Revisiting specific force loss in human permeabilized single skeletal muscle fibers obtained from older individuals.

Specific force (SF) has been shown to be reduced in some but not all studies of human aging using chemically skinned single muscle fibers. This may be due, in part, not only to the health status/physical activity levels of different older cohorts, but also from methodological differences in studying skinned fibers. The aim of the present study was to compare SF in fibers from older hip fracture patients (HFP), healthy master cyclists (MC), and healthy nontrained young adults (YA) using two different activating solutions. Quadriceps muscle samples and 316 fibers were obtained from HFPs (74.6 ± 4 years, n = 5), MCs (74.8 ± 1, n = 5), and YA (25.5 ± 2, n = 6). Fibers were activated (pCa 4.5, 15°C) in solutions containing either 60 mM N-tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid pH buffer (TES) or 20 mM imidazole. SF was determined by normalizing force to fiber cross-sectional area (CSA) assuming either an elliptical or circular shape and to fiber myosin heavy chain content. Activation in TES resulted in significantly higher MHC-I SF in all groups and YA MHC-IIA fibers, irrespective of normalization method. Although there were no differences in SF between the participant groups, the ratio of SF between the TES and imidazole solutions was lower in HFPs compared with YAs (MHC-I P < 0.05; MHC-IIA P = 0.055). Activating solution composition, as opposed to donor characteristics, had a more notable effect on single fiber SF. However, this two-solution approach revealed an age-related difference in sensitivity in HFPs, which was not shown in MCs. This suggests further novel approaches may be required to probe age/activity-related differences in muscle contractile quality.NEW & NOTEWORTHY Whether specific force (SF) decreases with advancing age in human single skeletal muscle fibers is uncertain. Equivocal published findings may be due to the different physical activity levels of the elderly cohorts studied and/or different chemical solutions used to measure force. We compared single fiber SF between young adults, elderly cyclists, and hip fracture patients (HFP) using two solutions. The solution used significantly affected force and revealed a difference in sensitivity of HFP muscle fibers.

The predominant stride-frequency for routine swimming in catsharks (Scyliorhinus canicula) generates high power at high efficiency in the red musculature.

Videos of free swimming of catsharks (Scyliorhinus canicula) were analysed to give values of swimming speed (units: FL (fish lengths) s-1), stride-length (forward movement in the direction of travel per cycle of body undulation (units: FL) and stride-frequency (units: s-1). Most of the swims (139 of 163, 85%) were at speeds less than 0.545 FL s-1 and were categorized as slow. The rest (24/163, 15%) were categorized as fast. Stride-lengths and stride-frequencies could be evaluated for 115 of the slow swims and 16 of the fast swims. We discuss the fast swim results, but there were so few fast swims that no firm conclusions could be made. As swim speed increased during slow swims, there was a strong increase stride-length [slope 0.965, P < 0.0001)] and a small increase in stride-frequency. Most stride-frequencies (70/115, 61%) were in the range 0.68-0.88 s-1. Previous experiments on red muscle isolated of catshark showed that in this range of frequencies of sinusoidal movement, high power was produced at high efficiency (Curtin and Woledge b). Lower frequencies gave less power and at higher frequencies the efficiency of energy conversion was lower. Thus, we conclude that during routine swimming catsharks choose a swimming speed that optimizes red muscle performance in terms of power and efficiency.

Methodological considerations in measuring specific force in human single skinned muscle fibres.

Chemically skinned fibres allow the study of human muscle contractile function in vitro. A particularly important parameter is specific force (SF), that is, maximal isometric force divided by cross-sectional area, representing contractile quality. Although SF varies substantially between studies, the magnitude and cause of this variability remains puzzling. Here, we aimed to summarize and explore the cause of variability in SF between studies. A systematic search was conducted in Medline, Embase and Web of Science databases in June 2020, yielding 137 data sets from 61 publications which studied healthy, young adults. Five-fold differences in mean SF data were observed. Adjustments to the reported data for key methodological differences allowed between-study comparisons to be made. However, adjustment for fibre shape, swelling and sarcomere length failed to significantly reduce SF variance (I2 = 96%). Interestingly, grouping papers based on shared authorship did reveal consistency within research groups. In addition, lower SF was found to be associated with higher phosphocreatine concentrations in the fibre activating solution and with Triton X-100 being used as a skinning agent. Although the analysis showed variance across the literature, the ratio of SF in single fibres containing myosin heavy chain isoforms IIA or I was found to be consistent across research groups. In conclusion, whilst the skinned fibre technique is reliable for studying in vitro force generation of single fibres, the composition of the solution used to activate fibres, which differs between research groups, is likely to heavily influence SF values.

Energy turnover in mammalian skeletal muscle in contractions mimicking locomotion: effects of stimulus pattern on work, impulse and energetic cost and efficiency.

Active muscle performs various mechanical functions during locomotion: work output during shortening, work absorption when resisting (but not preventing) lengthening, and impulse (force-time integral) whenever there is active force. The energetic costs of these functions are important components in the energy budget during locomotion. We investigated how the pattern of stimulation and movement affects the mechanics and energetics of muscle fibre bundles isolated from wild rabbits (Oryctolagus cuniculus). The fibres were from muscles consisting of mainly fast-twitch, type 2 fibres. Fibre length was held constant (isometric) or a sinusoidal pattern of movement was imposed at a frequency similar to the stride frequency of running wild rabbits. Duty cycle (stimulation duration×movement frequency) and phase (timing of stimulation relative to movement) were varied. Work and impulse were measured as well as energy produced as heat. The sum of net work (work output-work input) and heat was taken as a measure of energetic cost. Maximum work output was produced with a long duty cycle and stimulation starting slightly before shortening, and was produced quite efficiently. However, efficiency was even higher with other stimulation patterns that produced less work. The highest impulse (considerably higher than isometric impulse) was produced when stimulation started while the muscle fibres were being lengthened. High impulse was produced very economically because of the low cost of producing force during lengthening. Thus, locomotion demanding high work, high impulse or economical work output or impulse requires a distinct pattern of stimulation and movement.

Biomechanics of predator-prey arms race in lion, zebra, cheetah and impala.

The fastest and most manoeuvrable terrestrial animals are found in savannah habitats, where predators chase and capture running prey. Hunt outcome and success rate are critical to survival, so both predator and prey should evolve to be faster and/or more manoeuvrable. Here we compare locomotor characteristics in two pursuit predator-prey pairs, lion-zebra and cheetah-impala, in their natural savannah habitat in Botswana. We show that although cheetahs and impalas were universally more athletic than lions and zebras in terms of speed, acceleration and turning, within each predator-prey pair, the predators had 20% higher muscle fibre power than prey, 37% greater acceleration and 72% greater deceleration capacity than their prey. We simulated hunt dynamics with these data and showed that hunts at lower speeds enable prey to use their maximum manoeuvring capacity and favour prey survival, and that the predator needs to be more athletic than its prey to sustain a viable success rate.

The effect of minimalist footwear and instruction on running: an observational study.

BACKGROUND: It is not known whether the effects on altered running style which are attributed to minimalist footwear can be achieved by verbal instructions in standard running shoes (SRS). AIM: To explore the effect of Vibram FiveFingers (VFF) versus SRS plus running instruction on lower extremity spatiotemporal parameters and lower limb joint kinematics. METHODS: 35 healthy subjects (mean=30 years, 18 females) were assessed on two occasions with 3D motion analysis. At each session subjects ran on a treadmill (3.58 m/s) for 2 min in either VFF or SRS (randomised order); with and without running instruction. Differences between spatiotemporal parameters and lower limb joint kinematics between conditions were assessed using a 2x2 repeated-measures ANOVA. RESULTS: Wearing VFF significantly increased cadence (p<0.001) and reduced stride length (p<0.01). Prior to initial contact, both instruction and VFF significantly increased foot (p<0.001 and p=0.02, respectively) and ankle (p<0.001 and p=0.02, respectively) plantarflexion, while wearing VFF significantly increased knee extension (p=0.04). At initial contact, instruction significantly increased knee flexion (p=0.04), and foot (p=0.001) and ankle (p=0.03) plantarflexion. At mid-stance and toe-off, instruction significantly increased knee flexion (p=0.048 and p<0.001, respectively) and foot plantarflexion (p<0.001 and p=0.01, respectively). Instruction had a greater effect on increasing knee flexion (p=0.007) and plantarflexion angle (p<0.001) when subjects wore SRS and VFF, respectively. CONCLUSION: Alterations in spatiotemporal parameters observed when running in VFF are likely to be attributable to the minimalist footwear. However, the kinematic adaptations observed following instruction suggests that changes in joint angles previously attributed to minimalist footwear alone may be similarly achieved with instruction.

Does 'Kinesio tape' alter thoracolumbar fascia movement during lumbar flexion? An observational laboratory study.

BACKGROUND: Changes in thoracolumbar fascial thickness, structure and shear strain are associated with lower back pain (LBP). Therapeutic taping techniques such as Kinesio-Taping (KT) are increasingly used to treat LBP, albeit with variable effects and unclear mechanisms. However, evidence for quantifying how treatment effects in vivo fascia properties is inadequate. We therefore aimed to explore taping mechanisms using an in vivo ultrasound measurement. METHODS: Thoracolumbar ultrasound videos of known orientations and positions were taken from 12 asymptomatic participants (8 males and 4 females, aged 22.9 ± 3.59) while performing velocity-guided lumbar flexion with and without KT applied. An automated algorithm using cross-correlation to track contiguous tissue layers across sequential frames in the sagittal plane, was developed and applied to two movements of each subject in each taping condition. Differences of inter-tissue movements and paracutaneous translation at tissue boundaries were compared. RESULTS: Significant reduction in the mean movement of subcutaneous tissue during lumbar flexion before and after taping was found. There was no difference in other observed tissue layers. Tissue paracutaneous translations at three boundaries were significantly reduced during lumbar flexion when KT was applied (skin-subcutaneous: 0.25 mm, p < 0.01; subcutaneous-perimuscular tissue: 0.5 mm, p = 0.02; and perimuscular-muscle: 0.46, p = 0.05). No overall reduction in lumbar flexion was found (p = 0.10). CONCLUSIONS: KT reduced subcutaneous inter-tissue movement and paracutaneous translation in the superficial thoracolumbar fascia during lumbar flexion, and the relationship of such difference to symptomatic change merits exploration. Combining ultrasound data with muscle activation information may be useful to reveal potential mechanisms of therapeutic taping in patients with LBP.

Skinned fibres produce the same power and force as intact fibre bundles from muscle of wild rabbits.

Skinned fibres have advantages for comparing the muscle properties of different animal species because they can be prepared from a needle biopsy taken under field conditions. However, it is not clear how well the contractile properties of skinned fibres reflect the properties of the muscle fibres in vivo. Here, we compare the mechanical performance of intact fibre bundles and skinned fibres from muscle of the same animals. This is the first such direct comparison. Maximum power and isometric force were measured at 25 °C using peroneus longus (PL) and extensor digiti-V (ED-V) muscles from wild rabbits (Oryctolagus cuniculus). More than 90% of the fibres in these muscles are fast-twitch, type 2 fibres. Maximum power was measured in force-clamp experiments. We show that maximum power per volume was the same in intact (121.3 ± 16.1 W l(-1), mean ± s.e.m.; N=16) and skinned (122.6 ± 4.6 W l(-1); N=141) fibres. Maximum relative power (power/F(IM) Lo, where F(IM) is maximum isometric force and Lo is standard fibre length) was also similar in intact (0.645 ± 0.037; N=16) and skinned (0.589 ± 0.019; N=141) fibres. Relative power is independent of volume and thus not subject to errors in measurement of volume. Finally, maximum isometric force per cross-sectional area was also found to be the same for intact and skinned fibres (181.9 kPa ± 19.1; N=16; 207.8 kPa ± 4.8; N=141, respectively). These results contrast with previous measurements of performance at lower temperatures where skinned fibres produce much less power than intact fibres from both mammals and non-mammalian species.

Eccentric and concentric loading of the triceps surae: an in vivo study of dynamic muscle and tendon biomechanical parameters.

Triceps surae eccentric exercise is more effective than concentric exercise for treating Achilles tendinopathy, however the mechanisms underpinning these effects are unclear. This study compared the biomechanical characteristics of eccentric and concentric exercises to identify differences in the tendon load response. Eleven healthy volunteers performed eccentric and concentric exercises on a force plate, with ultrasonography, motion tracking, and EMG applied to measure Achilles tendon force, lower limb movement, and leg muscle activation. Tendon length was ultrasonographically tracked and quantified using a novel algorithm. The Fourier transform of the ground reaction force was also calculated to investigate for tremor, or perturbations. Tendon stiffness and extension did not vary between exercise types (P = .43). However, tendon perturbations were significantly higher during eccentric than concentric exercises (25%-40% higher, P = .02). Furthermore, perturbations during eccentric exercises were found to be negatively correlated with the tendon stiffness (R2 = .59). The particular efficacy of eccentric exercise does not appear to result from variation in tendon stiffness or extension within a given session. However, varied perturbation magnitude may have a role in mediating the observed clinical effects. This property is subject-specific, with the source and clinical time-course of such perturbations requiring further research.

The immediate effects of foot orthoses on hip and knee kinematics and muscle activity during a functional step-up task in individuals with patellofemoral pain.

BACKGROUND: Evidence shows that anti-pronating foot orthoses improve patellofemoral pain, but there is a paucity of evidence concerning mechanisms. We investigated the immediate effects of prefabricated foot orthoses on (i) hip and knee kinematics; (ii) electromyography variables of vastus medialis oblique, vastus lateralis and gluteus medius during a functional step-up task, and (iii) associated clinical measures. METHODS: Hip muscle activity and kinematics were measured during a step-up task with and without an anti-pronating foot orthoses, in people (n=20, 9 M, 11 F) with patellofemoral pain. Additionally, we measured knee function, foot posture index, isometric hip abductor and knee extensor strength and weight-bearing ankle dorsiflexion. FINDINGS: Reduced hip adduction (0.82°, P=0.01), knee internal rotation (0.46°, P=0.03), and decreased gluteus medius peak amplitude (0.9mV, P=0.043) were observed after ground contact in the 'with orthoses' condition. With the addition of orthoses, a more pronated foot posture correlated with earlier vastus medialis oblique onset (r=-0.51, P=0.02) whilst higher Kujala scores correlated with earlier gluteus medius onset (r=0.52, P=0.02). INTERPRETATION: Although small in magnitude, reductions in hip adduction, knee internal rotation and gluteus medius amplitude observed immediately following orthoses application during a task that commonly aggravates symptoms, offer a potential mechanism for their effectiveness in patellofemoral pain management. Given the potential for cumulative effects of weight bearing repetitions completed with a foot orthoses, for example during repeated stair ascent, the differences are likely to be clinically meaningful.

Gluteal muscle activation during the isometric phase of squatting exercises with and without a Swiss ball.

OBJECTIVES: Growing evidence supports hip muscle activation and strengthening exercise prescription to prevent and treat various lower limb injuries. Common prescriptions include single-legged and double-legged squatting, with and without a Swiss ball. We aimed to establish the effect of varying forms of squatting exercises on gluteal muscle activation. DESIGN AND SETTING: Observational laboratory study. PARTICIPANTS: Nineteen (11 male) healthy participants (28.4 +/- 2.7 years old) were compared using one-way repeated measures analysis of variance. MAIN OUTCOME MEASURES: Surface electromyography (EMG) measures of gluteus medius (GMed) and gluteus maximus (GMax) during the isometric phase of single-legged and double-legged squatting, with and without a Swiss ball. RESULTS: A greater percentage of maximal voluntary contraction (%MVC) during single-legged squatting was found compared to double-legged squatting for GMed (42 versus 9%MVC, p < 0.001) and GMax (35 versus 14%MVC, p < 0.001). Additionally, the Swiss ball increased GMax activity (42 versus 35%MVC, p = 0.026) and demonstrated a trend toward increased GMed activity (46 versus 42%MVC, p = 0.075) during the single-legged squat. CONCLUSIONS: These results indicate single-legged squatting may be more appropriate than double-legged squatting to facilitate strength gains of GMed and GMax. Additionally, the Swiss ball may be a useful adjunct to target gluteal muscle strengthening during single-legged squatting.

The effect of anti-pronation foot orthoses on hip and knee kinematics and muscle activity during a functional step-up task in healthy individuals: a laboratory study.

BACKGROUND: Greater frontal and transverse plane hip and knee motion, and delayed gluteus medius and vastus medialis oblique activation have frequently been identified in patellofemoral pain syndrome populations, whilst prefabricated anti-pronation foot orthoses have been reported to reduce symptoms. The aim of the study was to evaluate the effects of such orthoses on hip and knee kinematics, gluteal and vasti muscle activity, kinematic and electromyographic interactions alongside correlations with specific clinical measures. METHODS: Eighteen asymptomatic individuals (11 male 7 female) had measures taken of static foot posture and ankle range of motion. Hip muscle activity and kinematics were measured using electromyography and an active motion capture system during a step-up task. Order of testing with or without orthoses was determined using a coin toss. FINDINGS: Between condition paired t-tests indicated significantly reduced peak hip adduction angles (1.56°, P < 0.05) and significantly reduced knee internal rotation (1.3°, P < 0.05) in the orthoses condition. Reduced ankle dorsiflexion range of motion correlated with a reduction in hip adduction following the orthoses intervention (r = 0.59, P = 0.013). INTERPRETATION: The effects of prefabricated orthoses may be partially explained by kinematic alterations that occur proximal to the foot in the kinetic chain. These clinically and biomechanically relevant effects appear more evident in those with reduced underlying ankle motion. Further research is indicated using a symptomatic population to explore the clinical relevance of these observations.

The effect of neutral-cushioned running shoes on the intra-articular force in the haemophilic ankle.

BACKGROUND: The ankle continues to be one of the most affected joints in the haemophilia patient, and as cartilage damage progresses, the joint can feel unstable, painful and stiff. Anecdotally, patients often report that sports trainers can improve their pain and daily function, however the actual mechanism for this remains unclear. METHODS: Nine patients with ankle haemarthropathy and three controls were examined using 'CODAmotion' analysis and a force plate. Kinematic and kinetic variables of the hip, knee and ankle were recorded. Data was imported from CODA to Excel, where a programme using 2D modelling of the ankle joint forces was employed. This calculated intra-articular force from heel strike to toe-off. FINDINGS: The haemophilia group at midstance showed an increase in intra-articular force in the ankle when wearing the trainer compared to the shoe (P=<0.05). Overall the haemophilia cohort had an increased joint force in both the trainers and shoes, compared to controls. INTERPRETATION: The type of footwear worn by individuals with ankle arthropathy has a significant effect on the amount of force acting at the joint surface. Sports shoes, in providing better comfort and foot support, may facilitate an increased muscular activity around the ankle and therefore improved dynamic joint stability, accounting for why some patients with ankle arthropathy report less pain. Further research is needed to establish levels of acceptable force and the combined effects of orthotics and footwear.

Mechanical and energetic properties of papillary muscle from ACTC E99K transgenic mouse models of hypertrophic cardiomyopathy.

We compared the contractile performance of papillary muscle from a mouse model of hypertrophic cardiomyopathy [α-cardiac actin (ACTC) E99K mutation] with nontransgenic (non-TG) littermates. In isometric twitches, ACTC E99K papillary muscle produced three to four times greater force than non-TG muscle under the same conditions independent of stimulation frequency and temperature, whereas maximum isometric force in myofibrils from these muscles was not significantly different. ACTC E99K muscle relaxed slower than non-TG muscle in both papillary muscle (1.4×) and myofibrils (1.7×), whereas the rate of force development after stimulation was the same as non-TG muscle for both electrical stimulation in intact muscle and after a Ca²âº jump in myofibrils. The EC50 for Ca²âº activation of force in myofibrils was 0.39 ± 0.33 µmol/l in ACTC E99K myofibrils and 0.80 ± 0.11 µmol/l in non-TG myofibrils. There were no significant differences in the amplitude and time course of the Ca²âº transient in myocytes from ACTC E99K and non-TG mice. We conclude that hypercontractility is caused by higher myofibrillar Ca²âº sensitivity in ACTC E99K muscles. Measurement of the energy (work + heat) released in actively cycling heart muscle showed that for both genotypes, the amount of energy turnover increased with work done but with decreasing efficiency as energy turnover increased. Thus, ACTC E99K mouse heart muscle produced on average 3.3-fold more work than non-TG muscle, and the cost in terms of energy turnover was disproportionately higher than in non-TG muscles. Efficiency for ACTC E99K muscle was in the range of 11-16% and for non-TG muscle was 15-18%.

Power output of skinned skeletal muscle fibres from the cheetah (Acinonyx jubatus).

Muscle samples were taken from the gluteus, semitendinosus and longissimus muscles of a captive cheetah immediately after euthanasia. Fibres were 'skinned' to remove all membranes, leaving the contractile filament array intact and functional. Segments of skinned fibres from these cheetah muscles and from rabbit psoas muscle were activated at 20°C by a temperature-jump protocol. Step and ramp length changes were imposed after active stress had developed. The stiffness of the non-contractile ends of the fibres (series elastic component) was measured at two different stress values in each fibre; stiffness was strongly dependent on stress. Using these stiffness values, the speed of shortening of the contractile component was evaluated, and hence the power it was producing. Fibres were analysed for myosin heavy chain content using gel electrophoresis, and identified as either slow (type I) or fast (type II). The power output of cheetah type II fibre segments was 92.5±4.3 W kg(-1) (mean ± s.e., 14 fibres) during shortening at relative stress 0.15 (the stress during shortening/isometric stress). For rabbit psoas fibre segments (presumably type IIX) the corresponding value was significantly higher (P<0.001), 119.7±6.2 W kg(-1) (mean ± s.e., 7 fibres). These values are our best estimates of the maximum power output under the conditions used here. Thus, the contractile filament power from cheetah was less than that of rabbit when maximally activated at 20°C, and does not account for the superior locomotor performance of the cheetah.

Coronal plane hip muscle activation in football code athletes with chronic adductor groin strain injury during standing hip flexion.

BACKGROUND: Groin pain arising from adductor muscle injury is common amongst football code athletes and can result in significant time lost from sporting participation. The associated motor control deficits are not well understood. AIMS: The aim of this study was to better understand the coronal plane muscle activation patterns associated with chronic adductor injury. METHODOLOGY: Measures of muscle activation at various stages of the standing hip flexion manoeuvre were made with surface electromyography and motion capture in 9 male football code subjects with chronic adductor injury, and 9 matched controls. RESULTS: The gluteus medius to adductor longus activation ratio was significantly reduced in subjects with groin pain when the injured leg was either moving (F = 64.3, p < 0.001) or in stance phase (F = 32.4, p < 0.001) when compared to activity-matched uninjured subjects, equating to a difference varying between 20 and 40% depending on phase of movement. These differences were particularly due to decreased abductor muscle activation. No significant differences between the uninjured and injured side of patients was found. CONCLUSION: Football code athletes with groin pain exhibit significantly altered coronal plane muscle activation with comparison to uninjured subjects. These findings need to be taken into account when planning rehabilitation for these athletes.

Effects of whole body vibration on motor unit recruitment and threshold.

Whole body vibration (WBV) has been suggested to elicit reflex muscle contractions but this has never been verified. We recorded from 32 single motor units (MU) in the vastus lateralis of 7 healthy subjects (34 ± 15.4 yr) during five 1-min bouts of WBV (30 Hz, 3 mm peak to peak), and the vibration waveform was also recorded. Recruitment thresholds were recorded from 38 MUs before and after WBV. The phase angle distribution of all MUs during WBV was nonuniform (P < 0.001) and displayed a prominent peak phase angle of firing. There was a strong linear relationship (r = -0.68, P < 0.001) between the change in recruitment threshold after WBV and average recruitment threshold; the lowest threshold MUs increased recruitment threshold (P = 0.008) while reductions were observed in the higher threshold units (P = 0.031). We investigated one possible cause of changed thresholds. Presynaptic inhibition in the soleus was measured in 8 healthy subjects (29 ± 4.6 yr). A total of 30 H-reflexes (stimulation intensity 30% Mmax) were recorded before and after WBV: 15 conditioned by prior stimulation (60 ms) of the antagonist and 15 unconditioned. There were no significant changes in the relationship between the conditioned and unconditioned responses. The consistent phase angle at which each MU fired during WBV indicates the presence of reflex muscle activity similar to the tonic vibration reflex. The varying response in high- and low-threshold MUs may be due to the different contributions of the mono- and polysynaptic pathways but not presynaptic inhibition.

Millisecond-scale biochemical response to change in strain.

Muscle fiber contraction involves the cyclical interaction of myosin cross-bridges with actin filaments, linked to hydrolysis of ATP that provides the required energy. We show here the relationship between cross-bridge states, force generation, and Pi release during ramp stretches of active mammalian skeletal muscle fibers at 20°C. The results show that force and Pi release respond quickly to the application of stretch: force rises rapidly, whereas the rate of Pi release decreases abruptly and remains low for the duration of the stretch. These measurements show that biochemical change on the millisecond timescale accompanies the mechanical and structural responses in active muscle fibers. A cross-bridge model is used to simulate the effect of stretch on the distribution of actomyosin cross-bridges, force, and Pi release, with explicit inclusion of ATP, ADP, and Pi in the biochemical states and length-dependence of transitions. In the simulation, stretch causes rapid detachment and reattachment of cross-bridges without release of Pi or ATP hydrolysis.

The effect of eccentric and concentric calf muscle training on Achilles tendon stiffness.

OBJECTIVE: To compare in vivo effects of eccentric and concentric calf muscle training on Achilles tendon stiffness, in subjects without tendinopathy. METHODS: Thirty-eight recreational athletes completed 6 weeks eccentric (6 males, 13 females, 21.6 ± 2.2 years) or concentric training (8 males, 11 females, 21.1 ± 2.0 years). Achilles tendon stiffness, tendon modulus and single-leg jump height were measured before and after intervention. Exercise adherence was recorded using a diary. RESULTS: All data are reported as mean ± SD. Groups were matched for height and weight but the eccentric training group were more active at baseline (P < 0.05). Tendon stiffness was higher in the eccentrically trained group at baseline compared to the concentrically trained group (20.9 ± 7.3 N/mm v 13.38 ± 4.66 N/mm; P = 0.001) and decreased significantly after eccentric training (to 17.2 ( ± 5.9) N/mm (P = 0.035)). There was no stiffness change in the concentric group (P = 0.405). Stiffness modulus showed similar changes to stiffness. An inverse correlation was found between initial, and subsequent, reduction in stiffness (r = -0.66). Jump height did not change and no correlation between stiffness change and adherence was observed in either group (r = 0.01). CONCLUSIONS: Six weeks of eccentric training can alter Achilles tendon stiffness while a matched concentric programme shows no similar effects. Studies in patients with Achilles tendinopathy are warranted.

Muscle activity and acceleration during whole body vibration: effect of frequency and amplitude.

BACKGROUND: Whole body vibration may improve muscle and bone strength, power and balance although contradictory findings have been reported. Prolonged exposure may result in adverse effects. We investigated the effects of high (5.5 mm) and low (2.5mm) amplitude whole body vibration at various frequencies (5-30 Hz) on muscle activity and acceleration throughout the body. METHODS: Surface electromyographic activity was recorded from 6 leg muscles in 12 healthy adults (aged 31.3 (SD 12.4) years). The average rectified acceleration of the toe, ankle, knee, hip and head was recorded from 15 healthy adults (36 (SD 12.1) years) using 3D motion analysis. FINDINGS: Whole body vibration increased muscle activity 5-50% of maximal voluntary contraction with the greatest increase in the lower leg. Activity was greater with high amplitude at all frequencies, however this was not always significant (P<0.05-0.001). Activation tended to increase linearly with frequency in all muscles except gluteus maximus and biceps femoris. Accelerations throughout the body ranged from approximately 0.2 to 9 g and decreased with distance from the platform. Acceleration at the head was always < 0.33 g. The greatest acceleration of the knee and hip occurred at approximately 15 Hz and thereafter decreased with increasing frequency. INTERPRETATION: Above the knee at frequencies > 15 Hz acceleration decreased with distance from the platform. This was associated with increased muscle activity, presumably due to postural control and muscle tuning mechanisms. The minimal acceleration at the head reduces the likelihood of adverse reactions. The levels of activation are unlikely to cause hypertrophy in young healthy individuals but may be sufficient in weak and frail people.