Influence of heat stress and carbohydrate availability on substrate metabolism and exercise tolerance time in humans

Whilst the effects of environmental heat stress on the physiological responses of humans during exercise have been investigated for over half a century, the mechanisms responsible for fatigue during exercise in the heat are not well understood. There is increasing evidence that heat stress increases the reliance on carbohydrate (CHO), particularly muscle glycogen, as a fuel for prolonged exercise. The provision of CHO during exercise and during short-term recovery from exercise in the heat may theoretically offer some benefit. However, the literature available on the efficacy of CHO feedings during prolonged running in the heat is scarce. The aim of the experiments that are reported in this thesis were to investigate the effects of heat stress and CHO feeding regimens on substrate metabolism and exercise tolerance during prolonged running. An initial investigation revealed that the heat stress imposed by wearing a military protective clothing ensemble during prolonged running impaired exercise tolerance time and increased the reliance on CHO as a fuel. This response was associated with increases in circulating adrenaline and lactate concentrations, which may be indicative of an enhanced ß-adrenergic receptor stimulation of muscle glycogenolysis. Thus, further studies into the efficacy of CHO supplementation regimens during exercise and recovery from exercise in the heat were performed. Rehydration with a carbohydrate-electrolytes olution (CES) during a 4-h recovery period markedly increased total CHO utilisation and exercise tolerance during subsequent exercise in the heat (35°C) compared to a sweetened placebo. Whilst there was no difference in post-recovery exercise tolerance time after ingesting 55-g or 220-g of CHO within a CES, ingesting 220-g lead to a five-fold increase in estimated glycogen synthesis during recovery, which increased CHO availability and utilisation during subsequent exercise. Ingesting a 12.5% glucose solution attenuated the increased reliance on endogenous CHO stores during exercise in the heat, but the associated increases in thermal and cardiovascular strain and gastric discomfort may have been responsible for the impairment of exercise capacity. These findings suggest that increases in endogenous CHO metabolism occur in response to exogenous heat stress during prolonged running. Whilst CHO ingestion during short-term recovery periods are associated with favourable changes in glycogen synthesis and tolerance to subsequent exercise in the heat, ingestion of a hypertonic glucose solution during exercise in the heat, may impair exercise capacity.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Backward Double Integration is a Valid Method to Calculate Maximal and Sub-Maximal Jump Height

The backward double integration method uses one force plate and could calculate jump height for countermovement jumping, squat jumping and drop jumping by analysing the landing phase instead of the push-off phase. This study compared the accuracy and variability of the forward double integration (FDI), backwards double integration (BDI) and Flight Time + Constant (FT+C) methods, against the marker-based rigid-body modelling method. It was hypothesised that the jump height calculated using the BDI method would be equivalent to the FDI method, while the FT+C method would have reduced accuracy and increased variability during sub-maximal jumping compared to maximal jumping. Twenty-four volunteers performed five maximal and five sub-maximal countermovement jumps, while force plate and motion capture data were collected. The BDI method calculated equivalent mean jump heights compared to the FDI method, with only slightly higher variability (2–3 mm), and therefore can be used in situations where FDI cannot be employed. The FT+C method was able to account for reduced heel-lift distance, despite employing an anthropometrically scaled heel-lift constant. However, across both sub-maximal and maximal jumping, it had increased variability (1.1 cm) compared to FDI and BDI and should not be used when alternate methods are available.</p

The effect of foot orientation modifications on knee joint biomechanics during different activities

Introduction Foot position during daily activities can influence the magnitude and rate of knee joint loading [1]. Over time, increased loading can cause cumulative damage to the articulating surfaces of the knee joint, especially in people with existing knee osteoarthritis [2]. Knee joint loading is difficult to measure in vivo as the majority of knee loading is distributed on the medial compartment of the knee joint, therefore, knee adduction moment (KAM) is commonly used as a surrogate measure for knee joint loading [3].   Foot orientation is believed to have an impact on knee loading during daily activities such as walking and standing from a chair, altering the direction of the ground reaction force vector to reduce the adduction moment arm, relative to the knee joint [4]. However, limited studies have systematically explored the effect of foot orientation on KAM in activities other than walking, which is crucial for improving functional mobility and quality of life in this population beyond the lab. Therefore, this study aims to evaluate the effect of different foot orientations (toe-in, parallel and toe-out) on knee loading across several daily activities (walking, sit-to-stand, and stair climbing).   Methods Twenty-nine participants (56 ± 5 years, 170 ± 8 cm, 74 ± 14 kg) performed over-ground walking, stair climbing and sit-to-stand movements at their preferred constant speed under three foot conditions, 10° toe-in, 10° toe-out, neutral (0°). Participants performed walking and sit-to-stand on overground force plates, and stair climbing on a portable force plate embedded within the stairs. Each condition within each activity was repeated until five successful trials were obtained.   Three-dimensional kinematic (200 Hz) and kinetic data (1000 Hz) were recorded to obtain knee joint moments and foot progression angles. Foot progression angle was identified using the frontal angle of foot (defined as a 6DOF rigid body) to the global coordinate system (QTM). KAM was computed using inverse dynamics (Visual 3D) and normalised to body mass. Mean within-participant values were calculated for statistical analysis, with repeated measures ANOVA and Bonferroni post-hoc analysis used to compare the KAMs of three foot orientations across all activities.   Results KAMs during toe-in foot position were significantly lower than those under neutral foot position during walking (P = 0.011), stair climbing and sit-to-stand (P &lt; 0.001), while the KAMs during neutral foot position were significantly lower than those in toe-out foot position across all activities (P &lt; 0.001) (Fig 1). Figure 1: Median and interquartile, peak KAM for toe-out, toe-in and neutral foot position conditions during walking, stair climbing and sit-to-stand.   Discussion All results showed a significant decrease in peak KAM during the toe-in foot position condition compared to toe-out and neutral foot positions, which is consistent with previous gait studies. The results of this study indicate that toe-in gait can reduce knee joint loading not only during walking, but also in stair climbing and sit-to-stand activities.   The results of this study will be of help in gait retraining programme in clinics and rehabilitation aimed at minimising knee loading and joint pain to slow the progression of the disease. They may provide a range of clinical guidance for injury prevention in a healthy older population under the common contexts  of stair climbing and sit-to-stand, taking the technique outside the lab. Future studies should explore the effectiveness of altered foot orientation modifications on knee loading and pain reduction, in a patient population such as knee osteoarthritis.   References 1.   Valenzuela et al, J Sports Sci. Med, 15:50-56, 2016. 2.   Lynn et al, Clin Biomech, 23: 779-786, 2008. 3.   Manal et al, Osteoarthr. Cartil, 23:1107-1111, 2015. 4.   Rutherford et al, Osteoarthr. Cartil, 16:883-889, 2008.   Acknowledgements This project was funded by China Scholarship Council.</p