Peak impact accelerations during track and treadmill running.

To determine whether peak vertical and horizontal impact accelerations were different while running on a track or on a treadmill, 12 healthy subjects (average age 32.8 ± 9.8 y), were fitted with a novel, wireless accelerometer capable of recording triaxial acceleration over time. The accelerometer was attached to a custom-made acrylic plate and secured at the level of the L5 vertebra via a tight fitting triathlon belt. Each subject ran 4 miles on a synthetic, indoor track at a self-selected pace and accelerations were recorded on three perpendicular axes. Seven days later, the subjects ran 4 miles on a treadmill set at the individual runner's average pace on the track and the peak vertical and horizontal impact magnitudes between the track and treadmill were compared. There was no difference (P = .52) in the average peak vertical impact accelerations between the track and treadmill over the 4 mile run. However, peak horizontal impact accelerations were greater (P = .0012) on the track when compared with the treadmill. This study demonstrated the feasibility for long-term impact accelerations monitoring using a novel wireless accelerometer.

Infrapatellar Straps Decrease Patellar Tendon Strain at the Site of the Jumper's Knee Lesion: A Computational Analysis Based on Radiographic Measurements.

BACKGROUND: The impetus for the use of patellar straps in the treatment of patellar tendinopathy has largely been based on empirical evidence and not on any mechanistic rationale. A computational model suggests that patellar tendinopathy may be a result of high localized tendon strains that occur at smaller patella-patellar tendon angles (PPTAs). HYPOTHESIS: Infrapatellar straps will decrease the mean localized computational strain in the area of the patellar tendon commonly involved in jumper's knee by increasing the PPTA. STUDY DESIGN: Controlled laboratory study. METHODS: Twenty adult males had lateral weightbearing and nonweightbearing radiographs of their knees taken with and without 1 of 2 infrapatellar straps at 60° of knee flexion. Morphologic measurements of PPTA and patellar tendon length with and without the straps were used as input data into a previously described computational model to calculate average and maximum strain at the common location of the jumper's knee lesion during a simulated jump landing. RESULTS: The infrapatellar bands decreased the predicted localized strain (average and maximum) in the majority of participants by increasing PPTA and/or decreasing patellar tendon length. When both PPTA and patellar tendon length were altered by the straps, there was a strong and significant correlation with the change in predicted average localized strain with both straps. CONCLUSION: Infrapatellar straps may limit excessive patella tendon strain at the site of the jumper's knee lesion by increasing PPTA and decreasing patellar tendon length rather than by correcting some inherent anatomic or functional abnormality in the extensor apparatus. CLINICAL RELEVANCE: The use of infrapatellar straps may help prevent excessive localized tendon strains at the site of the jumper's knee lesion during a jump landing.

A preliminary study of patellar tendon torques during jumping.

The etiology of patellar tendinopathy (jumper's knee) has been attributed to a significant increase in patellar tendon torques associated with jumping. While some investigators have suggested that patellar tendon torques are greater during takeoff, little is known about the relative magnitudes of patellar tendon torques during takeoff and landing. We hypothesized that peak patellar tendon torques are greater in jump takeoff than in landing, and that there is a linear correlation between jump height and peak patellar tendon torque. Seven asymptomatic, recreational male athletes each performed a series of 21 jumps ranging from low to maximal height. A calibrated fiber-optic sensor, implanted transversely within the patellar tendon was used to measure the knee torque during takeoff and landing. There was no significant difference in the peak patellar tendon torque experienced during takeoff and landing within individuals. There was a moderate correlation (r = .64) between maximum takeoff patellar tendon torques and jump height. There was a weak correlation (r = .52) between maximum landing patellar tendon torques and jump height. There was a moderate correlation (r = .67) between maximum 60 degrees/s isokinetic extension torque and maximum jump height. The lack of a strong correlation between jump height and patellar tendon forces during take-off or landing suggests that these forces may be technique dependent. Therefore, modifying takeoff and/or landing techniques could reduce patellar tendon force and potentially lessen the incidence of patellar tendinopathy.

Patellar tendon strain is increased at the site of the jumper's knee lesion during knee flexion and tendon loading: results and cadaveric testing of a computational model.

BACKGROUND: Patellar tendinopathy (jumper's knee) is characterized by localized tenderness of the patellar tendon at its origin on the inferior pole of the patella and a characteristic increase in signal intensity on magnetic resonance imaging at this location. However, it is unclear why the lesion typically occurs in this area of the patellar tendon as surface strain gauge studies of the patellar tendon through the range of motion have produced conflicting results. HYPOTHESIS: The predicted patellar tendon strains that occur as a result of the tendon loads and patella-patellar tendon angles (PPTAs) experienced during a jump landing will be significantly increased in the area of the patellar tendon associated with patellar tendinopathy. STUDY DESIGN: Descriptive laboratory study. METHODS: A 2-dimensional, computational, finite element model of the patella-patellar tendon complex was developed using anatomic measurements taken from lateral radiographs of a normal knee. The patella was modeled with plane strain rigid elements, and the patellar tendon was modeled with 8-node plane strain elements with neo-Hookean material properties. A tie constraint was used to join the patellar tendon and patella. Patella-patellar tendon angles corresponding to knee flexion angles between 0 degrees and 60 degrees and patellar tendon strains ranging from 5% to 15% were used as input variables into the computational model. To determine if the location of increased strain predicted by the computational model could produce isolated tendon fascicle damage in that same area, 5 human cadaveric patella-patellar tendon-tibia specimens were loaded under conditions predicted by the model to significantly increase localized tendon strain. Pre- and posttesting ultrasound images of the patella-patellar tendon specimens were obtained to document the location of any injured fascicles. RESULTS: Localized tendon strain at the classic location of the jumper's knee lesion was found to increase in association with an increase in the magnitude of applied patellar tendon strain and a decrease in the PPTA. The principal stresses and strains predicted by the model for this localized area were tensile and not compressive in nature. Applying the tendon strain conditions and PPTA predicted by the model to significantly increase localized strain resulted in disruption of tendon fascicles in 3 of the 5 cadaveric specimens at the classic location of the patellar tendinopathy lesion. CONCLUSION: The localized increase in patellar tendon strain that occurs in response to the application of tendon loads and decreased PPTA could induce microdamage at the classic location of the jumper's knee lesion. CLINICAL RELEVANCE: The association of decreasing PPTA with increasing localized tendon strain would implicate the role of knee-joint angle as well as tendon force in the etiopathogenesis of jumper's knee.

Correlation between ground reaction force and tibial acceleration in vertical jumping.

Modern electronics allow for the unobtrusive measurement of accelerations outside the laboratory using wireless sensor nodes. The ability to accurately measure joint accelerations under unrestricted conditions, and to correlate them with jump height and landing force, could provide important data to better understand joint mechanics subject to real-life conditions. This study investigates the correlation between peak vertical ground reaction forces, as measured by a force plate, and tibial axial accelerations during free vertical jumping. The jump heights calculated from force-plate data and accelerometer measurements are also compared. For six male subjects participating in this study, the average coefficient of determination between peak ground reaction force and peak tibial axial acceleration is found to be 0.81. The coefficient of determination between jump height calculated using force plate and accelerometer data is 0.88. Data show that the landing forces could be as high as 8 body weights of the jumper. The measured peak tibial accelerations ranged up to 42 g. Jump heights calculated from force plate and accelerometer sensors data differed by less than 2.5 cm. It is found that both impact accelerations and landing forces are only weakly correlated with jump height (the average coefficient of determination is 0.12). This study shows that unobtrusive accelerometers can be used to determine the ground reaction forces experienced in a jump landing. Whereas the device also permitted an accurate determination of jump height, there was no correlation between peak ground reaction force and jump height.

The correlation of segment accelerations and impact forces with knee angle in jump landing.

Impact forces and shock deceleration during jumping and running have been associated with various knee injury etiologies. This study investigates the influence of jump height and knee contact angle on peak ground reaction force and segment axial accelerations. Ground reaction force, segment axial acceleration, and knee angles were measured for 6 male subjects during vertical jumping. A simple spring-mass model is used to predict the landing stiffness at impact as a function of (1) jump height, (2) peak impact force, (3) peak tibial axial acceleration, (4) peak thigh axial acceleration, and (5) peak trunk axial acceleration. Using a nonlinear least square fit, a strong (r = 0.86) and significant (p < or = 0.05) correlation was found between knee contact angle and stiffness calculated using the peak impact force and jump height. The same model also showed that the correlation was strong (r = 0.81) and significant (p < or = 0.05) between knee contact angle and stiffness calculated from the peak trunk axial accelerations. The correlation was weaker for the peak thigh (r = 0.71) and tibial (r = 0.45) axial accelerations. Using the peak force but neglecting jump height in the model, produces significantly worse correlation (r = 0.58). It was concluded that knee contact angle significantly influences both peak ground reaction forces and segment accelerations. However, owing to the nonlinear relationship, peak forces and segment accelerations change more rapidly at smaller knee flexion angles (i.e., close to full extension) than at greater knee flexion angles.

A sub-microwatt piezo-floating-gate sensor for long-term fatigue monitoring in biomechanical implants.

In this paper we describe an implementation of a novel fatigue monitoring sensor based on integration of piezoelectric transduction with floating gate avalanche injection. The miniaturized sensor enables continuous battery-less monitoring and time-to-failure predictions of biomechanical implants. Measured results from a fabricated prototype in a 0.5 microm CMOS process indicate that the device can compute cumulative statistics of electrical signals generated by piezoelectric transducer, while consuming less that 1 microW of power. The ultra-low power operation makes the sensor attractive for integration with poly-vinylidene difluoride (PVDF) based transducers that have already proven to be biocompatible.