The purpose of the four studies which comprise the present thesis, was to examine the relationship between intra-muscular aerobic adaptation and the rate of phosphocreatine (PCr) resynthesis following intense exercise. Study One required three male and two female volunteers 19.3 ± 1.4 years (mean age ± SD) to complete five weeks of endurance training using repeated plantar flexion of a randomly assigned leg (TRA). Subjects completed three training sessions a week for the first two weeks and then five sessions a week for the remainder of the training period; each session involved 30 min of exercise at a frequency of 30 contractions per min. Prior to, and again immediately following the training period, muscle was sampled from the m. gastrocnemius for the analysis of citrate synthase (CS) activity. Measures of PCr concentration ([PCr]) were acquired following a bout of plantar flexion using Phosphorus Nuclear Magnetic Resonance Spectroscopy (³¹P-NMR). Pre-training CS activity (±SD) was 12.02 ± 1.55 μmol•min¯¹•g¯¹ of protein and remained at a statistically similar level of 19.34 ± 10.46 μmol•min¯¹•g¯¹ of protein post-training (p > 0.05). Prior to training, [PCr] in the gastrocnemius muscle of the trained leg was reduced to 32.18 ± 13.57% of resting levels compared to 32.99 + 18.35% in the untrained control leg before recovering to 81.44 ± 11.09% and 93.66 ± 18.35% at the end of 10 min and 40 s of passive recovery in these two limbs respectively. There was no significant difference in [PCr] between the pre- and post- training condition for either limb at the end of exercise or at any time throughout the recovery period (p > 0.05). However, when the curve describing the initial 48 s of PCr recovery was transformed into a linear relationship, there was a trend for the mean slope posttraining to be steeper than pre-training (p = 0.07). However, recovery of PCr both pre and post- training was slower than had been expected which suggested that the Velcro strapping which held the exercising leg in place had impeded blood flow to the recovering leg.
The primary aim of Study Two was to determine whether subjects with an elevated VO₂ peak would be able to resynthesise PCr more quickly following intense exercise than subjects with lower VO₂ peak values. The investigation also presented the opportunity to address some of the limitations from Study One and to develop a protocol deemed more suitable to multiple muscle biopsies (which were intended to be used when the longitudinal training model was revisited in later work). Nine males and one female (mean age ± SD) 23.38 ± 5.01 years volunteered for Study Two and were assigned to either a high aerobic power (HAP) or low aerobic power (LAP) group depending on whether their one-legged cycling VO₂ peak was higher (HAP) or lower (LAP) than 40 ml•kg¯¹•min¯¹. ³¹P-NMR monitored [PCr] in the vastus lateralis muscle for 128 s immediately following a 96 s bout of leg extension in which mean pHi (±SD) fell to 6.83 ± 0.10 and 6.83 ± 0.09 at the end of exercise in the HAP and LAP group respectively (p > 0.05). At the same time, [PCr] (±SD) was reduced to 31.92 ± 8.82% and 29.35 ± 5.37% of resting levels before recovering to 89.79 ± 6.22% and 91.93 ± 1.87% in the HAP and LAP groups respectively. There was no difference in [PCr] between groups at the end of exercise (p > 0.05), however after 64 and 80 s of recovery, mean [PCr] was significantly higher in the HAP group (p < 0.05). Both the mean rate- (k) and time- (tc) constants of PCr resynthesis calculated over the 128 s of recovery for the HAP group (k = 1.26 ± 0.29 and tc = 49.82 ± 11.44) were statistically different (p < 0.05) to the rate- and time- constants for the LAP group (k = 1.00 ± 0.06 and tc = 60.37 ± 3.82). The results of Study Two demonstrated that HAP subjects were able to resynthesise PCr faster than LAP subjects following intense exercise.
The aim of Study Three was to examine possible changes in the rate of PCr resynthesis and potential increases in markers of aerobic power in response to seven weeks of one-legged endurance training. Six untrained volunteers 20.33 ± 3.14 years (mean age ± SD) performed an intense bout of leg extension prior to, and again immediately following training. In addition, muscle was sampled firom the vastus lateralis muscle for the analysis of CS and creatine kinase (CK) activity and capillary density. [PCr] was also monitored for five minutes and twenty seconds of passive recovery following 96 s of intense leg extension using ³¹ P-NMR. Subjects underwent endurance training for three sessions a week for seven weeks; this followed a test to determine each subject's pre- training peak aerobic power (PAP). Each training session required subjects to pedal with one leg against a resistance of 60% PAP for 31 min (70 rpm). Following this, subjects completed three, three min bouts against 80% PAP; each bout was separated by two min of passive recovery. CS activity was significantly increased as a result of endurance training (316.8 ± 48.6 U•g¯¹ protein vs. 185.2 + 57.37 U•g¯¹ protein; trained vs. control leg respectively, p < 0.05) while CK activity remained unchanged with training (p > 0.05). Capillary density when expressed as capillaries per muscle fibre (± SD) significantly increased as a result of training (1.76 ± 0.13 vs. 2.08 ± 0.11; pre- and post- training respectively, p < 0.01). There was no statistical difference in mean PCr levels during and following exercise before or after training. In addition, both the rate- and time- constants of PCr resynthesis following training were not statistically different from their pre-training value (p = 0.13 and p = 0.10 respectively). There was a significant relationship between CS activity and both k and tc (r = 0.63 and r = - 0.71 respectively, p < 0.05, n = 10) prior to endurance training. However, the significance of this relationship was not maintained following the seven weeks of training (r = 0.33 and r = - 0.28 respectively, p > 0.05, n = 10). The results of Study Three showed that CS activity in the trained leg was significantly higher when compared to the control leg, yet suggested that a difference such as this in the aerobic potential of skeletal muscle did not significantly enhance the rate of phosphocreatine resynthesis following intense exercise.
The aim of Study Four was to determine the effect of a period of detraining on the rate of PCr resynthesis following intense exercise and to compare possible changes with the time-course of the expected decline in the markers of oxidative metabolism. While CS activity was significantly reduced in response to inactivity (p < 0.05 compared to post-training values), capillary density (capillaries per mm² and capillaries per muscle fibre) remained statistically unchanged. [PCr] recovered with almost identical rate kinetics as the post-training test and clearly seven weeks of detraining had no significant influence on the rate of PCr resynthesis following intense exercise.
Collectively, the results of the four studies are difficult to interpret. Study Two strongly supported a causal link between VO₂ peak and the rate at which PCr is resynthesised. However, Study Three and Study Four were statistically inconclusive, with small sample size and large variability appearing to be the main reasons why the obtained results were difficult to explain. While there was slight tendency for the rate of PCr resynthesis to become more rapid following endurance training, further research is necessary to determine whether any relationship, that is either observed or theorised, is causal or coincidental in nature.