William J. Gonyea

Skeletal muscle fiber hyperplasia

Skeletal muscle enlargement in adult animals has been ascribed primarily to changes in fiber cross-sectional area (i.e., fiber hypertrophy); however, recent evidence from several laboratories suggests strongly that fiber hyperplasia contributes to muscle mass increases in adult animals and possibly human athletes. Scientists have used three models to study the cellular mechanisms of muscle enlargement: compensatory hypertrophy, stretch, and exercise. Each of these models has provided direct as well as indirect evidence supporting the occurrence of muscle fiber hyperplasia. Direct counts of muscle fibers using nitric acid digestion techniques have shown that both exercise and stretch overload result in significant increases (range = 9-52%) in fiber number. Indirect fiber counts using histological cross-sections have suggested fiber hyperplasia (range = 10-82%) in all three models. Additionally, the expression of embryonic myosin isoforms have provided indirect evidence for new fiber formation in stretch overloaded muscle. Furthermore, satellite cells have been shown to be involved in muscle fiber hyperplasia in stretch and exercise.

Exercise induced increases in muscle fiber number.

SummaryThe effect of weight-lifting, which induced muscular enlargement, on fiber number was tested in the flexor carpi radialis muscle by operantly conditioning 6 cats to flex their right wrist against increasing resistance for an average of 101 weeks. The left was used as a control. At the end of training, the cats were performing “one-arm“ lifts with an average of 57% of their body weight. There was an 11% greater muscle weight (P<0.01) and 9% (P<0.02) more fibers in the exercised muscles from the right limb than in the left. This study using a different method, supports our earlier observations that prolonged weight-lifting exercise significantly increases the total number of muscle fibers.

Muscular rehabilitation after orthognathic surgery

There is both a biologic and a clinical foundation for the use of therapeutic exercise after othognathic surgical procedures to restore normal function of the jaw muscles. This can be accomplished by a systematic plan Of occlusal and muscular rehabilitation after release of maxillomandibular fixation. Through the diligent application of physical therapy principles, the function of the masticatory muscles can be more efficiently rehabilitated after orthognathic surgery and maxillomandibular fixation.

Changes in muscle fiber size and composition in response to heavy-resistance exercise

Progressive resistance exercise was used to induce hypertrophy in the right palmaris longus muscle (PLM) of 16 cats. The left PLM served as the non-exercised intra-animal control. After an average 150 +/- 26.6 wk of training, left and right PLMs were removed and weighed. Muscle fibers were typed using standard histochemical techniques. Mean fiber cross-sectional area, connective tissue content, and muscle fiber length were determined. The right exercised PLM demonstrated a 24.2 +/- 6.9% increase in muscle mass. Mean muscle fiber cross-sectional area increased 11.0 +/- 7.3% in the exercised muscles. No change in connective tissue content, fiber length, or fiber type composition was observed. The results show that increases in muscle fiber cross-sectional area do not account for all the observed increases in muscle mass, and that other mechanisms, such as muscle fiber hyperplasia, may play a role in contributing to muscle mass increases.

Histochemical study of the masseter muscle in patients with vertical maxillary excess

This study was designed to examine the morphologic properties of the masseter muscle in individuals with vertical maxillary excess (VME). The deep surface of the anterior aspect of the superficial masseter was biopsied in nine patients at the time of corrective surgery. Individual muscle fibers were classified as slow-twitch-oxidative (SO), fast-twitch-oxidative-glycolytic (FOG), or fast-twitch-glycolytic (FG) based on staining characteristics using a battery of histochemical procedures. Most patients had normal tissue, although there were considerable differences in size and distribution of the three fiber types. Generally, SO fibers were the largest and most prevalent type of fiber. Two individuals, however, demonstrated evidence of muscle pathology that primarily affected the FG fiber population. Histochemical evidence of muscle pathology has not previously been reported in patients with VME, and the pattern of SO dominance observed in these patients may be different from the distribution that would be seen in the masseter of a person free of craniofacial deformities.

Masseter muscle adaptation following surgical correction of vertical maxillary excess

This study was designed to examine the adaptive response of the human masseter muscle following surgical correction of abnormal facial form. Biopsies of the deep surface of the anterior superficial masseter muscle were obtained from five patients demonstrating vertical maxillary excess (VME), one at the time of corrective surgery, and a second at a long-term postoperative time interval (mean, 8 months). Control biopsies were also obtained from five individuals (three cadavers and two patients) with normal dentofacial morphology. A standard regimen of histologic and histochemical staining was used to classify individual muscle fibers as either type 1, type 2, or intermediate. In both VME patients and normal subjects type 1 fibers predominated, with the control group displaying a higher percentage (50% vs. 43%). On the average, type 1 fibers also had the largest areas. Following surgery there was an increase in type 2 fibers (30% vs. 52%). Two patients receiving maxillary surgery only exhibited either no change in mean fiber area or only a slight increase. The three remaining patients who underwent concomitant maxillary and mandibular surgery all showed a significant decrease in mean fiber area. Two persons in the latter group also showed features consistent with a denervation-reinnervation process. The results of this study indicate that the human masseter responds to surgical manipulation of the jaws in a histochemically demonstrable manner, with the nature and magnitude of the response associated with the particular surgical procedure(s) performed.

Muscle fiber splitting in stretch-enlarged avian muscle

This study examined the role of longitudinal fiber splitting in enlarged anterior latissimus dorsi (ALD) muscle of adult quail. Muscle hypertrophy was induced using a model of progressive stretch overload (PSO) (5). After 16 and 28 d of PSO, muscle mass in the stretched ALD muscle increased significantly (P < 0.05) 188% and 294%, respectively, when compared with the intra-animal control muscle. Muscle length increased significantly (P < 0.05) in the stretched ALD muscle vs the intra-animal control by approximately 77% for both groups. Fiber number, which was assessed using direct counts after nitric acid digestion, did not change in the 16-d group; however, the 28-d stretched ALD muscle exhibited a 30% increase (P < 0.05) in fiber number vs the intra-animal control muscle. Furthermore, the frequency of splitting (i.e., branching) fibers was less than 0.3% in all muscles examined except the 28-d stretched ALD muscle. The 28-d stretched ALD muscle had 5.25% of its muscle fibers exhibiting split profiles. These results demonstrate that PSO produces effects unlike chronic stretch overload in that longitudinal fiber splitting may contribute significantly to an increase in fiber number.