc)

The prevalence of work-related muscle pain (WRMP) is large in the general population in the industrialized world. Despite significant advances over recent years in some research areas, the mechanisms of why WRMP occurs and the pathophysiological mechanisms behind the disorders are still unclear. One suggested explanation is that WRMP is caused initially by a limitation of the local muscle circulation and oxidative metabolism. There is a lack of objective methods to gauge the development and diagnosis of WRMP. Near infrared spectroscopy (NIRS) is a non-invasive technique that allows for determinations of oxygenation and blood flow. The purpose of this thesis was to evaluate NIRS (1) as a method for measuring muscle oxygenation and hemodynamics for the extensor carpi radialis (ECR) and trapezius descendens muscles (TD), and (2) to investigate whether variables measured by NIRS differed between patients diagnosed with WRMP and healthy subjects. Several variables of NIRS were produced and investigated. These included muscle oxygenation (StO2%), changes during contractions (ΔStO2%) and StO2% recovery (Rslope), total hemoglobin (HbT) as an indication of blood volume and its changes during contractions (ΔHbT). In addition, for the ECR, by applying an upper arm venous occlusion (VO) HbTslope increase as a surrogate of blood flow, and for both VO and arterial occlusion (AO) HHbslope increase (i.e. deoxyhemoglobin slope) as a surrogate of oxygen consumption were variables of interest. A first objective was to determine how StO2% and HbT responded to various contraction forces and how it related to muscle activation measured by electromyography (EMG). For both muscles isometric contractions of 10, 30, 50 and 70% of maximal voluntary contraction (MVC) were maintained for 20 s each by healthy males and females; additionally a 10% MVC contraction was sustained for 5 min. For the different contraction levels, predictable relationships were seen between ΔStO2% and force, and between ΔStO2% and EMG RMS amplitude. The general trend was a decrease in ΔStO2% with increasing force and increasing EMG. Females showed a tendency for a higher oxygen use (i.e., drop in StO2%) for the ECR over force levels than males and a higher RMS% MVC for the TD. For the 10% MVC contraction sustained for 5 min gender specific changes over time for HbT and RMS for the ECR, and for StO2% for the TD muscle were seen. A second objective was to determine the day-to-day reliability of NIRS variables for the ECR and TD muscles at group level (Pooled data) and at gender level (males and females). Measurements were performed on two occasions separated by 4-6 days and intraclass correlation coefficients (ICC) and limits of agreement (LOA) were determined as reliability and reproducibility indicators, respectively. Variables tested were ΔStO2% during submaximal isometric contractions of 10, 30, 50 and 70% MVC and StO2% recovery (Rslope) after contractions and after AO. For the ECR, HbTslope as an indication of blood flow (using VO) and HHbslope as a surrogate of oxygen consumption for both VO and AO were computed. For ΔStO2% for the ECR the highest ICC was at 30% MVC for both the pooled data and at gender level. For the TD ICCs were comparably high for 30, 50, 70 % MVC (for both muscles the ΔStO2% at 10% MVC showed the lowest ICC). Further, females showed a higher ICC than males for contraction levels of 50 and 70% MVC. For both muscles, LOA for ΔStO2% was lowest at 10% and highest at 50 and 70% MVC. For the ECR Rslope ICCs were high for all contraction levels, but was lower for AO; LOA was lowest at 70% MVC. For the TD, Rslope ICCs were also high for all contraction levels and LOA was lowest at 30 % MVC. ICC for HbTslope was the lowest of all variables tested. For HHbslope ICC was higher for AO than for VO, and LOA was lower for AO. A third objective was to determine if there were differences between healthy subjects and patients diagnosed with WRMP in ΔStO2% and ΔHbT responses during varying submaximal contractions (10, 30, 50 and 70% MVC), and StO2% recovery (Rslope) immediately after contractions and AO. Additional variables tested in the ECR at rest were HHbslope to indicate oxygen consumption (using AO) and HbTslope as an indication of blood flow. There were no differences between groups in ΔStO2% and ΔHbT variables during the contractions or Rslope in the recovery after contractions or AO. Furthermore, HbTslope was not different between groups However, oxygen consumption for the ECR and StO2% for the TD at rest were significantly greater for healthy subjects compared to patients. A fourth objective was to determine if there were differences in StO2% and HbT between healthy subjects and WRMP patients during a 12 min sustained contraction of 15 % MVC. In addition, the protocol included a recovery period of 30 min. Prior to contraction, as well as during the recovery period, HbTslope as a surrogate of blood flow was determined for the ECR. Neither the ECR nor the TD exhibited significant differences between groups for StO2% and HbT during the contraction. For the TD patients showed a lower StO2% value at rest and throughout the contraction than healthy subjects. For the ECR HbT during the sustained contraction the general trend was an initial decrease with gradual increase throughout the contraction for both groups. For HbTslope no differences were seen between patients and healthy subjects before the sustained contraction and during the recovery period for both muscles. NIRS is deemed a suitable technique for assessing physiological measurements of the upper extremity, including for day-to-day testing. NIRS was not able to distinguish between the patients with WRMP and controls. A concern in the thesis is the characteristics of the patient group in being equally active in recreational sports, actively working, and similar in muscle strength as controls. Thus, applying NIRS for studying a more severe patient group could yield different results.

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