PhD | Academic

Bone Morphological Changes to Loading History during Growth

Wed, 26 Feb 2020 00:00:00 +0000

Note: This work is currently under manuscript preparation so data will not be presented. Prelminary work of this study was presented at the 40th and 43rd Annual Meeting of the American Society of Biomechanics, the slides of which can be viewed from the “Featured Conference Presentations” section. This work was also funded through a NIH R21 grant and a grant through Center for Human Evolution and Devlopment at Penn State.

Overview: It is well-established that exercise during growth increases bone strength. Previous work in mice suggests that bone shape is similarly plastic, but very little research exists on the subject. Bone shape is important, as it acts like a system of levers to help us move. For example, bony protuberances and bone length can influence moment arm lengths, which impacts locomotor capability. The purpose of this study was to investigate if exercise during growth promotes changes to bone shape in addition to bone strength.

For this study, 30 helmeted guineafowl were split into a sedentary (SED) and exercise (EXE) group. SED birds were housed in small pens that restricted their ability to move. EXE birds were housed in large pens, which allowed plenty of movement freedom. EXE birds were also trained for 30 minutes per day, 4x per week, in which they performed short bursts of high-acceleration running. The protocol last for 14 weeks, at which point birds had almost reached skeletal maturity and were sacrificed. Bird carcasses were scanned using a DXA scanner to get a measure of full-body bone mineral content and density. The tarsometatarsus (TMT), analogous to a fused human foot excluding toes, was extracted scanned using a microCT scanner. Bone strength was assessed using cross-sectional properties at 50% of diaphysis length using ImageJ and the BoneJ plug-in. Bone shape was characterized using a 3D array of 159 custom landmarks (created using Avizo and ViewBox) for each TMT. Geometric morphometrics (i.e. general procrustes analysis followed by principal components analysis) were used to characterize differences in bone shape. Other measures of bone shape, such as length and hypotarsus width (analogous to the human heel or tibial tuberosity at the knee but for the ankle), were also taken. Procrustes ANOVA and t-tests on PC1 scores were used to test if groups could be distinguished. T-tests with Bonferonni corrections were used to test for group differences in linear measurements of bone, body size and composition, and bone composition. Geometric morphometrics and statistical analyses were performed in R.

Moment Arm Changes to Loading History during Growth

Wed, 26 Feb 2020 00:00:00 +0000

Note: This work is currently under manuscript preparation so data will not be presented. Preliminary work for this study was presented at the 44th Annual Meeting for the American Society of Biomechanics.

Overview: Moment arms are often linked to locomotor specialization due to their influence on torque generation, muscle length changes, and muscle shortening velocity. For example, animals adapted for sprinting (cheetas, greyhounds, human sprinters) were found to have small moment arms in their limbs, while animals known for digging tend to have larger moment arms. Small moment arms impair torque generation, but reduce muscle fiber strain and shortening velocity, which are beneficial for power production and rapid limb swing. Large moment arms, meanwhile, amplify a given muscle force to increase torque generation, and thus, are better suited to situations where strength is more important than speed. However, whether moment arms are immutable in response to exercise is largely unknown. Moment arm length is largely determined by joint structure, whose components of muscle and bone are known to be plastic in reponse to loading. Therefore, the purpose of this study was to investigate moment arm adaptability in response to loading history during growth.

In this study, 24 guineafowl were split evenly into 3 groups at 2 weeks-old. An exercise (EXE) group was stored in a large pen that allowed to ample opportunity to run aroud and jump to perches. A sedentary (SED) group was house in a small pen with a low ceiling to discourage movement, including jumping. The third group, called the botox (BTX) group, were house similarly to the SED birds, but also received bilateral intramuscular injections of botox into their plantarflexors to induce paralysis (i.e. muscle unloading). EXE and SED birds received injections of a saline solution instead. Injections were started at 7-8 weeks of age, and were given every 5 weeks for 20 weeks until birds had reached ~6 months of age. Birds were then sacrificed and the left hindlimb was isolated, with all musculature except plantarflexor tendon removed. Moment arms were measured using a tendon travel protocol, which is based on the concept of virtual work. This method assumes that joint work and muscle work are equal, so that moment arms are the ratio of the linear displacment of the dnon to the joint angle displacment (force is equal). Retroreflective markers were attached to the bones in order to use 3D motion capture to track joint movement. A linear transducer was attached to the tendon to measure linear displacement and to keep the tendon under constant tension. Each bird limb underwent 3 trials of 8-10 flexion/extension cycles. Linear vs. angular displacement plots were generated from data compiled across all 3 trials, and a 3rd order polynomial was fit to the data to calculate moment arms. A linear mixed model was used to test for group differences, as well as any group x angle interactions. Moment arm analyses were performed in MATLAB, while statistical analyses were run in R.

Muscle Architecture Changes to Loading History during Growth

Fri, 26 Oct 2018 00:00:00 +0000

Note: The work in this study was published in the Journal of Biomechanics in 2018. Work in this study was also presented at the 41st Annual Meeting of the American Society of Biomechanics, receiving the Journal of Biomechanics Award.

Overview: Muscle function is often studied in relation to locomotor function. For example, sprinters are known to have longer muscle fascicle lengths than non-sprinters, and longer fascicle lengths are correlated with faster times amongst sprinters. Longer fascicles are beneficial to sprinting to due their force-length and force-velocity effects that promote power production. However, it is unknown if the sprinter morphology is a result of genetics or training during adolescence. Therefore, the purpose of this study was to study the effects of high-acceleration training during growth on muscle morphology.

For this study, 30 helmeted guineafowl were split into a sedentary (SED) and exercise (EXE) group. SED birds were housed in small pens that restricted their ability to move. EXE birds were housed in large pens, which allowed plenty of movement freedom. EXE birds were also trained for 30 minutes per day, 4x per week, in which they performed short bursts of high-acceleration running. The protocol last for 14 weeks, at which point birds had almost reached skeletal maturity and were sacrificed. Bird carcasses were scanned using a DXA scanner to get a measure of full-body bone mineral content and density. Two muscles with differing architectures and functions were harvested for investigation. The iliotibialis lateralis pars postacetabularis (ILPO) is a large, hip extensor muscle ith long, parallel fibers and a short tendon. The lateral gastrocnemius (LG) is a plantarflexor at the angle with short, pennated fibers and a long tendon. During running, fibers in the ILPO undergo large length changes while fibers in the LG maintain their length. After formalin fixation, muscles were dissected in separate regions: ILPO into anterior and posterior regions, and LG into proximal, middle, and distal regions. Three muscle fibers were extracted, via digestion in nitric acid, were measured from each section using a macro-magnification camera, ImageJ, and MATLAB. Three sarcomere lengths were also measured from each muscle section using laser diffraction. Other measures included: muscle masses on fresh specimen, bone lengths X-ray, pennation angle, and physiological cross-sectional area (PCSA). Using R, a blocked ANOVA was used to test for exercise effects on optimal fiber lengths, limb length-normalized optimal fiber lengths, sarcomere length, and pennation angle while accounting for natural variations within the muscle (e.g. anterior fibers in ILPO are naturally shorter than posterior fibers). Repeated measures ANOVA was used to test for body mass differences across the experimental protocol using SAS. Meanwhile, t-tests with Bonferroni corrections were used for all other measures and also performed in R.

SED birds had 6% greater body mass than EXE birds, but body composition was not different between groups. SED birds also had 3% longer limbs. Muscle mass and pennation angle were not different between groups. EXE birds were found to have 12% longer optimal fiber lengths and 15% limb-length normalized optimal fibers lengths in ILPO. In the LG, a nonsignificant (p = 0.068) difference of 14% was found for optimal fiber lengths, while normalized optimal fiber lengths were significantly different. PCSA in the SED were larger in both the ILPO and LG (16% and 12% respectively), but these differences were not significant (p = 0.123).

The results of this study suggests that adult muscle morphology may be influenced by exercise during growth. Specifically, morphology may reflect the type of exercise performed. High-acceleration training induced longer muscle fascicles, which are preferably for high power movements. Meanwhile, muscle size was either unchanged or smaller as a result of training. Together, it seems that muscle priortized adaptations for strain and power production over isometric strength.

Relationship Between Muscle Stiffness & Sarcopenia

Tue, 07 Jul 2015 00:00:00 +0000

Note: Master’s thesis work.

Overview:
Sarcopenia, the loss of muscle mass with age, is a common problem in the elderly population. Although exercise is a common prescription, older adults have a decreased ability to gain muscle compared to young adults. Evidence also suggests muscles in older adults are stiffer, due to increased glycation in the extracellular matrix. Strain is a mechanical signal for hypertrophy and increased stiffness may cause muscle cells to experience less strain for any given load. Therefore, we hypothesized that response to exercise is impaired in older adults because of increased stiffness.

For this study, 19 rats were split into young (12 months) and old (32-33 months) groups. Each rat underwent 3 sets of 10 maximum eccentric dorsiflexions utilizing electrical stimulation and a dynamometer. Following sacrifice, two dorsiflexor muscles were harvesed - the tibialis anterior (TA) of both legs and extensor digitorum longus (EDL) of the non-exercised leg. The EDL underwent stiffness testing to determine the Young’s modulus of the muscle. The TAs were used in a Western blot analysis to determine cellular response. Focal adhesison kinase (FAK) is a protein that is phosphorylated with stretch, making it a good indicator of exercise response. The ratio of phosphorylated to total FAK was used a measure of cellular to response to exercise and termed FAK activity.

As a result, we found smaller and stiffer muscles in the old rats. There was also greater variability in these measures in the old rats. A negative relationship was also seen between muscle stiffness and size. In older rats, a negative relationship was seen between FAK activity and muscle stiffness, while a positive relationship was seen between FAK activity and muscle size.

Our results describe relationships between muscle size, muscle stiffness, and response to exercise in old age. This study implicates increased muscle stiffness as possible mechanism behind the development of sarcopenia. However, while results suggest the possibility of a relationship between sarcopenia and muscle stiffness, this study lacks the ability to establish a causal link.