Unit Length Changes Research Articles

Correcting Postural Deviations: An Evidence- Supported Shift in Paradigm

Somatic syndromes of the spinal region may develop secondary to postural dysfunction. Traditionally, emphasis has been placed on correcting postural asymmetries by attempting to change muscle-tendon unit length. The effectiveness of this approach in providing prolonged muscle-tendon unit length changes and optimal postural outcomes has met with controversy. The time has come for a redirection of goals in conservative care for postural dysfunction of the spinal regions. An emphasis needs to be placed on continuous, self-awareness training of postural correction that will facilitate lasting neuromuscular adaptations. The purpose of this review is to rationalize the need to veer from exercises attempting to adjust the muscle-tendon unit length and recognize the evidence-supported impact of simple self-correction strategies meant to facilitate neuromuscular adjustments and correct postural asymmetries.

Muscle-tendon unit length changes in knee extensors and flexors during alpine skiing

ABSTRACT This study characterised the thigh muscle-tendon unit length variations (MTUv) in elite alpine skiing. MTUv were modelled for seven muscles from knee and hip angles in 17 national team skiers during 1602 cycles (546 Slalom (SL), 908 Giant-Slalom (GS), 136 Super-Giant (SG) and 12 Downhill (DH) cycles). The biarticular rectus femoris showed a dynamic pattern on both legs, especially in SL. On the other side, vastii displayed a stable length on the inside leg in all disciplines (37–46% of the cycle), contrasting with their dynamic behaviour on the outside leg being quasi-static for only 3% (SL) to 27% (DH) of the cycle. SL showed the largest amplitude of MTUv followed by GS, SG and DH. For vastus lateralis, MTUv was ~60%Lr.s−1 in SL but only ~12%Lr.s−1 in DH. In SL, a fine desynchronisation between both joints led to nearly constant MTUv (slow lengthening lasting ~45-51% of the cycle) for the biarticular hamstrings separated by faster variations during turn switch from outside to inside leg. In summary, biarticular MTUv were not characterised by single-joint behaviours, suggesting that extrapolating contraction regimen from knee joint only is not accurate in alpine skiing.

Muscle-tendon unit length changes differ between young and adult sprinters in the first stance phase of sprint running.

The aim of this study was to compare young and adult sprinters on several biomechanical parameters that were previously highlighted as performance-related and to determine the behaviour of several muscle–tendon units (MTU) in the first stance phase following a block start in sprint running. The ground reaction force (GRF) and kinematic data were collected from 16 adult and 21 young well-trained sprinters. No difference between the groups was found in some of the previously highlighted performance-related parameters (ankle joint stiffness, the range of dorsiflexion and plantar flexor moment). Interestingly, the young sprinters showed a greater maximal and mean ratio of horizontal to total GRF, which was mainly attributed to a greater horizontal GRF relative to body mass and resulted in a greater change in horizontal centre of mass (COM) velocity during the stance phase in the young compared with the adult sprinters. Results from the MTU length analyses showed that adult sprinters had more MTU shortening and higher maximal MTU shortening velocities in all plantar flexors and the rectus femoris. Although previously highlighted performance-related parameters could not explain the greater 100 m sprinting times in the adult sprinters, differences were found in the behaviour of the MTU of the plantar flexors and rectus femoris during the first stance phase. The pattern of length changes in these MTUs provides ideal conditions for the use of elastic energy storage and release for power enhancement.

Slower Walking Speed in Older Men Improves Triceps Surae Force Generation Ability.

Older adults walk slower than young adults, but it is not known why. Previous research suggests that ankle plantarflexors may have a crucial role in the reduction of walking speed. The purpose of this study was to investigate age-related differences in triceps surae muscle-tendon function during walking to further investigate the role of plantarflexors in the age-related reduction of walking speed. Medial gastrocnemius and soleus muscle fascicle lengths were measured using ultrasound imaging during walking from 13 young (25 ± 4 yr) men at preferred walking speed and from 13 older (73 ± 5 yr) men at preferred speed and at the young men's preferred speed. Muscle-tendon unit lengths were calculated from joint kinematics, and tendinous tissue lengths were calculated by subtracting muscle lengths from muscle-tendon unit lengths. In addition, ground reaction forces and electromyographic activity of medial gastrocnemius and soleus were measured. In both medial gastrocnemius and soleus, it was observed that at preferred walking speed, older men used a narrower muscle fascicle operating range and lower shortening velocity at the estimated time of triceps surae peak force generation compared with young men. Fascicles also accounted for a lower proportion of muscle-tendon unit length changes during the stance phase in older compared with young men. Significant differences in triceps surae muscle function were not observed between age groups when compared at matched walking speed. In older men, walking at preferred speed allows triceps surae muscles to generate force with more favorable shortening velocity and to enhance use of tendinous tissue elasticity compared with walking at young men's preferred speed. The results suggest that older men may prefer slower walking speeds to compensate for decreased plantarflexor strength.

Introduction of a team-based care model in a general medical unit.

BackgroundAlberta Health Services is a provincial health authority responsible for healthcare for more than four million people. The organization recognized a need to change its care delivery model to make care more patient- and family-centred and use its health human resources more effectively by enhancing collaborative practice. A new care model including changes to how providers deliver care and skill mix changes to support the new processes was piloted on a medical unit in a large urban acute care hospital Evidence-based care processes were introduced, including an initial patient assessment and orientation, comfort rounds, bedside shift reports, patient whiteboards, Name Occupation Duty, rapid rounds, and team huddles. Small teams of nurses cared for a portion of patients on the unit. The model was intended to enhance safety and quality of care by allowing providers to work to full scope in a collaborative practice environment.MethodsWe evaluated the new model approximately one year after implementation using interviews with staff (n = 15), surveys of staff (n = 25 at baseline and at the final evaluation) and patients (n = 26 at baseline and 37 at the final evaluation), and administrative data pulled from organizational databases.ResultsStaff interviews revealed that overall, the new care processes and care teams worked quite well. Unit culture and collaboration were improved, as were role clarity, scope of practice, and patient care. Responses from staff surveys were also very positive, showing significant positive changes in most areas. Patient satisfaction surveys showed a few positive changes; scores overall were very high. Administrative data showed slight decreases in overall length of stay, 30-day readmissions, staff absenteeism, staff vacancies, and the overtime rate. We found no changes in unit length of stay, 30-day returns to emergency department, or nursing sensitive adverse events.ConclusionsConclusions from the evaluation were positive, providing initial support for the idea of the collaborative practice model vision for adult medical units across Alberta. There were also a few positive effects on patient care suggesting that models such as this one could improve the organization’s ability to deliver sustainable, high-quality, patient- and family-centred care without compromising quality.

Passive Muscle-Tendon Unit Gearing Is Joint Dependent in Human Medial Gastrocnemius.

Skeletal muscles change length and develop force both passively and actively. Gearing allows muscle fiber length changes to be uncoupled from those of the whole muscle-tendon unit. During active contractions this process allows muscles to operate at mechanically favorable conditions for power or economical force production. Here we ask whether gearing is constant in passive muscle; determining the relationship between fascicle and muscle-tendon unit length change in the bi-articular medial gastrocnemius and investigating the influence of whether motion occurs at the knee or ankle joint. Specifically, the same muscle-tendon unit length changes were elicited by rotating either the ankle or knee joint whilst simultaneously measuring fascicle lengths in proximal and distal muscle regions using B-mode ultrasound. In both the proximal and distal muscle region, passive gearing values differed depending on whether ankle or knee motion occurred. Fascicle length changes were greater with ankle motion, likely reflecting anatomical differences in proximal and distal passive tendinous tissues, as well as shape changes of the adjacent mono-articular soleus. This suggests that there is joint-dependent dissociation between the mechanical behavior of muscle fibers and the muscle-tendon unit during passive joint motions that may be important to consider when developing accurate models of bi-articular muscles.

Fractal dimension of superfluid turbulence: a random-walk toy model

This paper deals with the fractal dimension of a superfluid vortex tangle. It extends a previous model [J. Phys. A: Math. Theor. 43, 205501 (2010)] (which was proposed for very low temperature), and it proposes an alternative random walk toy model, which is valid also for finite temperature. This random walk model combines a recent Nemirovskii’s proposal, and a simple modelization of a self-similar structure of vortex loops (mimicking the geometry of the loops of several sizes which compose the tangle). The fractal dimension of the vortex tangle is then related to the exponents describing how the vortex energy per unit length changes with the length scales, for which we take recent proposals in the bibliography. The range between 1.35 and 1.75 seems the most consistent one.

Computational Models Predict Larger Muscle Tissue Strains at Faster Sprinting Speeds

Proximal biceps femoris musculotendon strain injury has been well established as a common injury among athletes participating in sports that require sprinting near or at maximum speed; however, little is known about the mechanisms that make this muscle tissue more susceptible to injury at faster speeds. This study aimed to quantify localized tissue strain during sprinting at a range of speeds. Biceps femoris long head (BFlh) musculotendon dimensions of 14 athletes were measured on magnetic resonance (MR) images and used to generate a finite-element computational model. The model was first validated through comparison with previous dynamic MR experiments. After validation, muscle activation and muscle-tendon unit length change were derived from forward dynamic simulations of sprinting at 70%, 85%, and 100% maximum speed and used as input to the computational model simulations. Simulations ran from midswing to foot contact. The model predictions of local muscle tissue strain magnitude compared favorably with in vivo tissue strain measurements determined from dynamic MR experiments of the BFlh. For simulations of sprinting, local fiber strain was nonuniform at all speeds, with the highest muscle tissue strain where injury is often observed (proximal myotendinous junction). At faster sprinting speeds, increases were observed in fiber strain nonuniformity and peak local fiber strain (0.56, 0.67, and 0.72 for sprinting at 70%, 85%, and 100% maximum speed). A histogram of local fiber strains showed that more of the BFlh reached larger local fiber strains at faster speeds. At faster sprinting speeds, peak local fiber strain, fiber strain nonuniformity, and the amount of muscle undergoing larger strains are predicted to increase, likely contributing to the BFlh muscle's higher injury susceptibility at faster speeds.

Effect of Heel Construction on Muscular Control Potential of the Ankle Joint in Running

The purpose of this study was to investigate the effect of heel construction on ankle joint mechanics during the early stance phase of running. Kinematic and kinetic parameters (ankle joint angles, angular velocities and joint moments, lever arms of ground reaction force, triceps surae muscle tendon unit lengths, and rates of muscle tendon unit length change) were calculated from 19 male subjects running at 3.3 m/s in shoes with different heel constructions. Increasing heel height and posterior wedging amplified initial plantar flexion velocity and range. The potential for a muscle to control the movement of a joint depends upon its ability to produce joint moments. Runners in this study showed decreased external eversion moments and an increase in eversion range. Maximum eversion angles were not significantly affected by shoe conditions. Without considerable tendon prestretch, joint moment generation potentials of triceps surae and deep plantar flexors might be inhibited due to rapid plantar flexion based on the force-velocity relationship. It could be speculated that increasing ankle inversion at heel strike could be a strategy to keep maximum eversion angles inside an adequate range, if joint moment generation potentials of deep plantar flexors are inhibited due to rapid plantar flexion.

Triceps surae muscle–tendon unit length changes as a function of ankle joint angles and contraction levels: The effect of foot arch deformation

The purpose of this study was to clarify how foot deformation affects the relationship between triceps surae muscle–tendon unit (MTU) length and ankle joint angle. For six women and six men a series of sagittal magnetic resonance (MR) images of the right foot were taken, and changes in MTU length (the displacement of the calcaneal tuberosity), foot arch angle, and ankle joint angle were measured. In the passive session, each subject's ankle joint was secured at 10° dorsiflexed position, neutral position (NP), and 10° and 20° plantar flexed positions while MR images were acquired. In the active session, each subject was requested to perform submaximal isometric plantar flexions (30%, 60%, and 80% of voluntary maximum) at NP. The changes in MTU length in each trial were estimated by two different formulae reported previously. The changes of the measured MTU length as a function of ankle joint angles observed in all trials of the active session were significantly ( p<0.05) larger than corresponding values in the passive session and by the estimation formulae. In the passive session, MTU length changes were significantly smaller than the estimated values when the ankle was plantar flexed. The foot arch angle increased as the contraction level increased from rest (117±4°) to 80% (125±3°), and decreased as the ankle was positioned further into plantar flexion in the passive session (115±3°). These results indicate that foot deformation profoundly affects the triceps surae MTU length–ankle joint angle relationship during plantar flexion.

Vortex length, vortex energy and fractal dimension of superfluid turbulence at very low temperature

By assuming a self-similar structure for Kelvin waves along vortex loops with successive smaller scale features, we model the fractal dimension of a superfluid vortex tangle in the zero temperature limit. Our model assumes that at each step the total energy of the vortices is conserved, but the total length can change. We obtain a relation between the fractal dimension and the exponent describing how the vortex energy per unit length changes with the length scale. This relation does not depend on the specific model, and shows that if smaller length scales make a decreasing relative contribution to the energy per unit length of vortex lines, the fractal dimension will be higher than unity. Finally, for the sake of more concrete illustration, we relate the fractal dimension of the tangle to the scaling exponents of amplitude and wavelength of a cascade of Kelvin waves.

The Relationship Between Passive Ankle Plantar Flexion Joint Torque and Gastrocnemius Muscle and Achilles Tendon Stiffness: Implications for Flexibility

Experimental laboratory study. We tested the hypothesis that the muscle fibers and the connective tendinous structures, combined in series, provide the resistance to passive joint movement at the ankle. We also determined the relative association between passive joint torque and each of these 2 elements. The reason for individual variation in joint flexibility or tightness is not clearly understood, but the influence of musculotendinous stiffness has been inferred. Each of the subjects (6 women and 6 men) was seated with the right knee extended and right ankle positioned at a 30 degrees , 20 degrees , 10 degrees , 0 degrees , -10 degrees , -20 degrees , and -30 degrees (0, neutral position, positive values reflecting plantar flexion) angle while passive plantar flexion torque was measured. The distal muscle-tendon junction of the medial gastrocnemius was visualized by ultrasonography, and its positional change was defined as muscle belly length change. The whole muscle-tendon unit length change was estimated from joint angle changes, from which Achilles tendon length change was estimated. Both the muscle belly and tendon were significantly elongated as the ankle was dorsiflexed (at 0 degrees the mean +/- SD muscle belly elongation was 10.3% +/- 1.8 %, and the tendon elongation was 2.8% +/- 1.2 %, of the initial length at 30 degrees of ankle plantar flexion), from which stiffness indices were determined both for muscle belly and tendon. The passive torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees was significantly correlated with the stiffness indices of the Achilles tendon (at 0 degrees , r2 = 0.70 and 0.62 for overall and specific stiffness, respectively; P<.05). A tendon stiffness index, separately obtained from tendon lengthening during maximal isometric contraction, was also correlated with passive ankle plantar flexion torque at 0 degrees , -10 degrees , -20 degrees , and -30 degrees (at 0 degrees , r2 = 0.76; P<.05). The specific stiffness index of the muscle belly was correlated (r2 = 0.47, P<.05) with the passive ankle plantar flexion torque at 0 degrees , but its overall stiffness index was not (r2 = 0.32, P>.05). Results suggest that extensibility of the muscle-tendon unit of the Achilles tendon for the most part is related to passive ankle plantar flexion joint torque.

Quantification of muscle fiber strain during in vivo repetitive stretch-shortening cycles

Muscles subjected to lengthening contractions exhibit evidence of subcellular disruption, arguably a result of fiber strain magnitude. Due to the difficulty associated with measuring fiber strains during lengthening contractions, fiber length estimates have been used to formulate relationships between the magnitude of injury and mechanical measures such as fiber strain. In such protocols, the series compliance is typically minimized by removing the distal tendon and/or preactivating the muscle. These in vitro and in situ experiments do not represent physiological contractions well where fiber strain and muscle strain may be disassociated; thus the mechanisms of in vivo muscle injury remain elusive. The purpose of this paper was to quantify fiber strains during lengthening contractions in vivo and assess the potential role of fiber strain in muscle injury following repetitive stretch-shortening cycles. Using intact New Zealand White rabbit dorsiflexors, fiber strain and joint torque were measured during 50 stretch-shortening cycles. We were able to show that fiber length changes are disassociated from muscle tendon unit length changes and that complex fiber dynamics during these cycles prevent easy estimates of fiber strains. In addition, fiber strains vary, depending on how they are defined, and vary from repetition to repetition, thereby further complicating the potential relationship between muscle injury and fiber strain. We conclude from this study that, during in vivo stretch-shortening cycles, the relationship between fiber strain and muscle injury is complex. This is due, in part, to temporal effects of repeated loading on fiber strain magnitude that may be explained by an increasing compliance of the contractile element as exercise progresses.

In vivo behavior of vastus lateralis muscle during dynamic performances

Length changes in vastus lateralis fascicles were measured in vivo using ultrasonography during one-legged squat jumps (SJ), counter movement jumps (CMJ), and drop jumps (DJ) in the sledge apparatus (n = 9). Patellar tendon forces were recorded simultaneously with an optic fiber technique from 4 subjects. Fascicle length changes were compared with muscletendon unit length changes calculated from kinematic recordings. In general, the tendomuscular and fascicle length changes demonstrated similar patterns. During SJ the fascicles showed shortening throughout the action while during CMJ and DJ they underwent stretch-shortening cycle. In DJ greater muscular activity in braking phase.

On the invariance of the hybrid position/force control

Recently, the noninvariance nature of the Raibert and Craig hybrid control scheme, based on the work of Mason and others, has been pointed out. In fact, on the basis of the screw theory, Lipkin and Duffy demonstrated that the selection of the position and force controlled degrees of freedom in the Raibert and Craig scheme, may give wrong results if a translation or change in unit length of the coordinate frame is performed. A general theoretical solution to this problem, called ‘kinestatic filtering’, has been given by Lipkin and Duffy. In this paper, the two approaches are summarized and discussed. First, the conditions in which the Mason filtering technique fails are determined and, second, the situations where the Lipkin and Duffy approach cannot be applied owing to degeneracy of the twist and wrench spaces, are reported. As a consequence of this analysis, a new invariant kinestatic filtering method is proposed. The method here presented is based on the original Mason approach and requires the definition of a task-dependent filter based on the knowledge of the position of the compliant frame. Examples are presented and discussed.

An analysis of receptor potential and tension of isolated cat muscle spindles in response to sinusoidal stretch.

In isolated cat muscle spindles the receptor potential responses of primary and secondary endings as well as tension responses to sinusoidal length changes in the steady state have been analysed. 1. At a given stimulus frequency, receptor potential per unit length change (receptor potential gain) in both primary and secondary endings is constant when displacement is less than about 10 micrometer. With larger stretches, receptor potential gain decreases approximately as a power function of displacement, the gain of primary endings decreasing more rapidly with increasing displacement than that of secondary endings. Tension per unit length change (tension gain) shows a similar constant range above which it also decreases as a power function of displacement. 2. In spite of the large reduction in gain at high displacement amplitudes, response wave forms remained essentially sinusoidal. The gain reduction results principally from a displacement-dependent non-linearity which has a rapid onset and slow decay. 3. Receptor potential and tension responses to small amplitude sinusoidal stretch depend, in a parallel manner, on the initial length of the preparation. 4. Both receptor potential and tension responses are highly dependent on frequency of sinusoidal stretch. In primary endings receptor potential gain increased as a power function of frequency over the range 0 . 01 to about 40 Hz, above which frequency the gain decreased; phase advance remained relatively constant up to 10 Hz then decreased to become a phase lag at higher frequency. In secondary endings receptor potential gain remained fairly constant between 0 . 01 and 1 Hz then rose as a power function of frequency but less steeply than in primary endings. 3. The possible mechanisms underlying these findings are discussed.

Determination of thermal-expansion characteristics of metals using strain gages

A method has been developed to measure thermal-expansion characteristics of metals using bonded resistance strain gages. The method allows rapid and accurate determination of expansion properties at similar or lower cost (depending on the particular application) than conventional dilatometric techniques. Other advantages include elimination of a need for perfectly flat sample material, elimination of specimen machining, and applicability to structures and components. To utilize this technique, the ‘apparent strain’ of the gage is determined by attaching it to a ‘standard’ material for which the thermal-expansion characteristics are accurately known and subtracting the known thermal response of the material from the total gage output. ‘Apparent strain’ is therefore the temperature-induced output of the gage when bonded to a material having a thermal-expansion coefficient of zero. When the gage is then attached to a test material and cycled through the same temperature range, this ‘apparent strain’ is subtracted from the total gage output to obtain the actual unit-length change of the test material. Using this technique, mean-expansion coefficients of experimental alloys were determined over the temperature range −320°F (−196°C) to room temperature.

Filament sliding and energy absorbed by the cross‐bridge in active muscle subjected to cycical length changes.

1. The effects of single and double cycles of stretch and release on the tension response and relative sliding movement of the actin and myosin filaments in active frog's muscle were investigated. 2. The cross-bridges linking the filaments together are able to accommodate a greater range of filament displacement before becoming detached during a second cycle stretch, providing it commences without delay following the preceding release: sarcomere 'give' then occurs for displacements of around 18 nm, as compared with 12 nm for a first cycle stretch. It is postulated that the difference arises because the myosin heads adopt different 'preferred' positions in the isometric steady-state and at the end of a previous release. 3. Muscle length-tension loops were recorded and used to measure the energy absorbed when a muscle is subjected to cycles of stretch and release. The work absorbed per unit length change increases with increasing displacement of the cross-bridges from their initial (isometric) steady-state position, up to the point at which sarcomere 'give' occurs (S2); thereafter it remains constant. 4. More work is absorbed during the first cycle of a double stretch-release combination than during the second. The greater amount absorbed during the first cycle is associated with a correspondingly greater amount of filament sliding in the period following sarcomere 'give'. Sarcomere length-tension loops were constructed and these showed that not less than 80-85% of the work done on a muscle is absorbed by the sarcomeres themselves. 5. A greater amount of work is done on stretching up to (but not beyond) S2 during second cycle stretch as compared to a first. The difference amounts about 1 mJ.m-2 per half-sarcomere. 6. The results are compatible with the mechanism for force production proposed by Huxley & Simmons (1973), in which each myosin head generates force in a number of stepping movements, from one attached state to another. It is concluded that (a) during an unloaded isotonic contraction the working 'stroke' of the head would result in a 10-13 nm relative sliding movement of the filaments, and (b) the potential energy difference separating the two 'preferred' states is 6-9.6 kT per cross-bridge, or 3-4.8 kT per S-1 sub-units, assuming that each one interacts simultaneously with the actin filament.