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.
