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.

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.

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.

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.

A study of force and cross-sectional area of adductor pollicis muscle in female hip fracture patients.

OBJECTIVES: To determine the extent of muscle weakness in older female hip fracture patients compared with healthy older and young women; to determine the extent to which this weakness is caused by a decline of the force produced per unit area of muscle rather than by a decline in muscle bulk; and to investigate the mechanism of the decline in force per unit area. DESIGN: This was an open study of three groups of subjects, two age matched older groups and one young group. SETTING: University College London, Royal Free Hospital, and St. Thomas's Hospital, London. PARTICIPANTS: Twenty-nine older female hip fracture patients (mean age 85.6 +/- 0.9 SEM), 18 healthy older women (mean age 84.7 +/- 1.2 SEM), and 43 young women (mean age 28.9 +/- 1.2 SEM). MEASUREMENTS: Adductor pollicis muscle maximum voluntary force (MVF) during isometric and pliometric contractions and cross-sectional area (CSA), body weight, height, and demi-span. RESULTS: Isometric MVF was lowest in the hip fracture group. In both older groups, isometric MVF and CSA were lower than in the young women. Only part of this weakness in the older groups could be explained by the smaller CSAs. The isometric force per unit area (MVF/CSA) was also lower in both older groups, the hip fracture patients again having the lowest values. Analysis of variance showed a significant difference between groups. The age-related declines in pliometric force were much less than the declines in isometric force. This resulted in an increase in the pliometric/isometric force ratio both for the hip fracture patients and for the healthy older women compared with that for young women. CONCLUSION: In comparison with the results from young women, the adductor pollicis muscles of female hip fracture patients were even weaker than those of healthy older women when normalized for muscle size. This decline in isometric MVF/CSA accounted for at least half of the overall weakness in the hip fracture patients. Inasmuch as pliometric force is maintained in situations where weakness is caused by a decline in the force produced per muscle cross-bridge, this is the likely mechanism of the declines in isometric MVF/CSA observed in this study.

Interpreting the relation between force and cross-sectional area in human muscle.

The maximum force a muscle can produce depends on its cross-sectional area (CSA). However, the exact interpretation of this relationship has been a matter of controversy. Recently, the controversy has centered on whether the measurements are best correlated using regression analysis or ratio standards. Applying regression analysis to this problem implies that all the experimental error is in the measurement of force. Thus, confusion may arise by failure to take account of errors in the measurement of CSA. Using a statistical model, we show how regression analysis can be misleading as error is introduced into the measurement of CSA as well as that of force. Because neither the errors in force nor CSA can be quantified in the experimental situation, we conclude that ratio standards are less likely to mislead although the accuracy of the result depends on the degree of correlation between force and CSA in the muscle measured.

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.

Muscle energetics during unfused tetanic contractions. Modelling the effects of series elasticity.

During an unfused tetanic contraction the contractile component stretches and then is stretched by the series elasticity in the muscle fibre during each tension oscillations. This causes the heat rate to increase, from increased metabolic rate, during the time when the contractile component is shortening. During the time when the contractile component is being stretched there is heat produced within the contractile component from dissipation of the work stored in the contractile component. A simulation is used to show that these effects are not negligible when the effects of shortening velocity on energy output rate is determined using unfused contraction. The overall effects resemble those that would be produced in a muscle if the effect of shortening velocity in accelerating the rate of cross-bridge cycling were reduced at low activation levels.

Power and efficiency: how to get the most out of striated muscle.

White muscle fibres from dogfish were stimulated during sinusoidal cycles of shortening and lengthening that mimic the in vivo pattern of movement. The results show that the timing of the stimulation relative to movement and also the frequency of the movement affect mechanical power output and efficiency. Maximum mechanical power is produced at a higher frequency of movement than maximum efficiency. The value of maximum power for a cycle of movement is less than half that produced during ramp shortening of fully active fibres. Higher efficiency is achieved during cyclic movement than during ramps, probably because work can be done with lower "overhead" costs for activation and because little ATP is used during lengthening.

The efficiency of energy conversion by swimming muscles of fish.

The efficiency of energy conversion by fish myotomal muscle undergoing sinusoidal length changes with brief periods of stimulation in each cycle has been found to be greater than the efficiency of the same type of muscle when tetanically stimulated. This finding is reviewed in relation to the well-known energetic properties of muscle undergoing shortening. It is suggested that an adaptation of the ideas of Lombardi & Piazzesi which were proposed to explain the behaviour of muscle during lengthening could be used to provide one possible explanation of this finding. Their theory proposes that crossbridges, when they have sufficient energy because they are strained, can detach to a crossbridge state which can rapidly reattach without having to split ATP first. This type of theory might also provide an explanation of the findings on ventricular energetics which are expressed in the time varying elastance model of Suga.

Shortening during stimulation vs. during relaxation. How do the costs compare?

White muscle fibres from dogfish were used to investigate the energetic cost of shortening by fully active muscle and by relaxing muscle. The muscle preparation was tetanized for 0.6 s and shortened by 1 mm (about 15% L0) at 7 mm/s (about 30% V0) either during stimulation or during relaxation. Isometric tetani at L0 were also investigated. Mechanical work was calculated from force and length change. Work + heat was taken as a measure of energetic cost. Both work and energetic cost were higher for shortening during stimulation than during relaxation. We also evaluated separately the work and heat associated with the contractile component and with the series elastic component. Work stored in the series elasticity could be completely recovered as external work when the shortening occurred during relaxation.

The energetics of work and heat production by single muscle fibres from the frog.

During active shortening the heat rate in isolated muscle fibres is greater than during isometric contraction, and increases with velocity of shortening (V), but at a decreasing rate as the maximum velocity (V0) is approached. For shortening at V less than 0.25 V0 the amount of extra heat produced during a period of shortening is proportional to the distance shortened, but for rapid shortening (V greater than 0.5 V0) the extra heat increases less than proportionally with the distance shortened. After a period of shortening the higher heat rate returns to the isometric level over a period of several hundred milliseconds. A similar period of increased heat rate is seen after a quick release. After shortening 10% of muscles slack length at 0.1 V0 the amount of heat produced during tension redevelopment is similar to that after a quick release. But after more rapid shortening there is less heat production than after a quick release.

Simultaneous heat and tension measurements from single muscle cells.

Simultaneous force and heat measurements were made in single cells from skeletal muscle of the frog during isometric twitches and tetani at 10 and 0 degree C. A Hill- Downing type thermopile of low heat capacity was used. In twitches, peak force development was found to be well correlated with heat production at both temperatures, during posttetanic twitch potentiation (at 10 degrees C) and during posttetanic twitch depression (at 0 degree C). In a twitch at 0 degree C, heat production started less than 14 msec after the stimulus had begun, before force development. As in whole muscle, the heat during a tetanus could be separated into two components: an early component produced at an exponentially decreasing rate, labile heat, and a steady rate, stable maintenance heat rate. Increasing temperature from 0 to 10 degrees C doubled the stable maintenance heat rate. At the higher temperature the time constant of labile heat production was halved and the quantity of labile heat decreased. When two tetani were given at 10 degrees C, a 5 min rest interval was required before the second tetanus produced the same force and heat as the first. At 0 degree C this interval was at least 10 min. With shorter intervals, both heat and force were depressed. At 10 degrees C both were depressed equally but at 0 degree C the effect on heat was greater than on force. At both temperatures labile heat was depressed to a greater extent than the stable maintenance heat rate. Results are interpreted in terms of possible calcium-parvalbumin interaction during a tetanus.

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.

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.