Suppression of Multiphoton Resonances in Driven Quantum Systems via Pulse Shape Optimization.

This Letter demonstrates control over multiphoton absorption processes in driven two-level systems, which include, for example, superconducting qubits or laser-irradiated graphene, through spectral shaping of the driving pulse. Starting from calculations based on Floquet theory, we use differential evolution, a general purpose optimization algorithm, to find the Fourier coefficients of the driving function that suppress a given multiphoton resonance in the strong field regime. We show that the suppression of the transition probability is due to the coherent superposition of high-order Fourier harmonics which closes the dynamical gap between the Floquet states of the two-level system.

Optimization of integrated polarization filters.

This study reports on the design of small footprint, integrated polarization filters based on engineered photonic lattices. Using a rods-in-air lattice as a basis for a TE filter and a holes-in-slab lattice for the analogous TM filter, we are able to maximize the degree of polarization of the output beams up to 98% with a transmission efficiency greater than 75%. The proposed designs allow not only for logical polarization filtering, but can also be tailored to output an arbitrary transverse beam profile. The lattice configurations are found using a recently proposed parallel tabu search algorithm for combinatorial optimization problems in integrated photonics.

Multiobjective optimization in integrated photonics design.

We propose the use of the parallel tabu search algorithm (PTS) to solve combinatorial inverse design problems in integrated photonics. To assess the potential of this algorithm, we consider the problem of beam shaping using a two-dimensional arrangement of dielectric scatterers. The performance of PTS is compared to one of the most widely used optimization algorithms in photonics design, the genetic algorithm (GA). We find that PTS can produce comparable or better solutions than the GA, while requiring less computation time and fewer adjustable parameters. For the coherent beam shaping problem as a case study, we demonstrate how PTS can tackle multiobjective optimization problems and represent a robust and efficient alternative to GA.

Validation of an EMG submaximal method to calibrate a novel dynamic EMG-driven musculoskeletal model of the trunk: Effects on model estimates.

BACKGROUND: Multijoint EMG-assisted optimization models are reliable tools to predict muscle forces as they account for inter- and intra-individual variations in activation. However, the conventional method of normalizing EMG signals using maximum voluntary contractions (MVCs) is problematic and introduces major limitations. The sub-maximal voluntary contraction (SVC) approaches have been proposed as a remedy, but their performance against the MVC approach needs further validation particularly during dynamic tasks. METHODS: To compare model outcomes between MVC and SVC approaches, nineteen healthy subjects performed a dynamic lifting task with two loading conditions. RESULTS: Results demonstrated that these two approaches produced highly correlated results with relatively small absolute and relative differences (<10 %) when considering highly-aggregated model outcomes (e.g. compression forces, stability indices). Larger differences were, however, observed in estimated muscle forces. Although some model outcomes, e.g. force of abdominal muscles, were statistically different, their effect sizes remained mostly small (ηG2 ≤ 0.13) and in a few cases moderate (ηG2 ≤ 0.165). CONCLUSION: The findings highlight that the MVC calibration approach can reliably be replaced by the SVC approach when the true MVC exertion is not accessible due to pain, kinesiophobia and/or the lack of proper training.

Beam shaping using genetically optimized two-dimensional photonic crystals.

We propose the use of two-dimensional (2D) photonic crystals (PhCs) with engineered defects for the generation of an arbitrary-profile beam from a focused input beam. The cylindrical harmonics expansion of complex-source beams is derived and used to compute the scattered wave function of a 2D PhC via the multiple scattering method. The beam shaping problem is then solved using a genetic algorithm. We illustrate our procedure by generating different orders of Hermite-Gauss profiles, while maintaining reasonable losses and tolerance to variations in the input beam and the slab refractive index.

Relative importance of expertise, lifting height and weight lifted on posture and lumbar external loading during a transfer task in manual material handling.

The objective of this study was to measure the effect size of three important factors in manual material handling, namely expertise, lifting height and weight lifted. The effect of expertise was evaluated by contrasting 15 expert and 15 novice handlers, the effect of the weight lifted with a 15-kg box and a 23-kg box and the effect of lifting height with two different box heights: ground level and a 32 cm height. The task consisted of transferring a series of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables (moments) while expertise had low impact. On the other hand, expertise showed a significant effect of posture variables on the lumbar spine and knees. All three factors are important, but for a reduction of external back loading, the focus should be on the lifting height and weight lifted. PRACTITIONER SUMMARY: The objective was to measure the effect size of three important factors in a transfer of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables but expertise was a major determinant in back posture.

Submaximal electromyography-driven musculoskeletal modeling of the human trunk during static tasks: Equilibrium and stability analyses.

Conventional electromyography-driven (EMG) musculoskeletal models are calibrated during maximum voluntary contraction (MVC) tasks, but individuals with low back pain cannot perform unbiased MVCs. To address this issue, EMG-driven models can be calibrated in submaximal tasks. However, the effects of maximal (when data points include the maximum contraction) and submaximal calibration techniques on model outputs (e.g., muscle forces, spinal loads) remain yet unknown. We calibrated a subject-specific EMG-driven model, using maximal/submaximal isometric contractions, and simulated different independent tasks. Both approaches satisfactorily predicted external moments (Pearson's correlation ∼ 0.75; relative error = 44%), and removing calibration tasks under axial torques markedly improved the model performance (Pearson's correlation âˆ¼ 0.92; relative error âˆ¼ 28%). Unlike individual muscle forces, gross (aggregate) model outputs (i.e., spinal loads, stability index, and sum of abdominal/back muscle forces) estimated from maximal and submaximal calibration techniques were highly correlated (r > 0.78). Submaximal calibration method overestimated spinal loads (6% in average) and abdominal muscle forces (11% in average). Individual muscle forces estimated from maximal and submaximal approaches were substantially different; however, gross model outputs (especially internal loads and stability index) remained highly correlated with small to moderate relative differences; therefore, the submaximal calibration technique can be considered as an alternative to the conventional maximal calibration approach.

The assessment of back muscle capacity using intermittent static contractions. Part II: validity and reliability of biomechanical correlates of muscle fatigue.

INTRODUCTION: In a previous paper, standard surface electromyographic (EMG) indices of muscle fatigue, which are based on the lowering of the median or mean frequencies of the EMG power spectrum in time, were applied during an intermittent absolute endurance test and were evaluated relative to criterion validity and test-retest reliability. The aims of this study were to assess mechanical and alternative EMG correlates of muscle fatigue. METHODS: Healthy subjects (44 males and 29 females; age: 20-55 yrs) performed three maximal voluntary contractions (MVC) and an endurance test while standing in a static dynamometer. Surface EMG signals were collected from four pairs of back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). The test, assessing absolute endurance (90 Nm torque), consisted of performing an intermittent extension task to exhaustion. Strength was defined as the peak MVC whereas our endurance criterion was defined as the time to reach exhaustion (Tend) during the endurance test. Mechanical indices quantifying physiological tremor and steadiness were computed from the dynamometer signals (L5/S1 extension moments) along with EMG indices presumably sensitive to variable load sharing between back muscle synergists during the endurance test. RESULTS: Mechanical indices were significantly correlated to Tend (r range: -0.47 to -0.53) but showed deceiving reliability results. Conversely, the EMG indices were correlated to Tend (r range: -0.43 to -0.63) with some of them particularly correlated to Strength (r=-0.72 to -0.81). In addition, their reliability results were acceptable (intra-class correlation coefficient >0.75; standard error of measurement <10% of the mean) in many cases. Finally, several analyses substantiated their physiological relevance. These findings imply that these new EMG indices could be used to predict absolute endurance as well as strength with the use of a single intermittent and time-limited (5-10min) absolute endurance test, a practical way to assess the back capacity of chronic low back pain subjects.

The assessment of back muscle capacity using intermittent static contractions. Part I - Validity and reliability of electromyographic indices of fatigue.

INTRODUCTION: Back muscle capacity is impaired in chronic low back pain patients but no motivation-free test exists to measure it. The aims of this study were to assess the reliability and criterion validity of electromyographic indices of muscle fatigue during an intermittent absolute endurance test. METHODS: Healthy subjects (44 males and 29 females; age: 20-55 yrs) performed three maximal voluntary contractions (MVC) and a fatigue test while standing in a static dynamometer. Surface EMG signals were collected from four pairs of back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). The fatigue test, assessing absolute endurance (90-Nm torque), consisted in performing an intermittent extension task to exhaustion. Strength was defined as the peak MVC whereas our endurance criterion was defined as the time to reach exhaustion (Tend) during the fatigue test. From the first five min (females) or ten min (males) of EMG data, frequency and time-frequency domain analyses were applied to compute various spectral indices of muscle fatigue. RESULTS: The EMG indices were more reliable when computed from the time-frequency domain than when computed from the frequency domain, but showed comparable correlation results (criterion validity) with Tend and Strength. Some EMG indices reached moderate to good correlation (range: 0.64-0.69) with Tend, lower correlations (range: 0.39-0.55) with Strength, and good to excellent between-day test-retest reliability results (intra-class correlation range: 0.75-0.83). The quantification of the spectral content more locally in different frequency bands of the power spectrum was less valid and reliable than the indices computed from the entire power spectrum. Differences observed among muscles were interpreted in light of specific neuromuscular activation levels that were observed during the endurance test. These findings supported the use of an intermittent and time-limited (5-10min) absolute endurance test, that is a practical way to assess the back capacity of chronic low back pain subjects, to assess absolute endurance as well as strength with the use of electromyographic indices of muscle fatigue.

A comparison of lumbar spine and muscle loading between male and female workers during box transfers.

There is a clear relationship between lumbar spine loading and back musculoskeletal disorders in manual materials handling. The incidence of back disorders is greater in women than men, and for similar work demands females are functioning closer to their physiological limit. It is crucial to study loading on the spine musculoskeletal system with actual handlers, including females, to better understand the risk of back disorders. Extrapolation from biomechanical studies conducted on unexperienced subjects (mainly males) might not be applicable to actual female workers. For male workers, expertise changes the lumbar spine flexion, passive spine resistance, and active/passive muscle forces. However, experienced females select similar postures to those of novices when spine loading is critical. This study proposes that the techniques adopted by male experts, male novices, and females (with considerable experience but not categorized as experts) impact their lumbar spine musculoskeletal systems differently. Spinal loads, muscle forces, and passive resistance (muscle and ligamentous spine) were predicted by a multi-joint EMG-assisted optimization musculoskeletal model of the lumbar spine. Expert males flexed their lumbar spine less (avg. 21.9° vs 30.3-31.7°) and showed decreased passive internal moments (muscle avg. 8.9% vs 15.9-16.0%; spine avg. 4.7% vs 7.1-7.8%) and increased active internal moments (avg. 72.9% vs 62.0-63.9%), thus producing a different impact on their lumbar spine musculoskeletal systems. Experienced females sustained the highest relative spine loads (compression avg. 7.3 N/BW vs 6.2-6.4 N/BW; shear avg. 2.3 N/BW vs 1.7-1.8 N/BW) in addition to passive muscle and ligamentous spine resistance similar to novices. Combined with smaller body size, less strength, and the sequential lifting technique used by females, this could potentially mean greater risk of back injury. Workers should be trained early to limit excessive and repetitive stretching of their lumbar spine passive tissues.

Active movement measurements of the shoulder girdle in healthy subjects with goniometer and tape measure techniques: a study on reliability and validity.

In response to the current need to develop objective measurement techniques for active movements of the shoulder girdle, this study had two goals: 1) to evaluate the reliability of the active range of motion (AROM) of shoulder girdle in elevation, protraction, and retraction with use of a goniometer and a tape measure and 2) to present the criterion-related validity of the reliable movements. Thirty healthy subjects performed two trials of each shoulder girdle AROM. They were assessed by two physical therapists, who used both evaluation techniques on two occasions. The generalizability theory was used to compute intratester (test-retest) and intertester reliability using dependability coefficients (phi) and standard errors of measurement (SEMs). The shoulder girdle movements were then evaluated with a movement analysis system (Optotrak 3020). The Pearson product-moment correlation coefficient (r) and paired t-tests were used to compare the results of clinical measurements with those of the movement analysis system. Overall, the reliability with the tape measure was good (phi = 0.78-0.91; SEM's = 0.7-1.1 cm) for the elevation motion. The goniometer was only reliable for intratester measures (phi = 0.78-0.79; SEM = 2.7 degrees) for the elevation motion. Retraction was reliable with the tape measure alone (phi = 0.76-0.84; SEM's = 0.5-0.7 cm). The criterion-related validity analysis revealed that measuring shoulder girdle elevation with the tape measure and goniometer, and retraction with the tape measure were not valid when evaluated against a gold standard, thus, both techniques have limitations in assessing true elevation and retraction measurements.

Coherent destruction of tunneling in graphene irradiated by elliptically polarized lasers.

Photo-induced transition probabilities in graphene are studied theoretically from the viewpoint of Floquet theory. Conduction band populations are computed for a strongly, periodically driven graphene sheet under linear, circular, and elliptic polarization. Features of the momentum spectrum of excited quasi-particles can be directly related to the avoided crossing of the Floquet quasi-energy levels. In particular, the impact of the ellipticity and the strength of the laser excitation on the avoided crossing structure-and on the resulting transition probabilities-is studied. It is shown that the ellipticity provides an additional control parameter over the phenomenon of coherent destruction of tunneling in graphene, allowing one to selectively suppress multiphoton resonances.

Gender influence on fatigability of back muscles during intermittent isometric contractions: a study of neuromuscular activation patterns.

BACKGROUND: Gender difference in the fatigability of muscles can be attributed to muscle mass (or strength) and associated level of vascular occlusion, substrate utilization, muscle composition, and neuromuscular activation patterns. The purpose of this study was to assess the role of neuromuscular activation patterns to explain gender differences in back muscle fatigability during intermittent isometric tasks. METHODS: Sixteen males and 15 females performed maximal voluntary contractions (Strength) and a fatigue test to exhaustion (fatigue criterion=time to exhaustion), while standing in a static dynamometer measuring L5/S1 extension moment. The fatigue test consisted of repetitions of an 8-s cycle (1.5 s ramp to reach 40% of maximal voluntary contraction +5s plateau at 40% of maximal voluntary contraction +1.5s rest). Surface electromyography signals were collected bilaterally from 4 back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10). FINDINGS: Males were stronger (P<0.05) than females (316, SD 82>196, SD 25 Nm) but showed significantly shorter time-to-exhaustion values (7.1, SD 5.2<13.0, SD 6.1 min.), the latter result being corroborated by electromyographic indices of fatigue. However, the gender effect on time to exhaustion disappeared when accounting for Strength, thus supporting the muscle mass hypothesis. Among the various electromyographic indices computed to assess neuromuscular activation patterns, the amount of alternating activity between homolateral and between contralateral muscles showed a gender effect (females>males). INTERPRETATION: These results support the muscle mass hypothesis as well as the neuromuscular activation hypothesis to explain gender differences in back muscle fatigability.

A biomechanical comparison between expert and novice manual materials handlers using a multi-joint EMG-assisted optimization musculoskeletal model of the lumbar spine.

Expertise is a key factor modulating the risk of low back disorders (LBD). Through years of practice in the workplace, the typical expert acquires high level specific skills and maintains a clean record of work-related injuries. Ergonomic observations of manual materials handling (MMH) tasks show that expert techniques differ from those of novices, leading to the idea that expert techniques are safer. Biomechanical studies of MMH tasks performed by experts/novices report mixed results for kinematic/kinetic variables, evoking potential internal effect of expertise. In the context of series of box transfers simulated by actual workers, detailed internal loads predicted by a multiple-joint EMG-assisted optimization lumbar spine model are compared between experts and novices. The results confirmed that the distribution of internal moments are modulated by worker expertise. Experts flexed less their lumbar spine and exerted more active muscle forces while novices relied more on passive resistance of the muscles and ligamentous spine. More specifically for novices, the passive contributions came from global extensor muscles, selected local extensor muscles, and passive structures of the lumbar spine (ligaments and discs). The distinctive distribution of internal forces was not concomitant with a similar effect on joint forces, these forces being dependent on external loading which was equivalent between experts and novices. From a safety standpoint, the present results suggest that experts were more efficient than novices in partitioning internal moment contributions to balance net (external) loading. Thus, safer handling practices might be seen as a result of experts׳ experience.

Surface electromyography assessment of back muscle intrinsic properties.

The purpose of this study was to assess (1) the reliability and (2) the sensitivity to low back pain status and gender of different EMG indices developed for the assessment of back muscle weakness, muscle fiber composition and fatigability. Healthy subjects (men and women) and chronic low back pain patients (men only) performed, in a static dynamometer, maximal and submaximal static trunk extension tasks (short and long duration) to assess weakness, fiber composition and fatigue. Surface EMG signals were recorded from four (bilateral) pairs of back muscles and three pairs of abdominal muscles. To assess reliability of the different EMG parameters, 40 male volunteers (20 controls and 20 chronic low back pain patients) were assessed on three occasions. Reliable EMG indices were achieved for both healthy and chronic low back pain subjects when specific measurement strategies were applied. The EMG parameters used to quantify weakness and fiber composition were insensitive to low back status and gender. The EMG fatigue parameters did not detect differences between genders but unexpectedly, healthy men showed higher fatigability than back pain patients. This result was attributed to the smaller absolute load that was attributed to the patients, a load that was defined relative to their maximal strength, a problematic measure with this population. An attempt was made to predict maximal back strength from anthropometric measurements but this prediction was prone to errors. The main difficulties and some potential solutions related to the assessment of back muscle intrinsic properties were discussed.

A biomechanical comparison of lifting techniques between subjects with and without chronic low back pain during freestyle lifting and lowering tasks.

OBJECTIVE: To evaluate if chronic low back pain patients perform manual material handling tasks differently from control subjects. DESIGN: Comparative study using a repeated measures design. BACKGROUND: No study evaluated the lifting technique of back pain patients relative to control subjects during free style lifting and lowering tasks. Previous findings suggest that lowering would be more hazardous than lifting to the low back. It would be interesting to evaluate if chronic low back pain patients behave differently than controls when lifting and lowering. METHODS: Thirty-three male subjects (18 controls, 15 suffering from non-specific chronic low back pain) participated. A 12-kg box was lifted (freestyle) from the floor to the hips (1) in front (symmetric task) or (2) to a shelf located at 90 degree on the right (asymmetric task) and was lowered back to the floor. A 3D biomechanical analysis involving the assessment of L5/S1 loading, posture of segments, inertial parameters, and EMG was performed. RESULTS: There was no difference between the groups for postural (trunk and lower limb angles), inertial (trunk velocity and acceleration), and L5/S1 loading (moments and compression) variables. The patients showed abnormally low left lumbar erector spinae (symmetric task, lowering) or high left thoracic erector spinae (all tasks) EMG activation. Significant Group x Action (lifting vs. lowering) interactions were also observed for some inertial and L5/S1 loading variables suggesting that the biomechanical differences detected between lifting and lowering may have a differential influence on the technique used by back pain patients and control subjects. CONCLUSIONS: The gross lifting technique of back pain patients was unaltered relative to controls but the activation of paraspinal muscles differed, suggesting that a more detailed biomechanical analysis, such as the use of EMG driven models, might be required to reveal lumbar impairments during lifting. RELEVANCE: To evaluate if chronic low back pain patients use naturally different lifting techniques to prevent pain exacerbation and damaged lumbar tissue overloading.

Back strength cannot be predicted accurately from anthropometric measures in subjects with and without chronic low back pain.

OBJECTIVES: (1) To develop a multiple regression equation using anthropometric measurements to predict back strength and (2) to estimate the effect of practice on the back strength results and back strength predictions. DESIGN: Comparative study with repeated measures performed on three days. BACKGROUND: The assessment of back muscle relative endurance (% maximal strength) requires the measurement of maximal back strength which is problematic with low back pain patients. METHODS: The back strength (L5/S1 static extension moment), age and 26 anthropometric parameters were obtained from 83 male volunteers [42 healthy subjects and 41 chronic low back pain patients] aged between 20 and 60 years. A subsample of 20 healthy subjects and 20 patients were assessed through three days of testing to evaluate the variations of back strength with practice. RESULTS: The final regression model (n=42 healthy subjects) explained 39% of the variance in back strength. Back strength increased with practice (Day1

Biomechanical analysis of a posterior transfer maneuver on a level surface in individuals with high and low-level spinal cord injuries.

OBJECTIVE: The purpose of this study was to determine the movement patterns and the muscular demand during a posterior transfer maneuver on a level surface in individuals with spinal cord injuries. DESIGN: Six participants with high-level spinal cord injury (C7 to T6) were compared to five participants with low-level spinal cord injury (T11 to L2) with partial or complete control of abdominal musculature. BACKGROUND: Developing an optimal level of independence for transfer activities figures among the rehabilitation goals of individuals with spinal cord injury. There has been no biomechanical study which specifically describes the posterior transfer maneuver. METHODS: Tridimensional kinematics at the elbow, shoulder, head and trunk, as well as surface electromyographic data of the biceps, triceps, anterior deltoid, posterior deltoid, pectoralis major, latissimus dorsi, trapezius and rectus abdominus muscles were recorded during the posterior transfer. To quantify the muscular demand, the electromyographic data were amplitude normalized to the peak value obtained from maximum voluntary contractions. The transfer was divided into pre-lift, lift, and post-lift phases for analysis. RESULTS: The duration of the lift phase was significantly shorter (P<0.05) for the high-level spinal cord injury (1.24; SD, 0.37 s) when compared to the low-level spinal cord injury (1.74; SD, 0.39 s). The patterns and magnitudes of the angular displacements were found similar between groups (P values: 0.45-0.98). However, the high-level spinal cord injury initiated the task from a forward flexed posture, whereas the low-level spinal cord injury adopted an almost upright alignment of the trunk. Higher muscular demands were calculated for all muscles among high-level spinal cord injury participants during the transfer when compared to the low-level spinal cord injury. However, only the anterior deltoid (high level=92.4%; low level=34.2%) and the pectoralis major (high level=109.8%; low level=25.6%) reached statistical significance during the lift phase.Conclusions. Participants with high-level spinal cord injury presented different movement characteristics and higher muscular demands during the posterior transfer than low-level spinal cord injury ones. This is probably to compensate for the additional trunk and upper limb musculature impairment. RELEVANCE: The findings of this study may help to develop guidelines of specific strengthening programs for the thoracohumeral, scapulothoracic and shoulder muscles designed to restore optimal transfer capacity in individuals with spinal cord injury. Furthermore, innovative rehabilitation programs targeting the ability to control the trunk could be beneficial for these individuals.

Controlling for out-of-plane lumbar moments during unidirectional trunk efforts: learning and reliability issues related to trunk muscle activation estimates.

To assess the electromyographic (EMG) activation of trunk muscle during exertions performed in one primary plane (sagittal, frontal, transverse), we previously proposed a protocol allowing minimizing out-of-plane efforts (coupled moments - CMs) with the use of a static dynamometer combined with a visual feedback system. The aims of this study were to go further by testing motor learning and reliability issues related to such a protocol. Three identical sessions were conducted, where maximal voluntary contractions and submaximal ramp contractions were performed in six different directions while standing in the dynamometer. Two feedback conditions were tested, the simple 1D-feedback in the primary plane and the full 3D-feedback in all planes simultaneously. Surface EMG signals were collected from back and abdominal muscles and EMG amplitude and CMs were computed during the ramp contractions. Providing a 3D feedback to minimize CMs did not improve EMG reliability or in other words, did not reduce the within-subject variability. Providing three assessment days had practically no effect (no learning) on CMs and EMG variables. Overall, the reliability of EMG was at best moderate. However, although this limits its use on an individual basis, it still allows within- and between-group comparisons for research applications.

Low amplitude characterization tests conducted at regular intervals can affect tendon mechanobiological response.

In bioreactor studies of tissue mechanobiology, characterizing changes in tissue quality is essential for understanding and predicting the response to mechanical stimuli. Unfortunately, current methods are often destructive and cannot be used at regular intervals on the same sample to characterize progression over time. Non-destructive methods such as low amplitude stress relaxation tests could be used, but then, the following dilemma comes into play: how can we accurately measure live tissue progression over time if the tissue is reacting to our measurement methods? In this study, we investigated the hypothesis that stress relaxation tests at physiological amplitudes conducted at regular intervals between stimulation periods do not modify tissue progression over time. Live, healthy tendons were subjected to mechanical stimuli inside a bioreactor for 3 days. The tendons were grouped based on the daily characterization protocol (24 or 0 stress relaxation tests) and their progression over time were compared. Stress relaxation tests at physiological amplitudes modified the tendon response to mechanical stimulation as observed through mechanical and histologic analyses. Possible solutions to eliminate or minimize the effect of stress relaxation tests are to use the mechanical stimuli to characterize tissue progression or to limit the number of stress relaxation tests.