Mechanical properties of mussel byssus threads

The byssus threads of the common mussel, Mytilus edulis L., have been tested mechanically and the results from the tests related to the ecology of the animal. The threads are mechanically similar to other crystalline polymers such as polyethylene having a modulus of about 108N m−2 and a long relaxation time. Resilience of 60% is similar to tendon; ultimate strain is about five times that of tendon at 0.44. The thread is laid down with a prestrain of 10% and so guys the mussel in position. Calculation shows that a mussel with 50 byssus threads would be able to resist all but severe winter storms

Frequency-domain analysis detects previously unidentified changes in ground reaction force with visually guided foot placement

Studies investigating the effect of targeting on gait have focused on the analysis of ground reaction force (GRF) within the time domain. Analysis within the frequency domain may be a more sensitive method for evaluating variations in GRF. The aim of the present study was to investigate the effect of visual targeting on GRF analyzed within the frequency domain. A within-subject repeated-measures design was used to measure the mediolateral, vertical, and antero-posterior components of the GRF of 11 healthy volunteers while walking at their own pace over a paper-covered walkway. A 30 x 24-cm target area was superimposed over a hidden Kistler force plate mounted at the midpoint of the walkway. GRF were recorded with and without the target and were analyzed within the frequency domain. Although visually guided foot placement has previously been undetected by traditional time-domain measures, targeting was found to significantly increase the frequency content of both the mediolateral (t10 = -4.07, p < 0.05) and antero-posterior (t10 = -2.52, p < 0.05) components of GRF. Consequently, it appears that frequency analysis is a more sensitive analytic technique for evaluating GRF. These findings have methodological implications for research in which GRF is used to characterize and assess anomalies in gait patterns

The effect of plantar fasciitis on vertical foot-ground reaction force

Despite the implication that mechanical overload is fundamental to the development of plantar fasciitis, gait analysis has revealed inconsistent findings regarding its effect on lower limb loading. The aim of the current study was to evaluate the regional vertical ground reaction force in patients with and without plantar fasciitis. A pressure platform was used to determine the vertical ground reaction force beneath the feet of 16 patients with, and an equivalent number of patients without, unilateral plantar fasciitis while completing 10 gait trials at a self-selected walking speed. The magnitude and timing of ground reaction force for the entire foot and for the regions of the rearfoot, midfoot, forefoot, and digits were measured for each limb. The findings indicate that patients with plantar fasciitis make gait adjustments that result in reduced force beneath the rearfoot and forefoot of the symptomatic foot. In addition, increased digital loading was observed in patients with plantar fasciitis suggesting that digital function plays an important, and previously unidentified, protective role

Tendon rehabilitation: isolated eccentric loading invokes a greater reduction in Achilles tendon thickness than concentric loading

Eccentric calf muscle exercise has been advocated as the treatment of choice in Achilles tendinopathy. However, mechanisms underlying the efficacy of eccentric, as opposed to concentric, exercise remain unknown. This research investigated the acute change in the sagittal thickness of the Achilles tendon (AT) in response to bouts of isolated eccentric or concentric calf muscle exercise. Eleven healthy males, without AT pathology (age, 25.9 ± 4.9 years; mass, 74.2 ± 11.8 kg), completed an exercise protocol involving isolated eccentric loading of the AT (ankle dorsi-flexion), while the contra-lateral AT experienced isolated concentric loading (ankle plantar-flexion). Six sets of 15 repetitions were performed against body weight, with an additional 20% bodyweight added via a backpack. AT thickness 2 cm proximal to the calcaneal insertion was determined from sagittal sonograms taken immediately prior to and following exercise. Consistent with earlier research, calf muscle exercise resulted in an abrupt decrease in AT thickness. However, isolated eccentric loading induced a significantly greater decrease (−20.8 ± 5.5%) than concentric loading (−5.3 ± 4.7%, p = 0.013). It is hypothesised that eccentric muscle action may invoke a differential stress field within the AT that results in a localised increase in collagen strain and extrusion of water from the tendon. Shear stress arising from such fluid flow may, in turn, stimulate tenocytes to produce matrix proteins that promote tendon remodelling, as has been shown in other collagen-rich tissues. The findings of this research have broad implications for the treatment of tendinopathy and provide greater insight into the clinically perceived benefit of eccentric over concentric exercise in AT rehabilitation

Incidental walking activity is sufficient to induce time-dependent conditioning of the Achilles tendon

The Achilles tendon has been seen to exhibit time-dependent conditioning when isometric muscle actions were of a prolonged duration, compared to those involved in dynamic activities, such as walking. Since, the effect of short duration muscle activation associated with dynamic activities is yet to be established, the present study aimed to investigate the effect of incidental walking activity on Achilles tendon diametral strain. Eleven healthy male participants refrained from physical activity in excess of the walking required to carry out necessary daily tasks and wore an activity monitor during the 24 h study period. Achilles tendon diametral strain, 2 cm proximal to the calcaneal insertion, was determined from sagittal sonograms. Baseline sonographic examinations were conducted at approximately 08:00 h followed by replicate examinations at 12 and 24 h. Walking activity was measured as either present (1) or absent (0) and a linear weighting function was applied to account for the proximity of walking activity to tendon examination time. Over the course of the day the median (min, max) Achilles tendon diametral strain was -11.4 (4.5, -25.4)%. A statistically significant relationship was evident between walking activity and diametral strain (P<0.01) and this relationship improved when walking activity was temporally weighted (AIC 131 to 126). The results demonstrate that the short yet repetitive loads generated during activities of daily living, such as walking, are sufficient to induce appreciable time-dependant conditioning of the Achilles tendon. Implications arise for the in vivo measurement of Achilles tendon properties and the rehabilitation of tendinopathy

Modelling activity dependent diametral strain in Achilles tendon

The Achilles tendon is a viscoelastic tissue that typically experiences 3500 ± 1700 cyclic loads per day from intermittent periods of ambulatory activity. Typically, peak tensile loads exceed three times body weight and average about 1500 N during stance, which lasts for approximately 0.6 s, followed by a 0.2 s unloaded swing phase. Viscoelastic materials respond to external load (stress) in a time-dependent manner commonly referred to as creep deformation (strain) and recover slowly when unloaded. This can be observed in vivo by monitoring changes in tendon diameter using quantitative ultrasonography and is referred to here as diametral strain. Diametral strains between −25% and +10% have been recorded over a 24 h period and are hypothesised to be associated with fluid movement within the tendon that corresponds with the creep and recovery histories. Changes in tendon diameter were taken at five times throughout a 24 h period in 11 subjects. Ambulatory activity was monitored as time stamped cadence periods by a Polar RS800sd module enabling time of day and activity duration to be used as indicators of creep and recovery histories. These archival records of diametral strain versus activity were then used to develop an adaptive non-linear viscoelastic model for interpolating tendon cross-sectional dimensions between observation points and from which, site specific volumetric flow rates can be estimated. This model has an error bound of less than 5% and has the potential for application to future studies linking fluid flow within the tissue to inherent biomechanical properties, injury status or pathological defects

In vivo visco-elastic properties of the heel fat pad during gait: is the heel pad at its physiological maximum during walking?

The fat pad of the human heel is a highly specialised adipose tissue, which is widely regarded to function as a shock absorber during the impact phase of gait. Although altered mechanical properties of the heel pad have been implicated in the development of several overuse injuries, in vivo measurement of the visco-elastic properties of the heel pad during dynamic activities, such as walking, remain largely unexplored. This study used cine-radiography synchronised with a pressure platform to obtain stress-strain data for the heel pad of nine healthy subjects (age, 46 ± 12 years; height, 1.67 ± 0.11 m; weight, 80.1 ± 10.4 kg) while walking at their preferred pace. The initial thickness and compressive strain of the fat pad were estimated from dynamic lateral radiographs, while the compressive stress was derived from peak pressure data. Principle visco-elastic parameters of the heel pad, including peak strain, secant modulii, hysteresis and time constants were estimated from the stress-strain curves. Real in vivo transient loading profiles associated with walking induce rapidly changing strain rates in the heel pad and create irregular stress-strain curves that differ notably from those previously reported for standard materials tests. Moreover, in vivo viscous properties of the heel pad at physiologically relevant strain rates fall between those commonly cited by studies using conventional quasi-static and ballistic loading methods, despite similar absolute deformations. Given that deformation of the fat pad (11 ± 2 mm) in the current study is similar to that previously reported for barefoot running, it would appear that the heel pad operates near its physiological maximum, even during walking

Errors in measuring sagittal arch kinematics of the human foot with digital fluoroscopy

Although fluoroscopy has been used to evaluate motion of the foot during gait, the accuracy and precision of fluoroscopic measures of osseous structures of the foot has not been reported in the literature. This study reports on a series of experiments that quantify the magnitude and sources of error involved in digital fluoroscopic measurements of the medial longitudinal arch. The findings indicate that with a global distortion correction procedure, errors arising from image distortion can be reduced threefold to 0.2° for angular measurements and to 0.1 mm for linear measures. The limits of agreement for repeated angular measures of the calcaneus and first metatarsal were ±0.5° and ±0.6°, indicating that measurement error was primarily associated with the manual process of digitisation. While the magnitude of the residual error constitutes about ±2.5% of the expected 20° of movement of the calcaneus and first metatarsal, out-of-plane rotation may potentially contribute the greatest source of error in fluoroscopic measures of the foot. However, even at the extremes of angular displacement (15°) reported for the calcaneum during running gait, the root mean square (RMS) error was only about 1°. Thus, errors associated with fluoroscopic imaging of the foot appear to be negligible when compared to those arising from skin movement artefact, which typically range between 1.5 and 4 mm (equating to errors of 2° to 17° for angular measures). Fluoroscopy, therefore, may be a useful technique for analysing the sagittal movement of the medial longitudinal arch during the contact phase of walking

Non-contact ACL injuries: association with clegg soil impact test values for sports fields, soil moisture and prevailing weather

Introduction: There is a recognised relationship between dry weather conditions and increased risk of anterior cruciate ligament (ACL) injury. Previous studies have identified 28 day evaporation as an important weather-based predictor of non-contact ACL injuries in professional Australian Football League matches. The mechanism of non-contact injury to the ACL is believed to increased traction and impact forces between footwear and playing surface. Ground hardness and the amount and quality of grass are factors that would most likely influence this and are inturn, related to the soil moisture content and prevailing weather conditions. This paper explores the relationship between soil moisture content, preceding weather conditions and the Clegg Soil Impact Test (CSIT) which is an internationally recognised standard measure of ground hardness for sports fields. Methodology: The 2.25 kg Clegg Soil Impact Test and a pair of 12 cm soil moisture probes were used to measure ground hardness and percentage moisture content. Five football fields were surveyed at 13 prescribed sites just before seven football matches from October 2008 to January 2009 (an FC Women's W-League team). Weather conditions recorded at the nearest weather station were obtained from the Bureau of Meteorology website and total rainfall less evaporation was calculated for 7 and 28 days prior to each match. All non-contact injuries occurring during match play and their location on the field were recorded. Results/conclusions: Ground hardness varied between CSIT 5 and 17 (x10G) (8 is considered a good value for sports fields). Variations within fields were typically greatest in the centre and goal areas. Soil moisture ranged from 3 to 40% with some fields requiring twice the moisture content of others to maintain similar CSIT values. There was a non-linear, negative relationship for ground hardness versus moisture content and a linear relationship with weather (R2, of 0.30 and 0.34, respectively). Three non-contact ACL injuries occurred during the season. Two of these were associated with hard and variable ground conditions

Achilles tendon loading patterns during barefoot walking and slow running on a treadmill: An ultrasonic propagation study

Measurement of tendon loading patterns during gait is important for understanding the pathogenesis of tendon "overuse" injury. Given that the speed of propagation of ultrasound in tendon is proportional to the applied load, this study used a noninvasive ultrasonic transmission technique to measure axial ultrasonic velocity in the right Achilles tendon of 27 healthy adults (11 females and 16 males; age, 26 ± 9 years; height, 1.73 ± 0.07 m; weight, 70.6 ± 21.2 kg), walking at self-selected speed (1.1 ± 0.1 m/s), and running at fixed slow speed (2 m/s) on a treadmill. Synchronous measures of ankle kinematics, spatiotemporal gait parameters, and vertical ground reaction forces were simultaneously measured. Slow running was associated with significantly higher cadence, shorter step length, but greater range of ankle movement, higher magnitude and rate of vertical ground reaction force, and higher ultrasonic velocity in the tendon than walking (P < 0.05). Ultrasonic velocity in the Achilles tendon was highly reproducible during walking and slow running (mean within-subject coefficient of variation < 2%). Ultrasonic maxima (P1, P2) and minima (M1, M2) were significantly higher and occurred earlier in the gait cycle (P1, M1, and M2) during running than walking (P < 0.05). Slow running was associated with higher and earlier peaks in loading of the Achilles tendon than walking