QUADRICEPS RECRUITMENT AND TWITCH PROPERTIES IN LEAN AND OBESE WOMEN BEFORE AND AFTER FATIGUING EXERCISE

Skeletal muscle strength, specifically that of the anti-gravitational muscles such as the quadriceps, is essential for performing dynamic tasks including walking, navigating stairs, and transitioning between sitting and standing. Obesity has been shown to negatively impact musculoskeletal function and decrease quadriceps strength relative to body weight. Additionally, obesity has been linked with higher rates of fatigue and lower strength outcomes following a test of muscular endurance of the quadriceps. Since obesity lowers relative strength, it is likely that quadriceps muscle weakness contributes to reduced physical function of dynamic tasks. However, to date, the precise mechanism for obesity-induced muscle weakness has not been fully elucidated. Recent studies reported that obesity lowers skeletal muscle force per unit of cross-sectional area (specific force); however, these data were collected during isometric muscle contractions, not dynamic muscle actions. In addition, body mass index has been shown to be negatively correlated with the diffusion properties of muscle, specifically muscles of the lower back and thigh. The relationship between body mass index and muscle diffusion properties has been suggested to be a result of greater intermuscular fat. Thus, since it has been demonstrated that those with obesity have lower muscular strength, and previous studies have revealed strength is related to diffusion properties of the muscle, it is possible that muscle diffusivity is correlated with muscle strength in normal weight (NW) and obese (OB) subjects. In addition, obesity has been previously reported to alter neuromuscular recruitment. Neuromuscular recruitment plays a role in muscular strength; and therefore, may be an important determinant of reduced quadriceps strength in obesity. The gold standard for evaluating neuromuscular control in relation to muscular strength is the interpolated twitch technique (ITT). Through an ITT protocol voluntary and electrically evoked muscle contractions are evaluated. Data examining the effects of obesity on voluntary and electrically evoked muscle contractions are limited, specifically in women. Thus, impaired neuromuscular recruitment may be a primary mechanism of obesity-induced muscle weakness. Due to the ascending manner voluntary muscular recruitment occurs, it is worth noting that type I muscle fibers (smaller, slower MU) are first activated and are fatigue resistant, compared to type II muscle fibers (larger, faster MU) which are more susceptible to fatigue. When compared to NW, OB subjects expressed significantly greater proportions of type II muscle fibers, suggesting those with obesity may display a greater amount of muscular fatigue than NW subjects. Muscular fatigue is defined by Gandevia et al. (2001) as any exercise-induced reduction in the ability of the muscle to generate force (1). Therefore, because daily activities are repetitive in nature it is important to quantify skeletal muscle fatigue in those with obesity, compared to NW subjects. Therefore, the purposes of this dissertation were to 1) compare voluntary recruitment and muscle twitch properties in NW and OB women before and after fatiguing exercise and 2) to determine the relationship of diffusion properties and contractile properties of the quadriceps in NW and OB women. We hypothesized that relative muscle strength (strength per unit of body mass) would be lower in OB women, compared to NW and relative muscle strength would be related to physical function performance. Next, it was hypothesized that maximal voluntary specific force would be lower in women with obesity, compared to NW, which would be compounded by fatiguing exercise. Third, we hypothesized voluntary activation would be lower in women with obesity, compared to NW, which would be compounded by fatiguing exercise. The fourth hypothesis of this project was that women with obesity would have a greater antagonist-agonist co-activation during knee extension exercise, compared to those who are NW. The fifth hypothesis was that muscle diffusion properties would be associated with quadriceps contractile function and that muscle diffusion properties would be different between OB women, compared to NW counterparts. Nine NW women (mean ± SD; body mass index BMI: 22.3 ± 2.5 kg·m-2; age: 26 ± 4.5y) and 10 OB women (BMI: 36.4 ± 5.1 kg·m-2; age: 32.7 ± 5.7y) participated in this study. Subjects were free from cardiovascular, metabolic, and pulmonary disease, did not participate in structured exercise (\u3e1 day per week), and were free from current or previous lower extremity injury. Each subject visited the University of Kentucky campus twice. During session one, each subject completed a body composition test via dual energy x-ray absorptiometry and anthropometric measurements and, each subject participated in an Magnetic Resonance Imaging (MRI) scan of the dominant leg to determine quadriceps fat-free cross-sectional area and collect diffusion tensor imaging parameters of the quadriceps. After the MRI each subject completed a familiarization session of the muscle strength testing on the isokinetic dynamometer. During session two, subjects completed the following tests: 1) physical function tasks, 2) knee extensor and knee flexor maximal voluntary isometric contractions, 3) knee extensor (75%) and knee flexor (50%) submaximal strength test, 4) pre- and post-exercise interpolated twitch experiment, and 5) three sets of 10 repetitions of concentric/eccentric knee extensor fatiguing exercise. We found relative strength was significantly related to the time it took subjects to complete the timed up and go test, however relative strength was not significantly related to the number of repetitions subjects completed in during the 30s sit-to-stand test or the time to complete a 40m walk. When evaluating muscle contractile function measures, we found maximal voluntary specific force was not significantly different between OB and NW women, but there was a main effect of time. Compared to pre-exercise, all post-exercise trails (recovery at 1 minute: REC1, recovery at 2 minutes: REC2, and recovery at 3 minutes: REC3) were significantly reduced. Peak twitch force, rate of force development, rate of relaxation, and post-activation potentiation were not significantly different in OB women, compared to NW, but all twitch properties showed a main effect of time. Compared to pre-exercise values, peak twitch force, rate of force development and post-activation potentiation were all significantly lesser during post-exercise trials and rate of relaxation was slower during post-exercise trials. Furthermore, we found voluntary activation was not significantly different between pre- and post-exercise trials. In addition, we found no significant differences in antagonist-agonist co-activation index between OB and NW women, but we did see a main effect of time. Compared to pre-exercise co-activation was significantly less during REC2 and REC3, but not REC1. Diffusion parameters were not significantly different between NW and OB. However, diffusion parameters were significantly correlated with pre-exercise twitch properties, but not maximal voluntary specific force or voluntary activation. Diffusion parameters and fatigability (% change in contractile function) was not significantly correlated. In conclusion, relative muscle strength, but not maximal voluntary specific force, was found to be a contributor of reduced physical function. Muscle contractile function and diffusion properties were not significantly different between groups. Due to the correlation between diffusion properties and muscle twitch properties, absence of difference in diffusion properties between groups may explain the null effect of group throughout this study. The most interesting finding of the present study is the correlation of muscle twitch properties and muscle diffusion parameters. Given the ability of diffusion measurements to be related to intramyocellular lipid content, fiber type, and strength, this may be an emerging area and technique to define muscle contractile quality