- Muscle-Tendon Lengthening
- Exercise Modality, Phosphorylation of p70
- Musculoskeletal Modeling & Computer Simulation Techniques
- An Investigation of the Interactions Between Lower Limb Bone Morphology, Limb Inertial Properties and Limb Dynamics
- Athletic Performance
- Soccer Throw-In
- Backpack Load Carriage
- Quantitative Ultrasound Imaging of Muscle-Tendon Units
- Lateral Force Transmission
- Dynamic Creep
- Achilles Tendon Stiffness and Hysteresis During Cyclic Loading
- Gastrocnemius/Soleus Muscle Force Contributions to Ankle Plantar Flexion Torque
- Ground Reaction Force Estimation from Hip Acceleration Data
- Injury Prevention
Summary of Selected Research Projects:
Research projects may be self contained and terminal or they may be part of a sequence of studies designed to answer fundamental questions. Our ultimate goal is to further our understanding of the interactions between physical activity, human movement performance, fitness and musculoskeletal injury risk. Research conducted to achieve this goal involves animal models if appropriate, computer modeling and movement simulation, instrumentation design, human testing.
Following are examples of some of the research conducted in the laboratory:
Background: Various neuromuscular disorders lead to joint stiffness and impaired joint function. Orthopaedic surgeons often intervene in these cases and surgically lengthen the muscle-tendon unit or units causing the joint stiffness. There are a variety of surgical procedures used to lengthen muscle-tendon units; however, controlled studies of the affects of these procedures on muscle-tendon performance have not been conducted.
Purpose: The purpose of this study was to investigate the use of an animal model to quantify differences in muscle-tendon structure and performance after being lengthened by one of two alternative procedures.
Methods: Ten rat gastrocnemius muscle-tendon (MT) units were lengthened ~3 mm by cutting the aponeurosis of the medial head. An additional 10 MT units were lengthened by performing a Z-cut to the Achilles tendon. A sham operation was performed on the contralateral MT unit of each rat. Following three weeks of recovery, muscle and tendon lengths and the force-ankle angle relationship were quantified. Data were compared between paired MT units within each rat and within each surgical intervention group.
Exercise Modality, Phosphorylation of p70s6k, and Skeletal Muscle Hypertrophy
Background: Skeletal muscle strength is essential for human movement performance and quality of life. Understanding the signaling mechanisms responsible for maintaining and increasing muscle strength is fundamental for identifying strategies to combat sarcopenia and to enhance muscle strength and movement performance.
Purpose: The objective of this study was to determine if p70s6k phosphorylation, a ribosomal protein suggested to be essential for up-regulation of exercise-induced muscle protein synthesis, was dependent on the mode of resistance exercise. The hypothesis tested was: resistance exercise results in p70s6k phosphorylation, independent of the mode of resistance exercise (e.g. isometric, eccentric).
Methods: Two groups (N=5 each) of Female Sprague Dawley rats, ~12 weeks old, were tested. Rats were anesthetized and indwelling electrodes used to stimulate the right hind limb muscles via the sciatic nerve. The tibialis anterior (TA) muscle of Group 1 rats were exposed to 3 sets of 10 isometric resistance repetitions while the TA of Group 2 rats were exposed to 3 sets of 10 lengthening resistance repetitions. Contralateral TA muscles served as non-exercised controls. Rats were euthanized 6 hours post exercise, muscles harvested and muscle samples processed to determine the extent of p70s6k phosphorylation.
Musculoskeletal Modeling & Computer Simulation Techniques
Are Fixed Limb Inertial Models Valid for Dynamic Simulations of Human Movement?
Background: During human movement, muscle activation and limb movement result in subtle changes in muscle mass distribution. Muscle mass redistribution can affect limb inertial properties and limb dynamics, but it is not currently known to what extent. The objectives of this study were to investigate: (1) how physiological alterations of muscle and tendon length affect limb inertial characteristics, and (2) how such changes affect dynamic simulations of human movement.
Methods: A digital model of a human leg, custom software, and Software for Interactive Musculoskeletal Modeling were used to simulate mass redistribution of muscle-tendon structures within a limb segment during muscle activation and joint movement. Thigh and shank center of mass and moments of inertia for different muscle activation and joint configurations were determined and compared. Limb inertial parameters representing relaxed muscles and fully active muscles were input into a simulated straight-leg movement to evaluate the effect inertial parameter variations could have on movement simulation results.
Findings: Muscle activation and limb movement altered limb segment center of mass and moments of inertia by less than 0.04 cm and 1.2% respectively. These variations in limb inertial properties resulted in less than 0.01% change in maximum angular velocity for a simulated straight-leg hip flexion task. These data demonstrate that, for the digital human leg model considered, assuming static quantities for segment center of masses and moments of inertia in movement simulations appears reasonable and induces minimal errors in simulated movement dynamics.
The Effects of Changing Bone and Muscle Size on Limb Inertial Properties and Limb Dynamics: A Computer Simulation
Methods: Musculoskeletal modeling and movement simulations were used to determine how changes in bone and muscle cross-sectional area (mass) affect human thigh and shank inertial properties, the maximum speed of unloaded single-leg cycling and the energy required to sustain submaximal single-leg cycling.
Findings: Depending on initial conditions, shank moments of inertia increased 61-72 kg-cm2 per kg added bone and 72-100 kg-cm2 per kg added muscle. Thigh moments of inertia increased 46-63 kg-cm2 per kg bone and 180-225 kg-cm2 per kg muscle. Maximum unloaded cycling velocity increased with increased muscle mass (~ 2.2-2.9 rpm/kg muscle), but decreased with increased cortical bone mass (~ 2.0-2.8 rpm/kg bone). The internal work associated with unloaded submaximal cycling increased with increased muscle mass (~0.42-0.48 J/kg muscle) and bone mass (~ 0.18-0.22 J/kg bone).
An Investigation of the Interactions Between Lower Limb Bone Morphology, Limb Inertial Properties and Limb Dynamics
Background: Bone mass and size clearly affect the safety and survival of wild animals as well as human beings, however, little is known about the interactions between bone size and movement dynamics.
Methods: A modeling approach was used to investigate the hypothesis that increased bone cortical area causes increased limb moments of inertia, decreased lower limb movement maximum velocities, and increased energy requirements to sustain submaximum lower limb locomotion movements. Custom software and digital data of a human leg were used to simulate femur, tibia, and fibula cortical bone area increases of 0%, 22%, 50%, and 80%. Limb segment masses, center of mass locations, and moments of inertia in the sagittal plane were calculated for each bone condition. Movement simulations of unloaded running and cycling motions were performed. Linear regression analyses were used to determine the magnitude of the effect cortical area has on limb moment of inertia, velocity, and the internal work required to move the limbs at a given velocity.
Rowing – Dry Land Training with Real-Time Feedback
Methods: Two iterations of this system were developed using a Concept II rowing ergometer. The first system was instrumented with a force transducer and potentiometer, four electrogoniometers attached to the athlete’s ankle, knee, hip, and elbow, and a data acquisition computer. The force transducer is used to quantify the athlete’s pulling force. The potentiometer signal is used to locate the position of the handle. The electrogoniometers provide signals proportional to joint angles. A link segment model of the human body is used to locate joint centers based on limb lengths and joint angles. The computer is used to collect and process all the transducer signals, perform the link segment calculations, and provide feedback to the coach or athlete in the form of a stick figure animation overlaid with kinematic and kinetic information.
A second iteration of the dry-land training rowing system was developed to improve the hardware and software interfaces for the user. The system consists of a Concept II rowing ergometer instrumented with a load cell and a series of potentiometers, a data acquisition computer, and custom software. Kinematic and kinetic rowing data are displayed in the form of a two-dimensional stick figure animation overlaid with kinematic and kinetic profiles. The software allows data to be saved and later replayed.
Findings: This system allows the coach and athlete to quickly review rowing mechanics, to evaluate the effects that technique changes have on the power produced by the athlete, and to identify how technique changes with fatigue.
Rowing – On-Water Performance
Purpose: It was hypothesized that a crew’s rowing performance was predictable based on their total propulsive power, synchrony (a real-time comparison of rower propulsive force magnitudes) and total drag contribution (a measure of the rowers’ effect on shell drag forces during the recovery), quantities calculated from individual rower’s force-time profiles and recovery kinematics.
Methods: A rowing pair was equipped with transducers to gather shell velocity, propulsive blade force, oar angular position and seat displacement. Eight subjects (4 port, 4 starboard) participated in two rounds of data collection. The first round pairings were random, while the second round pairings were assigned based on Round 1 results. Regression analysis and ANCOVA were used to test the validity of assumptions inherent in the predictive model and, if applicable, explore a linear model predicting rowing performance based on total propulsive power, synchrony and total drag contribution.
Findings: Total propulsive power, synchrony and total drag contribution were correlated and further were affected by pairing, violating assumptions inherent in the linear model. The original hypothesis was not supported based on these violations. Important findings include (1) performance cannot be predicted using the simple linear model proposed, (2) rowers’ force-time profiles are repeatable between trials, with some but not all rowers adapting their force-time profile dependent on their pair partner, presumably in an effort to increase the level of synchrony between the two, and 3) subtle biomechanical factors may play a critical role in performance.
Purpose: The objective of this study was to analyze the mechanics involved in executing a standard soccer throw-in to determine the anthropometric, strength, and coordination factors that contribute most to a long throw. Two hypotheses were tested: 1) athletes producing longer throws are characterized by having longer segment lengths (torso, upper arm, forearm) and/or joint strengths (hip, shoulder, and elbow), and 2) long throwers utilize the same limb kinematic and kinetic sequencing patterns as short throwers.
Methods: Twenty collegiate level soccer players participated in this study. Their maximum throw distance was recorded from three trials. Segment lengths, and back, shoulder, and elbow maximum isometric strengths were quantified. The correlations between throw distance and an athlete’s segment lengths and joint strengths were determined. Three-dimensional upper extremity kinematics and joint kinetics were quantified for six athletes (3 long and 3 short throwers).
Findings: Correlation coefficients between throw distance and segment lengths were all less than 0.45. Correlation coefficients between throw distance and hip, shoulder, and elbow strength were 0.55, 0.70, and 0.65, respectively. Kinematic and kinetic profiles of long and short throwers were not clearly different. Kinematic and kinetic profiles for long throwers were not always consistent with theoretical predictions. Results suggest that 1) joint strength but not segment lengths correlate with throw performance, and 2) most athletes can improve their throw distance by altering their movement strategy.
Backpack Load Carriage
Background: Backpacks provide an effective means of load carriage and hip belts are an important design feature of many backpacks, serving to reduce the loads carried by the shoulders. Custom molded hip belts are a recent design innovation intended to distribute loads over a greater contact area, thus improving backpack user comfort.
Purpose: The purpose of this study was to evaluate the efficacy of custom molded hip belts for reducing skin pressure during backpack usage.
Methods: Pressure levels developed under a standard and custom molded hip belt were quantified while subjects toted a mountaineering pack under controlled gait conditions. Six subjects (3 male, 3 female) stood, walked, ascended and descended stairs while carrying a gender-specific backpack loaded with 20.4 kg (45 lbs) and equipped either with a standard hip belt or a custom molded hip belt. Pressures from 29 locations under the right side of the hip belt and shoulder strap were recorded for at least three gait cycles of each gait and hip belt condition. Paired T-test analyses were used to test for differences in the peak pressure, mean peak pressure, and the variation in mean peak pressure between the two hip belts during different gait conditions.
Quantitative Ultrasound Imaging of Muscle-Tendon Units
Two ultrasound images of the gastrocnemius muscle, a muscle on the back of the lower leg, are shown to the right. The top image is from a relaxed muscle and the bottom image is from a contracted muscle. The two images illustrate how the orientation of the fascicles within the muscle change with muscle activation.
Fundamental Study of Quantitative Ultrasound
Background: Numerous studies have used ultrasonography to non-invasively quantify muscle-tendon (MT) behavior in-vivo, however the limitations of quantitative ultrasonography have not been thoroughly characterized and therefore conclusions drawn from such studies are questionable.
Methods: Common assumptions inherent in quantitative MT ultrasonography were tested and ultrasound limitations characterized to provide critical information needed to design valid ultrasound studies of MT units. The accuracy and resolution of a commercial ultrasound system used to quantify muscle-tendon deformation and strain were determined by comparing known marker location data with marker location data obtained from analysis of ultrasound images of a phantom, isolated muscle, tendon and fat, and the Achilles tendon of human cadaver specimens during simulated muscle loading.
Results: Results indicate that ultrasound systems have the resolution to make meaningful geometric and deformation measurements of tendon, but speed of sound (SOS) variation, image clarity, and probe location can create large measurement errors. Differences between the actual structure SOS and the SOS used by ultrasound systems (1540 m/s) to create an image can lead to thickness and cross-sectional area (CSA) errors. Structure thickness obtained from digitized ultrasound images were underestimated by 4.8% for tendon, 2.1% for muscle and overestimated by 7.5% for fat. Similar errors would result in CSA measurements. Under ideal conditions ultrasound systems can resolve adult human Achilles tendon strain to less than 0.2%. However in practice poor image quality created by rapid movement and/or structure orientation can reduce this resolution by making it difficult to accurately locate structure boundaries. Achilles tendon strains in a cadaver system with simulated muscle forces were typically determined to be accurate to within 0.2%.
Lateral Force Transmission
Background: Force generated within a muscle-tendon unit (either through muscle activation or stretch) is commonly thought to be transmitted serially from one structure to the next, referred to as myotendinous force transmission. However, there exist alternate pathways that employ intramuscular and intermuscular connective tissues to transmit force laterally, referred to as lateral force transmission (LFT). The extensiveness of LFT and the mechanisms responsible for it are poorly understood.
Methods: Lateral force transmission was investigated in normal and partially compromised passive skeletal muscle systems to determine the fraction of total system force that can be transferred laterally and to investigate possible mechanisms contributing to LFT. Chicken peroneus longus (PL)/middle gastrocnemius (MG)/fascia complexes were isolated taking care to maintain their normal connection to each other. They were attached to a testing fixture that allowed removal and reattachment of the distal end of the muscles. Tensile tests were conducted under three levels of tenotomy (i.e. both muscles attached, only PL attached, only MG attached) and three levels of fasciotomy (100%, 66% and 33% intact).
Methods: The Gastrocnemius-Achilles tendon complexes of 18 subjects were studied to quantify the dynamic creep response of the Achilles tendon in-vivo and the warm-up dose required for the Achilles tendon to achieve steady-state behavior. A custom testing chamber was used to determine each subject’s maximum voluntary contraction (MVC) during an isometric ankle plantar flexion effort. The subject’s right knee and ankle were immobilized for one hour. Subjects then performed over seven minutes of cyclic isometric ankle plantar flexion efforts equal to 25% to 35% of their MVC at a frequency of 0.75 Hz. Ankle plantar flexion effort and images from dual ultrasound probes located over the gastrocnemius muscle-Achilles tendon junction and the calcaneus-Achilles tendon junction were acquired for eight seconds at the start of each sequential minute of the activity. Ultrasound images were analyzed to quantify the average relative Achilles tendon strain at 25% MVC force (ε25%MVC) for each minute.
Achilles Tendon Stiffness and Hysteresis During Cyclic Loading
Methods: The gastrocnemius-Achilles tendon complexes of six subjects were studied in-vivo to quantify changes in Achilles tendon stiffness and hysteresis during the onset of light physical activity. Subjects performed more than 8 minutes of cyclic isometric ankle plantar flexion efforts equal to 25-35% of their MVC at a frequency of 0.75 Hz in a custom testing chamber. Achilles tendon force and length were quantified using a force transduction system and a dual probe ultrasound system, respectively. Achilles tendon stiffness and hysteresis were calculated from cyclic force-deformation responses.
Gastrocnemius/Soleus Muscle Force Contributions to Ankle Plantar Flexion Torque
Background: The soleus (sol) and gastrocnemius (gast) muscles provide the primary forces responsible for ankle plantar flexion. Understanding the relative and absolute contribution each muscle can make to plantar flexion torque (PFT) as a function of joint angles is fundamental to understanding the role these muscles play in various movements. This study was conducted to quantify the contribution that the sol and gast muscles can make to ankle PFT as a function of ankle and knee angles.
Methods: Collegiate level endurance athletes (6 males (21.5±0.8 years) and 6 females (22.5±2.1 years)) performed isometric maximum ankle plantar flexion efforts (IMAPFE) while positioned in a modified incline bench equipped with an ankle torque transduction system that allowed testing of various ankle-knee angle combinations. Subjects performed three IMAPFE at varying ankle angles (~65, 75, 90, 105, and 120 deg included angle) with their knee in a fully flexed position to minimize gast contributions and therefore isolate the sol muscle’s contribution to PFT. Subjects then performed three IMAPFE at varying knee angles (60, 90, 120, 150, 180 deg included angle) with their ankle fixed at the angle that the subject produced their greatest PFT during the previous testing. Gast muscle contributions to PFT were calculated as the difference in the average PFT determined from this testing and the average maximum PFT from the previous testing.
Ground Reaction Force Estimation from Hip Acceleration Data
Background: To address a variety questions pertaining to physical activity, simple methods are needed to quantify, outside a laboratory setting, the forces acting on the human body during daily activities. The purpose of this study was to develop a statistically based model to estimate peak vertical ground reaction force (pVGRF) during youth gait.
Methods: Repeated measures mixed effects regression models to estimate pVGRF from Biotrainer activity monitor (AM) acceleration in youth (girls 10-12, boys 12-14 years) while walking and running were developed.
Background: The main objective of this study was to test the hypothesis that: physically active adults who are susceptible to Achilles tendon overuse injuries experience higher Achilles tendon strains during an isometric maximum voluntary contraction of the ankle plantar flexors compared to people not susceptible to such an injury.
Methods: Nineteen subjects participated in this study (14 uninjured, 5 injured). Subjects performed controlled isometric ankle plantar flexion efforts in a custom testing chamber while force and muscle-tendon image data were collected simultaneously using a force transducer and a Hitachi EUB 6500 Ultrasound System (Hitachi Corporation) with dual EUP L53 linear probes. Force data were used to determine forces in the gastrocnemius-soleus-Achilles complex (GSATC). Ultrasound images were digitized to determine muscle and Achilles tendon deformation during loading. Achilles tendon strain during maximum isometric ankle plantar flexion was calculated and compared between the uninjured control group and the asymptomatic leg of the Achilles tendon injured group.
Findings: Strain in the Achilles tendon during maximum isometric ankle plantar flexion was higher in the injured group compared to the uninjured group (P=0.127). Tendon strain during isometric ankle plantar flexion may be a good predictor of a person’s risk for sustaining an Achilles tendon injury and may be useful quantity to track during injury prevention or rehabilitation interventions.
Anterior Cruciate Ligament
Background: Anterior cruciate ligament (ACL) injuries are one of the most common and potentially debilitating sports injuries. Approximately 70% of ACL injuries occur without contact and are believed to be preventable. Jump stop movements are associated with many non-contact ACL injuries. It was hypothesized that an athlete performing a jump stop movement can reduce their peak tibial shear force (PTSF), a measure of ACL loading, without compromising performance, by modifying their knee flexion angle, shank angle, and foot contact location during landing.
Methods: PTSF was calculated for fourteen female basketball players performing jump stops using their normal mechanics and mechanics modified to increase their knee flexion angle, decrease their shank angle relative to vertical and land more on their toes during landing. Every subject tested experienced drastic reductions in their PTSF (average reduction = 56.4%) using modified movement mechanics. The athletes maintained or improved their jump height with the modified movement mechanics (an average increase in jump height of 2.5 cm).