Per Aagaard

Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants, effective surface area, and the hydrolysis of feldspar

Analysis of experimental data reported by Lagache (1965, 1976), Evans (1965), Busenberg (1975), Busenberg and Clemency (1976), Holdren and Berner (1979), Siegel and Pfannkuch (1984), and Chou and Wollast (1984) with the aid of irreversible thermodynamics and transition state theory (Aagaard and Helgeson, 1977, 1982) suggests that at temperatures at least up to 650°C, the rate of both congruent and incongruent feldspar hydrolysis in aqueous solutions far from equilibrium at pH ≲ 10.6 − (2300/T), where T stands for temperature in kelvins, is a function solely of effective surface area and pH at constant pressure and temperature. At higher pH, the rate is apparently pH-independent up to ~pH 8 at 25°C, where it again becomes pH-dependent at higher pH. Observations of scanning electron micrographs indicate that the cross-sectional area of etch pits on hydrolyzed feldspar grains is of the order of 10−9 to 10−8 cm2 and that the ratio of the effective to total surface area (which may or may not change with reaction progress) ranges from <0.01 to 1, depending on the grain size, dislocation density, and the extent of comminution damage on the surfaces of the grains. Apparent rate constants retrieved from experimental data reported in the literature for feldspar hydrolysis in the lower pH-dependent range extend from ~10−13 to ~10−7 moles cm−2 sec−1 at temperatures from 25° to 200°C, which is consistent with activation enthalpies for albite and adularia of the order of 20 kcal mole−1. In contrast, the apparent rate constants for the pH-independent rate law range from ~10−16 to ~10−11 moles cm−2 sec−1 at temperatures from 25° to 650°C, which requires an activation enthalpy for adularia of ~ 9 kcal mole−1. These observations are consistent with surface control of reaction rates among minerals and aqueous solutions. The rate-limiting step in the pH-dependent case apparently corresponds at the lower end of the pH scale to breakdown of a protonated configuration of atoms on the surface of the reactant feldspar, but at higher pH the rate is limited by decomposition of an activated surface complex corresponding in stoichiometry to hydrous feldspar. In highly alkaline solutions, an activated complex containing hydroxyl ions apparently controls the rate of feldspar hydrolysis. Nevertheless, near equilibrium, regardless of pH the rate is proportional to the chemical affinity of the overall hydrolysis reaction.

A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture

1 In human pennate muscle, changes in anatomical cross‐sectional area (CSA) or volume caused by training or inactivity may not necessarily reflect the change in physiological CSA, and thereby in maximal contractile force, since a simultaneous change in muscle fibre pennation angle could also occur. 2 Eleven male subjects undertook 14 weeks of heavy‐resistance strength training of the lower limb muscles. Before and after training anatomical CSA and volume of the human quadriceps femoris muscle were assessed by use of magnetic resonance imaging (MRI), muscle fibre pennation angle (θp) was measured in the vastus lateralis (VL) by use of ultrasonography, and muscle fibre CSA (CSAfibre) was obtained by needle biopsy sampling in VL. 3 Anatomical muscle CSA and volume increased with training from 77.5 ± 3.0 to 85.0 ± 2.7 cm2 and 1676 ± 63 to 1841 ± 57 cm3, respectively (±s.e.m.). Furthermore, VL pennation angle increased from 8.0 ± 0.4 to 10.7 ± 0.6 deg and CSAfibre increased from 3754 ± 271 to 4238 ± 202 μm2. Isometric quadriceps strength increased from 282.6 ± 11.7 to 327.0 ± 12.4 N m. 4 A positive relationship was observed between θp and quadriceps volume prior to training (r = 0.622). Multifactor regression analysis revealed a stronger relationship when θp and CSAfibre were combined (R= 0.728). Post‐training increases in CSAfibre were related to the increase in quadriceps volume (r = 0.749). 5 Myosin heavy chain (MHC) isoform distribution (type I and II) remained unaltered with training. 6 VL muscle fibre pennation angle was observed to increase in response to resistance training. This allowed single muscle fibre CSA and maximal contractile strength to increase more (+16 %) than anatomical muscle CSA and volume (+10 %). 7 Collectively, the present data suggest that the morphology, architecture and contractile capacity of human pennate muscle are interrelated, in vivo. This interaction seems to include the specific adaptation responses evoked by intensive resistance training.

Load-displacement properties of the human triceps surae aponeurosis in vivo

1 The present investigation measured the load‐displacement and stress‐strain characteristics of the proximal and distal human triceps surae aponeurosis and tendon in vivo during graded voluntary 10 s isometric plantarflexion efforts in five subjects. 2 During the contractions synchronous real‐time ultrasonography of aponeurosis displacement, electromyography of the gastrocnemius, soleus and dorsiflexor muscles, and joint angular rotation were obtained. Tendon cross‐sectional area and moment arm were obtained from magnetic resonance (MR) images. Force and electromyography data from dorsiflexion efforts were used to examine the effect of coactivation. 3 Tendon force was calculated from the joint moments and tendon moment arm, and stress was obtained by dividing force by cross‐sectional area. Aponeurosis and tendon strain were obtained from the displacements normalised to tendon length. 4 Tendon force was 3171 ± 201 N, which corresponded to 2.6 % less than the estimated force when coactivation was accounted for (3255 ± 206 N). Aponeurosis displacement (13.9‐ 12.9 mm) decreased 30 % (9.6‐10.7 mm) when accounting for joint angular rotation (3.6 deg). Coactivation and angular rotation‐corrected stiffness yielded a quadratic relationship, R2= 0.98± 0.01, which was similar for the proximal (467 N mm−1) and distal (494 N mm−1) aponeurosis and tendon. Maximal strain and stress were 4.4‐5.6 % and 41.6 ± 3.9 MPa, respectively, which resulted in a Youngs modulus of 1048‐1474 MPa. 5 The mechanical properties of the human triceps surae aponeurosis and tendon in vivo were for the first time examined. The stiffness and Youngs modulus exceeded those previously reported for the tibialis anterior tendon in vivo, but were similar to those obtained for various isolated mammalian and human tendons.

Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a countermeasure

Aging is characterized by loss of spinal motor neurons (MNs) due to apoptosis, reduced insulin‐like growth factor I signaling, elevated amounts of circulating cytokines, and increased cell oxidative stress. The age‐related loss of spinal MNs is paralleled by a reduction in muscle fiber number and size (sarcopenia), resulting in impaired mechanical muscle performance that in turn leads to a reduced functional capacity during everyday tasks. Concurrently, maximum muscle strength, power, and rate of force development are decreased with aging, even in highly trained master athletes. The impairment in muscle mechanical function is accompanied and partly caused by an age‐related loss in neuromuscular function that comprise changes in maximal MN firing frequency, agonist muscle activation, antagonist muscle coactivation, force steadiness, and spinal inhibitory circuitry. Strength training appears to elicit effective countermeasures in elderly individuals even at a very old age (>80 years) by evoking muscle hypertrophy along with substantial changes in neuromuscular function, respectively. Notably, the training‐induced changes in muscle mass and nervous system function leads to an improved functional capacity during activities of daily living.

A New Concept For Isokinetic Hamstring: Quadriceps Muscle Strength Ratio

Conventionally, the hamstring:quadriceps strength ratio is calculated by dividing the maximal knee flexor (hamstring) moment by the maximal knee extensor (quadriceps) moment measured at identical angular velocity and contraction mode. The agonist-antagonist strength relationship for knee extension and flexion may, however, be better described by the more functional ratios of eccentric hamstring to concentric quadriceps moments (extension), and concentric hamstring to eccentric quadriceps moments (flexion). We compared functional and conventional isokinetic hamstring: quadriceps strength ratios and examined their relation to knee joint angle and joint angular velocity. Peak and angle-specific (50°, 40°, and 30° of knee flexion) moments were determined during maximal concentric and eccentric muscle contractions (10° to 90° of motion; 30 and 240 deg/sec). Across movement speeds and contraction modes the functional ratios for different moments varied between 0.3 and 1.0 (peak and 50°), 0.4 and 1.1 (40°), and 0.4 and 1.4 (30°). In contrast, conventional hamstring:quadriceps ratios were 0.5 to 0.6 based on peak and 50° moments, 0.6 to 0.7 based on 40° moment, and 0.6 to 0.8 based on 30° moment. The functional hamstring:quadriceps ratio for fast knee extension yielded a 1:1 relationship, which increased with extended knee joint position, indicating a significant capacity of the hamstring muscles to provide dynamic knee joint stability in these conditions. The evaluation of knee joint function by use of isokinetic dynamometry should comprise data on functional and conventional hamstring:quadriceps ratios as well as data on absolute muscle strength.

A mechanism for altered flexibility in human skeletal muscle.

1. We investigated the effect of a long‐term stretching regimen on the tissue properties and stretch tolerance of human skeletal muscle. 2. Resistance to stretch was measured as torque (in N m) offered by the hamstring muscle group during passive knee extension while electromyographic (EMG) activity, knee joint angle and velocity were continuously monitored during a standardized stretch manoeuvre. Seven healthy subjects were tested before and after a 3 week training period using two separate protocols. Protocol 1 consisted of a slow stretch at 0.087 rad s‐1 to a predetermined angle followed by a 90 s holding phase. Subjects were brought to the same angle before and after the training period. Protocol 2 was a similar stretch, but continued to the point of pain. 3. During protocol 1 the torque rose during the stretch and then declined during the holding phase. EMG activity was small and did not change significantly during the protocol. No significant differences in stiffness, energy and peak torque about the knee joint were seen as a result of the training. During protocol 2 the angle to which the knee could be extended was significantly increased as a result of the training. This was accompanied by a comparable increase in peak torque and energy. EMG activity was small and not affected by training. 4. It is concluded that reflex EMG activity does not limit the range of movement during slow stretches and that the increased range of motion achieved from training is a consequence of increased stretch tolerance on the part of the subject rather than a change in the mechanical or viscoelastic properties of the muscle.

Training-induced changes in neural function

AAGAARD, P. Training-induced changes in neural functions. Exerc. Sport Sci. Rev., Vol. 31, No. 2, pp. 61–67, 2003. Adaptive changes can occur in the nervous system in response to training. Electromyography studies have indicated adaptation mechanisms that may contribute to an increased efferent neuronal outflow with training, including increases in maximal firing frequency, increased excitability and decreased presynaptic inhibition of spinal motor neurons, and downregulation of inhibitory pathways.

Myosin heavy chain IIX overshoot in human skeletal muscle.

The distribution of myosin heavy chain (MHC) isoforms, fiber type composition, and fiber size of the vastus lateralis muscle were analyzed by sodium dodecylsulfate polymerase gel electrophoresis (SDS‐PAGE), ATPase histochemistry, and immunocytochemistry in a group of adult sedentary men before and after 3 months of heavy‐load resistance training and, subsequently, after 3 months of detraining. Following the period of resistance training, MHC IIX content decreased from 9.3 ± 2.1% to 2.0 ± 0.8% (P < 0.01), with a corresponding increase in MHC IIA (42.4 ± 3.9% vs. 49.6 ± 4.0% [P < 0.05]). Following detraining the amount of MHC IIX reached values that were higher than before and after resistance training (17.2 ± 3.2% [P < 0.01]). Changes in fiber type composition resembled the changes observed in MHC isoform content. Significant hypertrophy was observed for the type II fibers after resistance training. Maximal isometric quadriceps strength increased after resistance training, but returned to pretraining levels after detraining. The present results suggest that heavy‐load resistance training decreases the amount of MHC IIX while reciprocally increasing MHC IIA content. Furthermore, detraining following heavy‐load resistance training seems to evoke an overshoot in the amount of MHC IIX to values markedly higher than those observed prior to resistance training.

Biomechanical responses to repeated stretches in human hamstring muscle in vivo

To examine stiffness, energy, and passive torque in the dynamic and static phases of a stretch maneuver in the human hamstring muscle in vivo we used a test- retest protocol and a repeated stretches protocol. Re sistance to stretch was defined as passive torque (in newton-meters) offered by the hamstring muscle group during passive knee extension as measured using an isokinetic dynamometer with a modified thigh pad. In 13 uninjured subjects, the knee was passively ex tended to a predetermined final position (0.0875 rad/ sec, dynamic phase) where it remained stationary for 90 seconds (static phase). The test-retest protocol in cluded two tests administered 1 hour apart. On a sep arate occasion, five consecutive static stretches were administered separated by 30 seconds and followed by a sixth stretch 1 hour later. For the test-retest phase, stiffness and energy in the dynamic phase and passive torque in the static phase did not differ and yielded correlations of r = 0.91 to 0.99. During the static phase, passive torque declined in both tests (P < 0.0001). For the repeated stretches, decreases were observed for energy (P < 0.01) and stiffness (P < 0.05) in the dynamic phase and for passive torque (P < 0.0001) in the static phase. However, the decline in the variables returned to baseline within 1 hour. The data show that the method employed is a useful tool for measuring biomechanical variables during a stretch maneuver. This may provide a more detailed method to examine skeletal muscle flexibility.

Load-displacement properties of the human triceps surae aponeurosis and tendon in runners and non-runners

The load‐displacement and stress–strain characteristics of the human triceps surae tendon and aponeurosis, in vivo, was examined during graded maximal voluntary plantarflexion efforts in runners who trained 80 km/ week or more and age‐matched non‐runners. Synchronous real‐time ultrasonography of triceps surae tendon and aponeurosis displacement, electromyography of the gastrocnemius, soleus and dorsiflexor muscles, and joint angular rotation were obtained. Tendon cross‐sectional area and ankle joint moment arm were obtained from magnetic resonance imaging. Tensile tendon force was calculated from the joint moments and tendon moment arm and stress was obtained by dividing force by cross‐sectional area. Strain was obtained from the displacements normalized to tendon length. Antagonist coactivation and small amounts of ankle joint rotation significantly affected tensile tendon force and aponeurosis and tendon displacement, respectively (P < 0.01). Plantarflexion moment was similar in runners (138 ± 27 Nm, mean ± SEM) and non‐runners (142 ± 17 Nm). Tendon moment arm was alike in non‐runner (58.3 ± 0.2 mm) and runners (55.1 ± 0.1 mm). Similarly, there was no difference in tendon tensile force between runners (2633 ± 465 N) and non‐runners (2556 ± 401 N). The cross‐sectional area of the Achilles tendon was larger in runners (95 ± 3 mm2) than non‐runners (73 ± 3 mm2) (P < 0.01). The load‐deformation data yielded similar stiffness (runners 306 ± 61 N/mm, non‐runners 319 ± 42 N/mm). The maximal strain and stress was 4.9 ± 0.8% and 38.2 ± 9.8 MPa in non‐runners and 4.1 ± 0.8% and 26.3 ± 5.1 MPa in runners. The larger tendon cross‐sectional area in trained runners suggests that chronic exposure to repetitive loading has resulted in a tissue adaptation.