Respiratory muscle power (RMP) is the main drive for respiration in health and disease. Negative correlation was found between age and RMP for both sexes. Height, obesity and physical training have been shown to increase RMP. The use of RMP in asthma as a tool of investigation and as a predictor of training has not been investigated. Asthma diagnosis when mild or intermittent is a clinical challenge in between attacks, and the common pulmonary function tests (PFTs) are not very sensitive. An extensive study has been performed on the RMP in normal individuals, in asthmatics as a new diagnostic tool, in trained police officers, and in obesity. Methods: A total of 653 subjects from Gezira and Khartoum states, Sudan were included. Two hundred and sixty normal adult volunteers both sexes, aged 16-60 years have performed RMP measurement (Maximum Expiratory and Inspiratory Pressures, MEP and MIP) by mouth pressure meter and pulmonary function (FVC, FEV1 & PEFR) by microplus spirometer. RMP and PFTs in 30 young adults aged 16 -23 years were compared with 30 old subjects aged 65-85 years. Respiratory muscle thickness at the end of quiet and maximum inspiration were measured using ultrasound device .Fifty asthmatic and 50 matching controls have performed reversibility test and spontaneous variability by RMP and PFTs. Fifty nine trained policemen and 28 policewomen were compared with 61 matching control. Diaphragmatic thickness and RMP was performed in 10 chronic asthmatics, matched with 10 controls. Obesity and RMP was studied in 52 obese individuals and 43 matching controls. Results: Both RMP and PFTs in males were significantly higher than in females (mean MIP (cmH2O) and FEV1 (L) were 111±21 and 3.7 ± 0.5 in males compared to 75.8± 16.3 and 2.8 ± 0.3 in females .Both RMP and PFTs values decreased with age in both sexes (mean MIP and FVC (L) in young subjects were 114.3±28.4 and 4.1± 0.5 compared to 78±19 and 2.9±0.5 in old subjects. The sensitivity of MEP and MIP was better than FEV1 and PEFR in asthma diagnosis by reversibility test (49% & 71% for MEP & MIP, and 40% & 31% for FEV1 and PEFR) and in variability test (53% & 61% for MEP & MIP and 40% & 46% for FEV1 and PEFR). MEP and MIP significantly increased in obese subjects (p=0.01 and 0.00 respectively) and insignificant difference (p=0.73) was found in lung function between obese subjects and controls (Mean FVC (L), MEP and MIP (cmH2O) were 3.58 ±0.9, 109±29.8 and 78±17.2 respectively in obese subjects compared to 3.63±0.7, 94.6 ±27.7 and 67.8±18.7 respectively in non-obese controls). The diaphragm muscles thickness was significantly increased in chronic asthmatics compared to healthy controls (diaphragm thickness in quiet inspiration was 4 mm in asthmatics and 3.1 mm in controls). RMP and PFTs were found significantly higher in trainee polices officers compared to controls (mean MIP and FVC in policemen were 106.9 ±28.2 and 3.96 ± 0.5 compared to 88.1 ±31 and 3.58± 0.8 in controls). In conclusions: RMP differences between males and females and in younger and old adults can explain the differences in their lung function values. RMP reversibility and variability is a new potential diagnostic tool for asthma. Obese and trained subjects have Higher RMP which could explain their lung function values.