Increased Vertical Impact Forces and Altered Running Mechanics with Softer Midsole Shoes

To date it has been thought that shoe midsole hardness does not affect vertical impact peak forces during running. This conclusion is based partially on results from experimental data using homogeneous samples of participants that found no difference in vertical impact peaks when running in shoes with different midsole properties. However, it is currently unknown how apparent joint stiffness is affected by shoe midsole hardness. An increase in apparent joint stiffness could result in a harder landing, which should result in increased vertical impact peaks during running. The purpose of this study was to quantify the effect of shoe midsole hardness on apparent ankle and knee joint stiffness and the associated vertical ground reaction force for age and sex subgroups during heel-toe running. 93 runners (male and female) aged 16-75 years ran at 3.33 ± 0.15 m/s on a 30 m-long runway with soft, medium and hard midsole shoes. The vertical impact peak increased as the shoe midsole hardness decreased (mean(SE); soft: 1.70BW(0.03), medium: 1.64BW(0.03), hard: 1.54BW(0.03)). Similar results were found for the apparent ankle joint stiffness where apparent stiffness increased as the shoe midsole hardness decreased (soft: 2.08BWm/º x 100 (0.05), medium: 1.92 BWm/º x 100 (0.05), hard: 1.85 BWm/º x 100 (0.05)). Apparent knee joint stiffness increased for soft (1.06BWm/º x 100 (0.04)) midsole compared to the medium (0.95BWm/º x 100 (0.04)) and hard (0.96BWm/º x 100 (0.04)) midsoles for female participants. The results from this study confirm that shoe midsole hardness can have an effect on vertical impact force peaks and that this may be connected to the hardness of the landing. The results from this study may provide useful information regarding the development of cushioning guidelines for running shoes.     

Shoe midsole hardness, sex and age effects on lower extremity kinematics during running

Previous studies investigating the effects of shoe midsole hardness on running kinematics have often used male subjects from within a narrow age range. It is unknown whether shoe midsole hardness has the same kinematic effect on male and female runners as well as runners from different age categories. As sex and age have an effect on running kinematics, it is important to understand if shoe midsole hardness affects the kinematics of these groups in a similar fashion. However, current literature on the effects of sex and age on running kinematics are also limited to a narrow age range distribution in their study population. Therefore, this study tested the influence of three different midsole hardness conditions, sex and age on the lower extremity kinematics during heel-toe running. A comprehensive analysis approach was used to analyze the lower-extremity kinematic gait variables for 93 runners (male and female) aged 16-75 years. Participants ran at 3.33±0.15 m/s on a 30 m-long runway with soft, medium and hard midsoles. A principal component analysis combined with a support vector machine showed that running kinematics based on shoe midsole hardness, sex, and age were separable and classifiable. Shoe midsole hardness demonstrated a subject-independent effect on the kinematics of running. Additionally, it was found that age differences affected the more dominant movement components of running compared to differences due to the sex of a runner.     

Computer simulation of the effects of shoe cushioning on internal and external loading during running impacts

Biomechanical aspects of running injuries are often inferred from external loading measurements. However, previous research has suggested that relationships between external loading and potential injury-inducing internal loads can be complex and nonintuitive. Further, the loading response to training interventions can vary widely between subjects. In this study, we use a subject-specific computer simulation approach to estimate internal and external loading of the distal tibia during the impact phase for two runners when running in shoes with different midsole cushioning parameters. The results suggest that: (1) changes in tibial loading induced by footwear are not reflected by changes in ground reaction force (GRF) magnitudes; (2) the GRF loading rate is a better surrogate measure of tibial loading and stress fracture risk than the GRF magnitude; and (3) averaging results across groups may potentially mask differential responses to training interventions between individuals.     

Effects of Forefoot Shoe on Knee and Ankle Loading during Running in Male Recreational Runners

Objectives: Although overuse running injury risks for the ankle and knee are high, the effect of different shoe designs on Achilles tendon force (ATF) and Patellofemoral joint contact force (PTF) loading rates are unclear. Therefore, the primary objective of this study was to compare the ATF at the ankle and the PTF and Patellofemoral joint stress force (PP) at the knee using different running shoe designs (forefoot shoes vs. normal shoes). Methods: Fourteen healthy recreational male runners were recruited to run over a force plate under two shoe conditions (forefoot shoes vs. normal shoes). Sagittal plane ankle and knee kinematics and ground reaction forces were simultaneously recorded. Ankle joint mechanics (ankle joint angle, velocity, moment and power) and the ATF were calculated. Knee joint mechanics (knee joint angle velocity, moment and power) and the PTF and PP were also calculated. Results: No significant differences were observed in the PTF, ankle plantarflexion angle, ankle dorsiflexion power, peak vertical active force, contact time and PTF between the two shoe conditions. Compared to wearing normal shoes, wearing the forefoot shoes demonstrated that the ankle dorsiflexion angle, knee flexion velocity, ankle dorsiflexion moment extension, knee extension moment, knee extension power, knee flexion power and the peak patellofemoral contact stress were significantly reduced. However, the ankle dorsiflexion velocity, ankle plantarflexion velocity, ankle plantarflexion moment and Achilles tendons force increased significantly. Conclusions: These findings suggest that wearing forefoot shoes significantly decreases the patellofemoral joint stress by reducing the moment of knee extension, however the shoes increased the ankle plantarflexion moment and ATF force. The forefoot shoes effectively reduced the load on the patellofemoral joint during the stance phase of running. However, it is not recommended for new and novice runners and patients with Achilles tendon injuries to wear forefoot shoes.     

How do rocker-soled shoes influence the knee adduction moment in people with knee osteoarthritis? An analysis of biomechanical mechanisms

The primary objective was to examine mechanisms behind previously observed changes in the knee adduction moment (KAM) with rocker-soled shoes, in participants sub-grouped according to whether they experienced an immediate decrease, or increase, in peak KAM. In subgroups where frontal plane knee ground reaction force (GRF) lever-arm emerged as a significant predictor, a secondary aim was to examine biomechanical factors that contributed to change in this parameter. Thirty individuals with symptomatic, radiographic knee osteoarthritis (OA) underwent 3D gait analysis in unstable rocker-soled shoes and non-rocker-soled shoes. Multiple regression analyses, within each subgroup, examined relationships between changes in frontal plane knee-GRF lever arm and frontal plane resultant GRF magnitude and changes in peak KAM and KAM impulse between shoe conditions. In the subgroup that decreased peak KAM with rocker-soled shoes (n = 23), change in knee-GRF lever arm and frontal plane GRF magnitude at peak KAM together were significant predictors of change in peak KAM; however, only change in mean knee-GRF lever arm significantly predicted change in KAM impulse. Decreased medial GRF magnitude, increased lateral trunk lean towards the stance limb and reduced varus/increased valgus hip-knee-ankle angle were associated with a lower knee-GRF lever arm in this group, with rocker-soled shoes. In contrast, none of the independent variables predicted changes in KAM in the subgroup who increased peak KAM with rocker-soled shoes (n = 7).     

The influence of footwear on foot motion during walking and running

There are evidences to suggest that wearing footwear constrains the natural barefoot motion during locomotion. Unlike prior studies that deduced foot motions from shoe sole displacement parameters, the aim of this study was to examine the effect of footwear motion on forefoot to rearfoot relative motion during walking and running. The use of a multi-segment foot model allowed accurate both shoe sole and foot motions (barefoot and shod) to be quantified. Two pairs of identical sandals with different midsole hardness were used. Ten healthy male subjects walked and ran in each of the shod condition.The results showed that for barefoot locomotion there was more eversion of the forefoot and it occurred faster than for shod locomotion. In this later condition, the range of eversion was reduced by 20% and the rate of eversion in late stance by 60% in comparison to the barefoot condition. The sole constrained both the torsional (eversion/inversion) and adduction range of motion of the foot. Interestingly, during the push-off phase of barefoot locomotion the rate and direction of forefoot torsion varied between individuals. However, most subjects displayed a forefoot inversion direction of motion while shod. Therefore, this experiment showed that the shoes not only restricted the natural motion of the barefoot but also appeared to impose a specific foot motion pattern on individuals during the push-off phase. These findings have implications for the matching of footwear design characteristics to individual natural foot function.     

The effect of muscle stiffness and damping on simulated impact force peaks during running.

It has been frequently reported that vertical impact force peaks during running change only minimally when changing the midsole hardness of running shoes. However, the underlying mechanism for these experimental observations is not well understood. An athlete has various possibilities to influence external and internal forces during ground contact (e.g. landing velocity, geometrical alignment, muscle tuning, etc.). The purpose of this study was to discuss one possible strategy to influence external impact forces acting on the athlete's body during running, the strategy to change muscle activity (muscle tuning). The human body was modeled as a simplified mass-spring-damper system. The model included masses of the upper and the lower bodies with each part of the body represented by a rigid and a non-rigid wobbling mass. The influence of mechanical properties of the human body on the vertical impact force peak was examined by varying the spring constants and damping coefficients of the spring-damper units that connected the various masses. Two types of shoe soles were modeled using a non-linear force deformation model with two sets of parameters based on the force-deformation curves of pendulum impact experiments. The simulated results showed that the regulation of the mechanical coupling of rigid and wobbling masses of the human body had an influence on the magnitude of the vertical impact force, but not on its loading rate. It was possible to produce the same impact force peaks altering specific mechanical properties of the system for a soft and a hard shoe sole. This regulation can be achieved through changes of joint angles, changes in joint angular velocities and/or changes in muscle activation levels in the lower extremity. Therefore, it has been concluded that changes in muscle activity (muscle tuning) can be used as a possible strategy to affect vertical impact force peaks during running.     

Local plantar pressure relief in therapeutic footwear: design guidelines from finite element models

A major goal of therapeutic footwear in patients with pain or those at risk for skin injury is to relieve focal loading under prominent metatarsal heads. One frequent approach is to place plugs of compliant material into the midsole of the shoe. This study investigated 36 plug designs, a combination of three materials, six geometries, and two placements using a two-dimensional (2D) finite element model. Realistic loading conditions were obtained from plantar pressures (PP) recorded during walking in five subjects who wore control midsoles manufactured using Microcell Puff. Measured peak pressures underneath the second metatarsal head were similar to the results of the control model. PP obtained from simulations with the plugs built into a firm midsole were compared to the simulation results of the control midsole. Large plugs (e.g. 40 mm width), made out of Microcell Puff Lite or Plastazote Medium, placed at peak pressure sites, resulted in highest reductions in peak pressures (18-28%). Smaller plugs benefited from tapering when placed at high pressure areas. Case studies were completed on a healthy male subject and a diabetic female patient to address the efficacy of a plug design favored by our simulations (pressure based placement, 40 x 20 mm, Plastazote Medium). Successful reductions of second metatarsal head pressures were observed with a mediolateral load redistribution that was not represented by our model. 2D computer simulations allowed systematic investigation of plug properties without the need for high volume experimentation on human subjects and established basic guidelines for plug selection. In particular, plugs that are placed based on plantar pressure measurements were proven to be more effective when compared to those positioned according to the projection of the bony landmark on the foot-shoe plantar contact area.     

Antagonist muscle co-contraction during a double-leg landing maneuver at two heights

Background: Knee injuries are common during landing activities. Greater landing height increases peak ground reaction forces (GRFs) and loading at the knee joint. As major muscles to stabilize the knee joint, Quadriceps and Hamstring muscles provide internal forces to attenuate the excessive GRF. Despite the number of investigations on the importance of muscle function during landing, the role of landing height on these muscles forces using modeling during landing is not fully investigated. Methods: Participant-specific musculoskeletal models were developed using experimental motion analysis data consisting of anatomic joint motions and GRF from eight male participants performing double-leg drop landing from 30 and 60 cm. Muscle forces were calculated in OpenSim and their differences were analyzed at the instances of high risk during landing i.e. peak GRF for both heights. Results: The maximum knee flexion angle and moments were found significantly higher from a double-leg landing at 60 cm compared to 30 cm. The results showed elevated GRF, and mean muscle forces during landing. At peak GRF, only quadriceps showed significantly greater forces at 60 cm. Hamstring muscle forces did not significantly change at 60 cm compared to 30 cm. Conclusions: Quadriceps and hamstring muscle forces changed at different heights. Since hamstring forces were similar in both landing heights, this could lead to an imbalance between the antagonist muscles, potentially placing the knee at risk of injury if combined with small flexion angles that was not observed at peak GRF in our study. Thus, enhanced neuromuscular training programs strengthening the hamstrings may be required to address this imbalance. These findings may contribute to enhance neuromuscular training programs to prevent knee injuries during landing.     

A mechanics comparison between landing from a countermovement jump and landing from stepping off a box

It is common practice to study jump landing mechanics by having subjects step off a box set at a certain height instead of landing from a jump. This practice assumes that the landing mechanics are similar between stepping off a box and a countermovement jump as long as the heights can be matched. The mechanics of the two methods had never been compared when landing from identical heights. Thus, the purpose of this study was to compare the mechanics of landing from a countermovement jump to landing from a step-off. Participants performed three maximal countermovement jumps. The mechanics of one countermovement jump was compared with a center of mass fall height matched step-off landing. The step-off landing showed a more rapid time to peak ground reaction force (GRF) in both genders and greater GRF peak and loading rate in males only. No difference was observed between joint angles at initial contact; however, the countermovement jump showed significantly greater joint flexion angles at peak GRF for both genders. EMG showed greater muscle activity during the countermovement jump condition in all subjects. It was concluded that countermovement jump landings are different from step-off landings; thus, results from analyses involving step-off landings should be taken with caution if the aim is to relate them to landing from a jump.     

Effect of landing height on frontal plane kinematics, kinetics and energy dissipation at lower extremity joints

Lack of the necessary magnitude of energy dissipation by lower extremity joint muscles may be implicated in elevated impact stresses present during landing from greater heights. These increased stresses are experienced by supporting tissues like cartilage, ligaments and bones, thus aggravating injury risk. This study sought to investigate frontal plane kinematics, kinetics and energetics of lower extremity joints during landing from different heights. Eighteen male recreational athletes were instructed to perform drop-landing tasks from 0.3- to 0.6-m heights. Force plates and motion-capture system were used to capture ground reaction force and kinematics data, respectively. Joint moment was calculated using inverse dynamics. Joint power was computed as a product of joint moment and angular velocity. Work was defined as joint power integrated over time. Hip and knee joints delivered significantly greater joint power and eccentric work (p<0.05) than the ankle joint at both landing heights. Substantial increase (p<0.05) in eccentric work was noted at the hip joint in response to increasing landing height. Knee and hip joints acted as key contributors to total energy dissipation in the frontal plane with increase in peak ground reaction force (GRF). The hip joint was the top contributor to energy absorption, which indicated a hip-dominant strategy in the frontal plane in response to peak GRF during landing. Future studies should investigate joint motions that can maximize energy dissipation or reduce the need for energy dissipation in the frontal plane at the various joints, and to evaluate their effects on the attenuation of lower extremity injury risk during landing.     

A comparison of forefoot stiffness in running and running shoe bending stiffness

This study characterizes the stiffness of the human forefoot during running. The forefoot stiffness, defined as the ratio of ground reaction moment to angular deflection of the metatarsophalangeal joint, is measured for subjects running barefoot. The joint deflection is obtained from video data, while the ground reaction moment is obtained from force plate and video data. The experiments show that during push-off, the forefoot stiffness rises sharply and then decreases steadily, showing that the forefoot behaves not as a simple spring, but rather as an active mechanism that exhibits a highly time-dependent stiffness. The forefoot stiffness is compared with the bending stiffness of running shoes. For each of four shoes tested, the shoe stiffness is relatively constant and generally much lower than the mean human forefoot stiffness. Since forefoot stiffness and shoe bending stiffness act in parallel (i.e., are additive), the total forefoot stiffness of the shod foot is dominated by that of the human foot.     

Effects of insoles and additional shock absorption foam on the cushioning properties of sport shoes

The purpose of this study was to investigate the effects of insoles and additional shock absorption foam on the cushioning properties of various sport shoes with an impact testing method. Three commercial sport shoes were used in this study, and shock absorption foam (TPE5020; Vers Tech Science Co. Ltd., Taiwan) with 2-mm thickness was placed below the insole in the heel region for each shoe. Eight total impacts with potential energy ranged from 1.82 to 6.08 J were performed onto the heel region of the shoe. The order of testing conditions was first without insole, then with insole, and finally interposing the shock absorption foam for each shoe. Peak deceleration of the striker was measured with an accelerometer attached to the striker during impact. The results of this study seemed to show that the insole or additional shock absorption foam could perform its shock absorption effect well for the shoes with limited midsole cushioning. Further, our findings showed that insoles absorbed more, even up to 24-32% of impact energy under low impact energy. It seemed to indicate that insoles play a more important role in cushioning properties of sport shoes under a low impact energy condition.     

Landing from different heights: Biomechanical and neuromuscular strategies in trained gymnasts and untrained prepubescent girls

The purpose of this study was to examine the biomechanics of the lower limb, during landing in female prepubertal gymnasts and prepubertal untrained girls, aged 9–12 years. Ten healthy participants were included in each group and performed five landings from 20, 40, and 60 cm. Kinematics, ground reaction forces (GRF) and electromyogram (EMG) from the lateral gastrocnemius, tibialis anterior, and vastus lateralis are presented. Gymnasts had higher vertical GRF and shorter braking phase during landing. Compared to untrained girls, gymnasts exhibited for all examined drop heights more knee flexion before and at ground contact, but less knee flexion at maximum knee flexion position. Especially when increasing drop heights the gymnasts activated their examined muscles earlier, and generally they had higher pre- and post landing EMG amplitudes normalized to the peak EMG at 60 cm drop height. Furthermore, gymnasts had lower antagonist EMG for the tibialis anterior compared to untrained girls, especially when landing from higher heights. It is concluded that the landing strategy preferred by gymnasts is influenced by long-term and specialized training and induces a stiffer landing pattern. This could have implications in injury prevention, which requires further investigation.     

Heel-shoe interactions and the durability of EVA foam running-shoe midsoles

A finite element analysis (FEA) was made of the stress distribution in the heelpad and a running shoe midsole, using heelpad properties deduced from published force-deflection data, and measured foam properties. The heelpad has a lower initial shear modulus than the foam (100 vs. 1050 kPa), but a higher bulk modulus. The heelpad is more non-linear, with a higher Ogden strain energy function exponent than the foam (30 vs. 4). Measurements of plantar pressure distribution in running shoes confirmed the FEA. The peak plantar pressure increased on average by 100% after 500 km run. Scanning electron microscopy shows that structural damage (wrinkling of faces and some holes) occurred in the foam after 750 km run. Fatigue of the foam reduces heelstrike cushioning, and is a possible cause of running injuries.     

Kinematic adaptations to a variable stiffness shoe: mechanisms for reducing joint loading

A recently described variable-stiffness shoe has been shown to reduce the adduction moment and pain in patients with medial-compartment knee osteoarthritis. The mechanism associated with how this device modifies overall gait patterns to reduce the adduction moment is not well understood. Yet this information is important for applying load modifying intervention for the treatment of knee osteoarthritis. A principal component analysis (PCA) was used to test the hypothesis that there are differences in the frontal plane kinematics that are correlated with differences in the ground reaction forces (GRFs) and center of pressure (COP) for a variable-stiffness compared to a constant-stiffness control shoe. Eleven healthy adults were tested in a constant-stiffness control shoe and a variable-stiffness shoe while walking at self-selected speeds. The PCA was performed on trial vectors consisting of all kinematic, GRF and COP data. The projection of trial vectors onto the linear combination of four PCs showed there were significant differences between shoes. The interpretation of the PCs indicated an increase in the ankle eversion, knee abduction and adduction, decreases in the hip adduction and pelvic obliquity angles and reduced excursion of both the COP and peak medial-lateral GRFs for the variable-stiffness compared to the control shoe. The variable-stiffness shoe produced a unique dynamic change in the frontal plane motion of the ankle, hip and pelvis that contributed to changes in the GRF and COP and thus reduced the adduction moment at a critical instant during gait suggesting a different mechanism that was seen with fixed interventions (e.g. wedges).     

Orthotic effect of a stabilising mechanism in the surface of gymnastic mats on foot motion during landings.

The purpose of this study was to examine two hypotheses: (a) mat hardness affects foot motion during landing; (b) the influence of a surface stabilising interface integrated in a mat on foot motion is detectable. Two studies were carried out: In the first one, six female gymnasts performed barefoot landings from different falling heights onto three mats having different hardness. In the second study, a stabilising mechanism was integrated in the surface of three new mats with different hardness. Three high speed video cameras (250Hz) captured the motion of the left leg and foot. These were modelled by means of a four rigid body system. The maximal eversion at the ankle joint was not influenced by the different mats (hard: 4.6 degrees +/-1.9 to 9.3 degrees +/-3.4, medium: 3.1 degrees +/-2.7 to 7.4 degrees +/-3.5, soft: 4.8 degrees +/-2.1 to 8.4 degrees +/-3.5). The soft mat without the stabilised surface showed higher eversion values (p<0.05) between forefoot and rearfoot (medial joint: hard: 5.1 degrees +/-3.2 to 7.3 degrees +/-3.3, medium: 6.9 degrees +/-3.1 to 7.5 degrees +/-2.9, soft: 12.7 degrees +/-4.1 to 13.4 degrees +/-3.3; lateral joint: hard: 8.5 degrees +/-3.1 to 9.7 degrees +/-1.1, medium: 9.5 degrees +/-2.6 to 11.2 degrees +/-3.3, soft: 12.1 degrees +/-2.3 to 15.7 degrees +/-3.3). For the mats with the surface stabilising interface, the different hardness did not cause any significant differences in maximal eversion values at the medial (hard: 1.5 degrees +/-3.3 to 5.5 degrees +/-4.5, medium: 1.3 degrees +/-3.5 to 5.1 degrees +/-3.6, soft: 0.7 degrees +/-4.9 to 5.4 degrees +/-4.2) nor at the lateral (hard: 11.3 degrees +/-4.2 to 17.3 degrees +/-4.2, medium: 12.3 degrees +/-4.8 to 17.1 degrees +/-3.7, soft: 11.5 degrees +/-4.6 to 17.1 degrees +/-4.3) forefoot joints. The structure of the mat and the consequent deformation hollow did not influence the kinematics of the ankle joint during landings, but it influenced the motion at the medial and the lateral forefoot joints. By means of a stabilised surface, it is possible to reduce the influence of mat deformation on the maximal eversion between forefoot and rearfoot.     

An analysis of the mechanisms for reducing the knee adduction moment during walking using a variable stiffness shoe in subjects with knee osteoarthritis

Variable stiffness shoes that have a stiffer lateral than medial sole may reduce the external knee adduction moment (EKAM) and pain during walking in patients with medial compartment knee osteoarthritis (OA). However, the mechanism by which EKAM may be reduced in the OA knee with this intervention remains unclear. Three hypotheses were tested in this study: (1) The reduction in EKAM during walking with the variable stiffness shoe is associated with a reduction in GRF magnitude and/or (2) frontal plane lever arm. (3) A reduction in frontal plane lever arm occurs either by moving the center of pressure laterally under the shoe and/or by dynamically reducing the medial component of GRF. Thirty-two subjects (20 male, 12 female; age: 58.7 ± 9.3 years; height: 1.62 ± 0.08 m; mass: 81.3 ± 14.6 kg) with medial compartment knee osteoarthritis were studied walking in a gait laboratory. The frontal plane lever arm was significantly reduced (1.62%, 0.07%ht, p=0.02) on the affected side while the magnitude of the GRF was not significantly changed. The reduction in the lever arm was weakly correlated with a medial shift in the COP. However, the combined medial shift in the COP and reduction in the medial GRF explained 50% of the change of the frontal plane lever arm. These results suggest that the medial shift in the COP at the foot produced by the intervention shoe stimulates an adaptive dynamic response during gait that reduces the frontal plane lever arm.     

Physiological cost of running while wearing spring-boots

The purpose of the study was to investigate the physiological cost of running in spring-boots compared with running in running shoes at different speeds. During testing, subjects (n = 7) completed running trials while wearing spring-boots and running shoes. Three speed conditions (2.23, 2.68, and 3.13 m.s(-1)) were completed per shoe condition (i.e., spring-boots and running shoes). Rate of oxygen consumption (Vo(2)), heart rate (HR), rating of perceived exertion (RPE), and stride frequency were recorded for each condition. Order of shoe conditions was balanced, with speeds tested continuously from slow to fast. There was no difference in Vo(2), HR, or RPE between shoe conditions across speeds (p > 0.05). Stride frequency was lower during running in spring-boots vs. running shoes at each speed (speed of spring-boots vs. running shoes for 2.23 m x s(-1): 69.9 +/- 2.9 strides x min(-1) vs. 75.6 +/- 3.5 strides x min(-1); for 2.68 m x s(-1): 71.3 +/- 5.2 strides x min(-1) vs. 79.4 +/- 5.0 strides x min(-1); for 3.13 m x s(-1): 73.6 +/- 7.3 strides x min(-1) vs. 83.1 +/- 8.2 strides x min(-1); p < 0.05). Despite the added mass to the lower extremity and change in stride frequency during running in spring-boots, the physiological cost of running was similar to that of running in running shoes. Exercising while running in spring-boots may provide less impact force with no change in running economy.     

A variable-stiffness shoe lowers the knee adduction moment in subjects with symptoms of medial compartment knee osteoarthritis

The purpose of this study was to evaluate the effectiveness of variable-stiffness shoes in lowering the peak external knee adduction moment during walking in subjects with symptomatic medial compartment knee osteoarthritis. The influence on other lower extremity joints was also investigated. The following hypotheses were tested: (1) variable-stiffness shoes will lower the knee adduction moment in the symptomatic knee compared to control shoes; (2) reductions in knee adduction moment will be greater at faster speeds; (3) subjects with higher initial knee adduction moments in control shoes will have greater reductions in knee adduction moment with the intervention shoes; and (4) variable-stiffness shoes will cause secondary changes in the hip and ankle frontal plane moments. Seventy-nine individuals were tested at self-selected slow, normal, and fast speeds with a constant-stiffness control shoe and a variable-stiffness intervention shoe. Peak moments for each condition were assessed using a motion capture system and force plate. The intervention shoes reduced the peak knee adduction moment compared to control at all walking speeds, and reductions increased with increasing walking speed. The magnitude of the knee adduction moment prior to intervention explained only 11.9% of the variance in the absolute change in maximum knee adduction moment. Secondary changes in frontal plane moments showed primarily reductions in other lower extremity joints. This study showed that the variable-stiffness shoe reduced the knee adduction moment in subjects with medial compartment knee osteoarthritis without the discomfort of a fixed wedge or overloading other joints, and thus can potentially slow the progression of knee osteoarthritis.