A quantitative and dynamic analysis of skeletal muscle structure and function can guide training protocols and optimize interventions for rehabilitation and disease. While technologies exist to measure body composition, techniques are still needed for quantitative, long-term functional imaging of muscle at the bedside. We evaluate whether diffuse optical spectroscopic imaging (DOSI) can be used for long-term assessment of resistance training (RT). DOSI measures of tissue composition were obtained from 12 adults before and after 5 weeks of training and compared to lean mass fraction (LMF) from dual-energy X-ray absorptiometry (DXA). Significant correlations were detected between DXA LMF and DOSI-measured oxy-hemo/myoglobin, deoxy-hemo/myoglobin, total-hemo/myoglobin, water, and lipid. RT-induced increases of ∼6% in oxy-hemo/myoglobin (3.4±1.0  μM, p=0.00314) and total-hemo/myoglobin (4.9±1.1  μM, p=0.00024) from the medial gastrocnemius were detected with DOSI and accompanied by ∼2% increases in lean soft tissue mass (36.4±12.4  g, p=0.01641) and ∼60% increases in 1 rep-max strength (41.5±6.2  kg, p=1.9E-05). DOSI measures of vascular and/or muscle changes combined with correlations between DOSI and DXA suggest that quantitative diffuse optical methods can be used to evaluate body composition, provide feedback on long-term interventions, and generate new insight into training-induced muscle adaptations.