The biomechanics of running in athletes with previous hamstring injury: A case-control study.

Hamstring injury is prevalent with persistently high reinjury rates. We aim to inform hamstring rehabilitation by exploring the electromyographic and kinematic characteristics of running in athletes with previous hamstring injury. Nine elite male Gaelic games athletes who had returned to sport after hamstring injury and eight closely matched controls sprinted while lower limb kinematics and muscle activity of the previously injured biceps femoris, bilateral gluteus maximus, lumbar erector spinae, rectus femoris, and external oblique were recorded. Intergroup comparisons of muscle activation ratios and kinematics were performed. Previously injured athletes demonstrated significantly reduced biceps femoris muscle activation ratios with respect to ipsilateral gluteus maximus (maximum difference -12.5%, P = 0.03), ipsilateral erector spinae (maximum difference -12.5%, P = 0.01), ipsilateral external oblique (maximum difference -23%, P = 0.01), and contralateral rectus femoris (maximum difference -22%, P = 0.02) in the late swing phase. We also detected sagittal asymmetry in hip flexion (maximum 8°, P = 0.01), pelvic tilt (maximum 4°, P = 0.02), and medial rotation of the knee (maximum 6°, P = 0.03) effectively putting the hamstrings in a lengthened position just before heel strike. Previous hamstring injury is associated with altered biceps femoris associated muscle activity and potentially injurious kinematics. These deficits should be considered and addressed during rehabilitation.

Mechanics of myosin function in white muscle fibres of the dogfish, Scyliorhinus canicula.

The contractile properties of muscle fibres have been extensively investigated by fast perturbation in sarcomere length to define the mechanical characteristics of myofilaments and myosin heads that underpin refined models of the acto-myosin cycle. Comparison of published data from intact fast-twitch fibres of frog muscle and demembranated fibres from fast muscle of rabbit shows that stiffness of the rabbit myosin head is only ∼62% of that in frog. To clarify if and how much the mechanical characteristics of the filaments and myosin heads vary in muscles of different animals we apply the same high resolution mechanical methods, in combination with X-ray diffraction, to fast-twitch fibres from the dogfish (Scyliorhinus canicula). The values of equivalent filament compliance (C(f)) measured by X-ray diffraction and in mechanical experiments are not significantly different; the best estimate from combining these values is 17.1 ± 1.0 nm MPa(−1). This value is larger than Cf in frog, 13.0 ± 0.4 nm MPa(−1). The longer thin filaments in dogfish account for only part of this difference. The average isometric force exerted by each attached myosin head at 5°C, 4.5 pN, and the maximum sliding distance accounted for by the myosin working stroke, 11 nm, are similar to those in frog, while the average myosin head stiffness of dogfish (1.98 ± 0.31 pN nm(−1)) is smaller than that of frog (2.78 ± 0.30 pN nm(−1)). Taken together these results indicate that the working stroke responsible for the generation of isometric force is a larger fraction of the total myosin head working stroke in the dogfish than in the frog.

Menopause alters temperature sensitivity of muscle force in humans.

Isometric maximum voluntary force (MVF) of the adductor pollicis and first dorsal interosseous muscles was measured in 11 pre- and 11 post-menopausal (Pre-M and Post-M) human subjects. The temperature of the hand varied in the range 18°-38°C by water immersion and skin temperature was recorded. MVF at each temperature was expressed relative to the value at skin temperature above 35°C to give MVF(REL). The form of the relation between MVF(REL) and temperature was different in the Pre-M and Post-M groups (p < 0.01). In the Pre-M group the maximum value of MVF(REL) occurred near 30°C and force fell at both higher and lower temperatures. In the Post-M group MVF(REL) showed an approximately linear decline with cooling across the whole temperature range. The maximum value of MVF(REL) for the Post-M group was near 35°C. The values of MVF(REL) for the Post-M group were significantly lower than for the Pre-M group at temperatures between 18° and 30°C.

Is the efficiency of mammalian (mouse) skeletal muscle temperature dependent?

Myosin crossbridges in muscle convert chemical energy into mechanical energy. Reported values for crossbridge efficiency in human muscles are high compared to values measured in vitro using muscles of other mammalian species. Most in vitro muscle experiments have been performed at temperatures lower than mammalian physiological temperature, raising the possibility that human efficiency values are higher than those of isolated preparations because efficiency is temperature dependent. The aim of this study was to determine the effect of temperature on the efficiency of isolated mammalian (mouse) muscle. Measurements were made of the power output and heat production of bundles of muscle fibres from the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles during isovelocity shortening. Mechanical efficiency was defined as the ratio of power output to rate of enthalpy output, where rate of enthalpy output was the sum of the power output and rate of heat output. Experiments were performed at 20, 25 and 30◦C. Maximum efficiency of EDL muscles was independent of temperature; the highest value was 0.31}0.01 (n =5) at 30◦C. Maximum efficiency of soleus preparations was slightly but significantly higher at 25 and 30◦C than at 20◦C; the maximum mean value was 0.48±0.02 (n =7) at 25◦C. It was concluded that maximum mechanical efficiency of isolated mouse muscle was little affected by temperature between 20 and 30◦C and that it is unlikely that differences in temperature account for the relatively high efficiency of human muscle in vivo compared to isolated mammalian muscles.

Sustained performance by red and white muscle fibres from the dogfish Scyliorhinus canicula.

The mechanical performance of red and white muscle fibres from dogfish was compared during a long series of contractions with sinusoidal movement or under isometric conditions at 12 degrees C (normal in vivo temperature). Power output was measured during sinusoidal movement at 0.75 Hz and peak-to-peak amplitude about 12% L(0). Tetanus duty cycle was 33% (0.44 s) at phase -8% (first stimulus at 0.107 s before shortening started). Initially, the red fibres produced only about one third as much power as the white fibres, 6.57+/-0.63 W kg(-1) wet mass (mean +/- s.e.m.) and 18.3+/-2.3, respectively. Red fibres were better at sustaining power output; it declined rapidly to about 60% of its initial value and then remained relatively steady for up to 450 cycles of movement. Force during shortening declined, but force during stretch did not increase: force always relaxed to a low value before stretch started. By contrast, net power output by white fibres declined rapidly to zero within about 50 cycles. Two changes contributed: decline in force during shortening and an increase in force during stretch because relaxation became progressively less complete during the series of contractions. In isometric series (0.44 s stimulation every 1.33 s, cycle frequency 0.75 Hz), red and white fibres sustained peak isometric force similarly; in the 50th cycle force was 59+/-3% and 56+/-4% of initial values. The time required for force to relax to 10% of its maximum value decreased during the series for red fibres and increased for white fibres.

Effect of phosphate and temperature on force exerted by white muscle fibres from dogfish.

Effects of Pi (inorganic phosphate) are relevant to the in vivo function of muscle because Pi is one of the products of ATP hydrolysis by actomyosin and by the sarcoplasmic reticulum Ca(2+) pump. We have measured the Pi sensitivity of force produced by permeabilized muscle fibres from dogfish (Scyliorhinus canicula) and rabbit. The activation conditions for dogfish fibres were crucial: fibres activated from the relaxed state at 5, 12, and 20 degrees C were sensitive to Pi, whereas fibres activated from rigor at 12 degrees C were insensitive to Pi in the range 5-25 mmol l(-1). Rabbit fibres activated from rigor were sensitive to Pi. Pi sensitivity of force produced by dogfish fibres activated from the relaxed state was greater below normal body temperature (12 degrees C for dogfish) in agreement with what is known for other species. The force-temperature relationship for dogfish fibres (intact and permeabilized fibres activated from relaxed) showed that at 12 degrees C, normal body temperature, the force was near to its maximum value.

Inferring crossbridge properties from skeletal muscle energetics.

Work is generated in muscle by myosin crossbridges during their interaction with the actin filament. The energy from which the work is produced is the free energy change of ATP hydrolysis and efficiency quantifies the fraction of the energy supplied that is converted into work. The purpose of this review is to compare the efficiency of frog skeletal muscle determined from measurements of work output and either heat production or chemical breakdown with the work produced per crossbridge cycle predicted on the basis of the mechanical responses of contracting muscle to rapid length perturbations. We review the literature to establish the likely maximum crossbridge efficiency for frog skeletal muscle (0.4) and, using this value, calculate the maximum work a crossbridge can perform in a single attachment to actin (33 x 10(-21) J). To see whether this amount of work is consistent with our understanding of crossbridge mechanics, we examine measurements of the force responses of frog muscle to fast length perturbations and, taking account of filament compliance, determine the crossbridge force-extension relationship and the velocity dependences of the fraction of crossbridges attached and average crossbridge strain. These data are used in combination with a Huxley-Simmons-type model of the thermodynamics of the attached crossbridge to determine whether this type of model can adequately account for the observed muscle efficiency. Although it is apparent that there are still deficiencies in our understanding of how to accurately model some aspects of ensemble crossbridge behaviour, this comparison shows that crossbridge energetics are consistent with known crossbridge properties.

Temperature change as a probe of muscle crossbridge kinetics: a review and discussion.

Following the ideas introduced by Huxley (Huxley 1957, Prog. Biophys. Biophys. Chem. 7, 255-318), it is generally supposed that muscle contraction is produced by temporary links, called crossbridges, between myosin and actin filaments, which form and break in a cyclic process driven by ATP splitting. Here we consider the interaction of the energy in the crossbridge, in its various states, and the force exerted. We discuss experiments in which the mechanical state of the crossbridge is changed by imposed movement and the energetic consequence observed as heat output and the converse experiments in which the energy content is changed by altering temperature and the mechanical consequences are observed. The thermodynamic relationship between the experiments is explained and, at the first sight, the relationship between the results of these two types of experiment appears paradoxical. However, we describe here how both of them can be explained by a model in which mechanical and energetic changes in the crossbridges occur in separate steps in a branching cycle.

Effects of UCP3 genotype, temperature and muscle type on energy turnover of resting mouse skeletal muscle.

Uncoupling protein 3 (UCP3) is a mitochondrial transporter protein which, when over-expressed in mice, is associated with increased metabolic rate, increased feeding and low body weight. This phenotype probably reflects the increased levels of UCP3 partially uncoupling mitochondrial respiration from cellular ATP demands. Consistent with that, mitochondria isolated from muscles of mice that over-express UCP3 are less tightly coupled than those from wild-type mice but the degree of uncoupling is not modulated by likely physiological regulatory factors. To determine whether this also applies to intact muscle fibres, we tested the hypothesis that UCP3 constitutively (i.e. in an unregulated fashion) uncouples mitochondria in muscles from mice that over-expressed human UCP3 (OE mice). The rate of heat production of resting muscles was measured in vitro using bundles of fibres from soleus and extensor digitorum longus muscles of OE, wild-type (WT) and UCP3 knock-out mice. At 20 degrees C, the only significant effect of genotype was that the rate of heat production of OE soleus (3.04+/-0.16 mW g(-1)) was greater than for WT soleus (2.31+/-0.05 mW g(-1)). At physiological temperature (35 degrees C), the rate of heat production was independent of genotype and equal to the expected in vivo rate for skeletal muscles of WT mice. We conclude that at 35 degrees C, the transgenic UCP3 was not constitutively active, but at 20 degrees C in slow-twitch muscle, it was partially activated by unknown factors. The physiological factor(s) that activate mitochondrial uncoupling by UCP3 in vivo was either not present or inactive in resting isolated muscles.

Manual landmark identification and tracking during the medial rotation test of the shoulder: an accuracy study using three-dimensional ultrasound and motion analysis measures.

Palpation of movement is a common clinical tool for assessment of movement in patients with musculoskeletal symptoms. The purpose of this study was to measure the accuracy of palpation of shoulder girdle translation during the medial rotation test (MRT) of the shoulder. The translation of the gleno-humeral and scapulo-thoracic joints was measured using both three-dimensional ultrasound and palpation in order to determine the accuracy of translation tracking during the MRT of the shoulder. Two movements of 11 normal subjects (mean age 24 (SD=4), range 19-47 years) were measured. The agreement between measures was good for scapulo-thoracic translation (r=0.83). Gleno-humeral translation was systematically under estimated (p=0.03) although moderate correlation was found (r=0.65). These results indicate that translation of the measured joints can be tracked by palpation and further tests of the efficacy of palpation tracking during musculoskeletal assessment may be warranted.

Energy turnover for Ca2+ cycling in skeletal muscle.

The majority of energy consumed by contracting muscle can be accounted for by two ATP-dependent processes, cross-bridge cycling and Ca(2+) cycling. The energy for Ca(2+) cycling is necessary for contraction but is an overhead cost, energy that cannot be converted into mechanical work. Measurement of the energy used for Ca(2+) cycling also provides a means of determining the total Ca(2+) released from the sarcoplasmic reticulum into the sarcoplasm during a contraction. To make such a measurement requires a method to selectively inhibit cross-bridge cycling without altering Ca(2+) cycling. In this review, we provide a critical analysis of the methods used to partition skeletal muscle energy consumption between cross-bridge and non-cross-bridge processes and present a summary of data for a wide range of skeletal muscles. It is striking that the cost of Ca(2+) cycling is almost the same, 30-40% of the total cost of isometric contraction, for most muscles studied despite differences in muscle contractile properties, experimental conditions, techniques used to measure energy cost and to partition energy use and in absolute rates of energy use. This fraction increases with temperature for amphibian or fish muscle. Fewer data are available for mammalian muscle but most values are similar to those for amphibian muscle. For mammalian muscles there are no obvious effects of animal size, muscle fibre type or temperature.

Oestrogen status in relation to the early training responses in human thumb adductor muscles.

AIMS: The aims of this study were to identify the mechanisms for the early response to training in women of different oestrogen status and to determine whether any oestrogen and exercise effects on these would be additive. METHODS: We monitored training (ten 5-s contractions per day for 12 weeks)-induced changes in the size, strength, voluntary activation capacity and index of crossbridge force state (i.e. rapid stretch to isometric torque ratio), in the thumb adductor muscles of postmenopausal [eight who had never used, and 14 who were using, hormone replacement therapy (HRT)] and seven premenopausal eumenorrhoeic women. The contralateral untrained muscle was used as a control. RESULTS: There was a significant effect of oestrogen status on the magnitude of training-induced strength increment, with the non-HRT postmenopausal group exhibiting the greatest benefits (28 +/- 6%, P = 0.024) from training. There were no significant or commensurate changes in either cross-sectional area or voluntary activation capacity. The index of crossbridge force state improved most in the no-HRT group (19 +/- 7%, P < 0.05). CONCLUSIONS: Presence, rather than absence of oestrogen, is associated with relatively higher muscle function which limits the potential for any further training-induced increments in muscle performance, as would be expected if the muscle strengthening actions of training and oestrogen share a common, partially saturable physiological pathway. The mechanism that is involved in the early training-induced strength increment in the three differing oestrogen groups cannot be due to increased size or recruitment. It would appear instead that increased motor unit firing frequency is involved.

Activity in three parts of the quadriceps recorded isometrically at two different knee angles and during a functional exercise.

The purpose of this study was to evaluate individual differences in three parts of the quadriceps activated isometrically at 60 degrees and 90 degrees of knee flexion, and during a functional activity involving both concentric and eccentric muscle work. Surface EMG amplitudes were therefore recorded from oblique parts of vastus medialis (VMO) and vastus lateralis (VLO) and from rectus femoris (RF). VMO and VLO showed less activation at 60 degrees than at 90 degrees, but in RF there was no difference between the two angles. In the second experiment, where 11 subjects stepped on and off a stool; these amplitudes were compared with those from a maximal isometric voluntary contraction at 90 degrees of knee flexion. For VMO & VLO the normalised peak amplitude in stepping up was 1.41 +/- 0.12, & 1.46 +/- 0.15 respectively, showing that higher activity is necessary during concentric contractions. These two results suggest that the motor control of VMO/VLO may be different from the bulk of quadriceps. Our findings have implications for patellofemoral function.

Energy storage during stretch of active single fibres from frog skeletal muscle.

Heat production and force were measured during tetani of single muscle fibres from anterior tibialis of frog. During stimulation fibres were either kept under isometric conditions, or were stretched or allowed to shorten (at constant velocity) after isometric force had reached its plateau value. The energy change was evaluated as the sum of heat and work (work = integral of force with respect to length change). Net energy absorption occurred during stretch at velocities greater than about 0.35 L0 s-1 (L0 is fibre length at resting sarcomere length 2.10 microm). Heat produced by 1 mm segments of the fibre was measured simultaneously and separately; energy absorption is not an artefact due to patchy heat production. The maximum energy absorption, 0.092 +/- 0.002 P0L0 (mean +/- S.E.M., n = 8; where P0 is isometric force at L0), occurred during the fastest stretches (1.64 L0 s-1) and amounted to more than half of the work done on the fibre. Energy absorption occurred in two phases. The amount in the first phase, 0.027 +/- 0.003 P0L0 (n = 32), was independent of velocity beyond 0.18 L0 s-1. The quantity absorbed in the second phase increased with velocity and did not reach a limiting value in the range of velocities used. After stretch, energy was produced in excess of the isometric rate, probably from dissipation of the stored energy. About 34 % (0.031 P0L0/0.092 P0L0) of the maximum absorbed energy could be stored elastically (in crossbridges, tendons, thick, thin and titin filaments) and by redistribution of crossbridge states. The remaining energy could have been stored in stretching transverse, elastic connections between myofibrils.

Isometric and isovelocity contractile performance of red muscle fibres from the dogfish Scyliorhinus canicula.

Maximum isometric tetanic force produced by bundles of red muscle fibres from dogfish, Scyliorhinus canicula (L.), was 142.4+/-10.3 kN m(-2) (N=35 fibre bundles); this was significantly less than that produced by white fibres 289.2+/-8.4 kN m(-2) (N=25 fibre bundles) (means +/- S.E.M.). Part, but not all, of the difference is due to mitochondrial content. The maximum unloaded shortening velocity, 1.693+/-0.108 L(0) s(-1) (N=6 fibre bundles), was measured by the slack-test method. L(0) is the length giving maximum isometric force. The force/velocity relationship was investigated using a step-and-ramp protocol in seven red fibre bundles. The following equation was fitted to the data: [(P/P(0))+(a/P(0))](V+b)=[(P(0)(*)/P(0))+(a/P(0))]b, where P is force during shortening at velocity V, P(0) is the isometric force before shortening, and a, b and P(0)(*) are fitted constants. The fitted values were P(0)(*)/P(0)=1.228+/-0.053, V(max)=1.814+/-0.071 L(0) s(-1), a/P(0)=0.269+/-0.024 and b=0.404+/-0.041 L(0) s(-1) (N=7 for all values). The maximum power was 0.107+/-0.005P(0)V(max) and was produced during shortening at 0.297+/-0.012V(max). Compared with white fibres from dogfish, the red fibres have a lower P(0) (49%) and V(max) (48%), but the shapes of the force/velocity curves are similar. Thus, the white and red fibres have equal capacities to produce power within the limits set by the isometric force and maximum velocity of shortening of each fibre type. A step shortening of 0.050+/-0.003L(0) (N=7) reduced the maximum isometric force in the red fibres' series elasticity to zero. The series elasticity includes all elastic structures acting in series with the attached cross-bridges. Three red fibre bundles were stretched at a constant velocity, and force (measured when length reached L(0)) was 1.519+/-0.032P(0). In the range of velocities used here, -0.28 to -0.63V(max), force varied little with the velocity.

A variable inertial system for measuring the contractile properties of human muscle.

PURPOSE: A flywheel system of variable inertia is described for inferring the mechanical properties of human muscle during a single explosive movement. METHODS: The system consists of a lightweight aluminum disk mounted on a shaft onto which a driving cog is mounted. The inertia of the system can be varied from 0.024 to 0.69 kg.m(2) by attaching semicircular steel plates to the disk. A rotary encoder detects displacement of the wheel with a resolution of 1 degrees. Digital signals from the encoder are collected using an A/D converter interfaced to a PC. The data are then processed for the calculation of torque, velocity, power, work done, and acceleration. The mechanical properties of the muscles employed are inferred from calculations of flywheel displacement, time, and force. In addition, a pretension release mechanism can be incorporated into the system to allow isometric force to be developed before movement. This can increase power generation at the low inertias where the time of contraction is typically less than 200 ms. Seven subjects were test-retested using the device. Measures of both average and peak power were made. RESULTS: When mounted in the apparatus described by Bassey and Short, the maximum values for peak and average power were on average 965 +/- 103 and 448 +/- 47 W, respectively. Upon retesting, these results were found to be reliable (cv = 3.3% and 3.0%, respectively). CONCLUSIONS: The inertial system described has been shown to have validity in reproducibility and provided a suitable method of determining a number of muscle output properties during short-term single exertions. This tool could prove useful in a research or clinical setting and may also prove useful as a training device as it negates the need for a strain gauge or goniometer attachment.