Samantha L. Winter

Fatigue reduces the complexity of knee extensor torque fluctuations during maximal and submaximal intermittent isometric contractions in man

Healthy physiological systems generate time series possessing complex structures, as seen for example in heart rate variability, respiratory rate and gait. A loss of complexity in physiological time series has been associated with system dysfunction, and this loss is a characteristic feature of torque output from the ageing neuromuscular system. We sought to determine the effect of neuromuscular fatigue on the complexity of knee extensor torque output in healthy young humans performing repeated maximal and submaximal contractions. Fatigue resulted in a substantial loss of knee extensor torque complexity, with the noise in the torque signal becoming increasingly Brownian in character. Complexity has been associated with system adaptability, and the fatigue‐induced loss of complexity, the physiological origin of which is obscure, may contribute to the inability to sustain physical exercise.

Changes in segmental inertial properties with age

The purpose of this study was to examine how the limb segment inertial parameters vary across the decades from the 1920s to the 1970s. Sixty-six males participated in this study, ranging in age from 20 to 79 years. Pre-screening ensured that all subjects were healthy. The inertial properties of the segments were determined by modeling each segment as series of geometric solids. A multivariate analysis of variance (ANOVA) revealed statistically significant differences between decade age groups for the upper arm, forearm, shank, and thigh (p<0.01). Subsequent ANOVAs revealed statistically significant differences for all the inertial properties for the upper arm, the center of mass location for the forearm, and segment mass for the thigh. Linear regression lines were fit to the data so that each inertial parameter for each segment could be predicted by subjects age, with the slope of this regression line indicating the trend in the data. These trends were statistically significant for all forearm inertial parameters, thigh mass and longitudinal moment of inertia, and forearm center of mass location. The changes for the thigh, upper arm, and forearm were consistent with the changes, which would accompany a change in muscle mass with aging. Resultant joint moments were computed for a set of gait data using inertial properties reflective of the subjects from the age extremes in the study. The resulting differences in the knee and hip moments, young versus old, were all less than 4.5%.

The expression of the skeletal muscle force-length relationship in vivo: a simulation study.

The force-length relationship is one of the most important mechanical characteristics of skeletal muscle in humans and animals. For a physiologically realistic joint range of motion and therefore range of muscle fibre lengths only part of the force-length curve may be used in vivo, i.e. only a section of the force-length curve is expressed. A generalised model of a mono-articular muscle-tendon complex was used to examine the effect of various muscle architecture parameters on the expressed section of the force-length relationship for a 90 degrees joint range of motion. The parameters investigated were: the ratio of tendon resting length to muscle fibre optimum length (L(TR):L(F.OPT)) (varied from 0.5 to 11.5), the ratio of muscle fibre optimum length to average moment arm (L(F.OPT):r) (varied from 0.5 to 5), the normalised tendon strain at maximum isometric force (c) (varied from 0 to 0.08), the muscle fibre pennation angle (theta) (varied from 0 degrees to 45 degrees) and the joint angle at which the optimum muscle fibre length occurred (phi). The range of values chosen for each parameter was based on values reported in the literature for five human mono-articular muscles with different functional roles. The ratios L(TR):L(F.OPT) and L(F.OPT):r were important in determining the amount of variability in the expressed section of the force-length relationship. The modelled muscle operated over only one limb at intermediate values of these two ratios (L(TR):L(F.OPT)=5; L(F.OPT):r=3), whether this was the ascending or descending limb was determined by the precise values of the other parameters. It was concluded that inter-individual variability in the expressed section of the force-length relationship is possible, particularly for muscles with intermediate values of L(TR):L(F.OPT) and L(F.OPT):r such as the brachialis and vastus lateralis. Understanding the potential for inter-individual variability in the expressed section is important when using muscle models to simulate movement.

Optoelectronic Plethysmography in Clinical Practice and Research: A Review

Background: Optoelectronic plethysmography (OEP) is a non-invasive motion capture method to measure chest wall movements and estimate lung volumes. Objectives: To provide an overview of the clinical findings and research applications of OEP in the assessment of breathing mechanics across populations of healthy and diseased individuals. Methods: A bibliographic research was performed with the terms “opto-electronic plethysmography,” “optoelectronic plethysmography,” and “optoelectronic plethysmograph” in 50 digital library and bibliographic search databases resulting in the selection of 170 studies. Results: OEP has been extensively employed in studies looking at chest wall kinematics and volume changes in chest wall compartments in healthy subjects in relation to age, gender, weight, posture, and different physiological conditions. In infants, OEP has been demonstrated to be a tool to assess disease severity and the response to pharmacological interventions. In chronic obstructive pulmonary disease patients, OEP has been used to test if patients can dynamically hyperinflate or deflate their lungs during exercise. In neuromuscular patients, respiratory muscle strength and chest kinematics have been analyzed. A widespread application of OEP is in tailoring post-operative pulmonary rehabilitation as well as in monitoring volume increases and muscle contributions during exercise. Conclusions: OEP is an accurate and validated method of measuring lung volumes and chest wall movements. OEP is an appropriate alternative method to monitor and analyze respiratory patterns in children, adults, and patients with respiratory diseases. OEP may be used in the future to contribute to improvements in the therapeutic strategies for respiratory conditions.

Comparison of male and female lower limb segment inertial properties

Many studies have examined human segmental inertial parameters, but these studies have focused more on male rather than female data. The purpose of this study was to determine the lower limb segmental inertial parameters for a large sample (n>1500) of both males and females. The participants in this study were those measured as part of a survey of the anthropometry of US army personnel. The sample comprised 1774 males (mean height 1.756±0.079 m, mean mass of 78.49±0.11 kg, and mean age of 27.21±6.81 years), and 2208 females (mean height 1.629±0.072 m, mean mass of 62.01±0.08 kg, and mean age of 26.18±5.70 years). Anthropometric measurements were used to determine the inertial properties of the lower limb segments by modeling them as series of geometric solids. An analysis of variance revealed that the normalized inertial parameters for each of the segments were statistically significantly different (p>0.001) between the two groups. The time for each segment to swing through the range of motion of the swing phase of gait, produced shorter swing times for the male segments. The differences between the segmental inertial properties for the sexes have implications for how these parameters are customized to experimental subjects.

Optical measurement of breathing: Algorithm volume calibration and preliminary validation on healthy trained subjects

The use of optical technologies may be beneficial when measuring breathing biomechanics. The purpose of this study was twofold: i) to enhance the optoelectronic plethysmography (OEP) algorithm performance for the volume estimation by the use of a novel volume calibration procedure and ii) to compare the OEP volumes gained by a commercial optoelectronic system against actual respiratory volumes measured by a breath-by-breath gas analyzer (BbB). The OEP volume algorithm calibration was performed by the use of a novel volume calibration procedure based on both a calibrator device that delivered known volumes changes and one ad-hoc designed software for the static and dynamic calibration analysis. OEP algorithm threshold, accuracy, repeatability and the volume algorithm calibration were investigated. Tidal volume (VT) measurements performed simultaneously by the calibrated OEP algorithm and BbB analyzer were compared. VT measured simultaneously by OEP and BbB was collected during submaximal exercise tests in five trained healthy participants in two conditions (with hunched shoulders and in normal shoulder position). The two methods were compared by linear regression and Bland-Altman analysis in both positions. The average difference between methods and the discrepancy were calculated. The OEP-BbB correlation was high in both positions, R2=0.92 and R2=0.97 for hunch and normal one, respectively. Bland-Altman analysis demonstrated that OEP algorithm systematic difference was lower than 100mL. The limits of agreement assessed in both positions are comparable. The difference between measurements suggesting that OEP may be a useful tool to analyze chest wall volume changes and breathing mechanics during intense exercise.The use of optical technologies may be beneficial when measuring breathing biomechanics. The purpose of this study was twofold: i) to enhance the optoelectronic plethysmography (OEP) algorithm performance for the volume estimation by the use of a novel volume calibration procedure and ii) to compare the OEP volumes gained by a commercial optoelectronic system against actual respiratory volumes measured by a breath-by-breath gas analyzer (BbB). The OEP volume algorithm calibration was performed by the use of a novel volume calibration procedure based on both a calibrator device that delivered known volumes changes and one ad-hoc designed software for the static and dynamic calibration analysis. OEP algorithm threshold, accuracy, repeatability and the volume algorithm calibration were investigated. Tidal volume (VT) measurements performed simultaneously by the calibrated OEP algorithm and BbB analyzer were compared. VT measured simultaneously by OEP and BbB was collected during submaximal exercise tests in five trained healthy participants in two conditions (with hunched shoulders and in normal shoulder position). The two methods were compared by linear regression and Bland-Altman analysis in both positions. The average difference between methods and the discrepancy were calculated. The OEP-BbB correlation was high in both positions, R2=0.92 and R2=0.97 for hunch and normal one, respectively. Bland-Altman analysis demonstrated that OEP algorithm systematic difference was lower than 100mL. The limits of agreement assessed in both positions are comparable. The difference between measurements suggesting that OEP may be a useful tool to analyze chest wall volume changes and breathing mechanics during intense exercise.

Loss of knee extensor torque complexity during fatiguing isometric muscle contractions occurs exclusively above the critical torque

The complexity of knee extensor torque time series decreases during fatiguing isometric muscle contractions. We hypothesized that because of peripheral fatigue, this loss of torque complexity would occur exclusively during contractions above the critical torque (CT). Nine healthy participants performed isometric knee extension exercise (6 s of contraction, 4 s of rest) on six occasions for 30 min or to task failure, whichever occurred sooner. Four trials were performed above CT (trials S1-S4, S1 being the lowest intensity), and two were performed below CT (at 50% and 90% of CT). Global, central, and peripheral fatigue were quantified using maximal voluntary contractions (MVCs) with femoral nerve stimulation. The complexity of torque output was determined using approximate entropy (ApEn) and the detrended fluctuation analysis-α scaling exponent (DFA-α). The MVC torque was reduced in trials below CT [by 19 ± 4% (means ± SE) in 90%CT], but complexity did not decrease [ApEn for 90%CT: from 0.82 ± 0.03 to 0.75 ± 0.06, 95% paired-samples confidence intervals (CIs), 95% CI = -0.23, 0.10; DFA-α from 1.36 ± 0.01 to 1.32 ± 0.03, 95% CI -0.12, 0.04]. Above CT, substantial reductions in MVC torque occurred (of 49 ± 8% in S1), and torque complexity was reduced (ApEn for S1: from 0.67 ± 0.06 to 0.14 ± 0.01, 95% CI = -0.72, -0.33; DFA-α from 1.38 ± 0.03 to 1.58 ± 0.01, 95% CI 0.12, 0.29). Thus, in these experiments, the fatigue-induced loss of torque complexity occurred exclusively during contractions performed above the CT.

Comparison of marker models for the analysis of the volume variation and thoracoabdominal motion pattern in untrained and trained participants

Respiratory assessment and the biomechanical analysis of chest and abdomen motion during breathing can be carried out using motion capture systems. An advantage of this methodology is that it allows analysis of compartmental breathing volumes, thoraco-abdominal patterns, percentage contribution of each compartment and the coordination between compartments. In the literature, mainly, two marker models are reported, a full marker model of 89 markers placed on the trunk and a reduced marker model with 32 markers. However, in practice, positioning and post-process a large number of markers on the trunk can be time-consuming. In this study, the full marker model was compared against the one that uses a reduced number of markers, in order to evaluate (i) their capability to obtain respiratory parameters (breath-by-breath tidal volumes) and thoracoabdominal motion pattern (compartmental percentage contributions, and coordination between compartments) during quiet breathing, and (ii) their response in different groups such as trained and untrained, male and female. Although tests revealed strong correlations of the tidal volume values in all the groups (R2 > 0.93), the reduced model underestimated the trunk volume compared with the 89 marker model. The highest underestimation was found in trained males (bias of 0.43 L). The three-way ANOVA test showed that the model did not influence the evaluation of compartmental contributions and the 32 marker model was adequate to distinguish thoracoabdominal breathing pattern in the studied groups. Our findings showed that the reduced marker model could be used to analyse the thoracoabdominal motion in both trained and untrained populations but performs poorly in estimating tidal volume.

Effects of ipsilateral and contralateral fatigue and muscle blood flow occlusion on the complexity of knee‐extensor torque output in humans

What is the central question of this study? We addressed the question “what role do central and peripheral fatigue mechanisms play in the fatigue‐induced loss of isometric torque complexity?” What is the main finding and its importance? When the contralateral limb is fatigued, the complexity of isometric torque output is unaffected even if the blood flow to the contralateral limb is occluded, which suggests that neither central fatigue nor afferent feedback from ischaemic muscle influences the complexity of torque output in an otherwise fresh muscle.

Classifying the variability in impact and active peak vertical ground reaction forces during running using DFA and ARFIMA models

The vertical ground reaction force (VGRF) during rear-foot striking running typically exhibits peaks referred to as the impact peak and the active peak; their timings and magnitudes have been implicated in injury. Identifying the structure of time-series can provide insight into associated control processes. The purpose here was to detect long-range correlations associated with the time from first contact to impact peak (TIP) and active peak (TAP); and the magnitudes of impact (IPM) and active peaks (APM) using a Detrended Fluctuation Analysis, and Auto-Regressive Fractionally Integrated Moving Average models. Twelve subjects performed an 8min trial at their preferred running speed on an instrumented treadmill. TIP, TAP; IPM, and APM were identified from the VGRF profile for each footfall. TIP and TAP time-series did not demonstrate long-range correlations, conversely IPM and APM time-series did. Short range correlations appeared as well as or instead of long range correlations for IPM. Conversely pure powerlaw behaviour was demonstrated in 11 of the 24 time series for APM, and long range dependencies along with short range correlations were present in a further 9 time series. It has been hypothesised that control mechanisms for IPM and APM are different, these results support this hypothesis.