This document presents a theoretical method for analyzing the performance of wireless body sensors in terms of channel capacity and bit error rate when experiencing slow and fast fading effects. Analytical expressions are developed for channel capacity and BER for different propagation scenarios, including implant-to-implant, implant-to-body surface, and body-surface-to-body-surface. Numerical results using realistic channel models indicate high outage probability and degraded capacity due to complex propagation on body surfaces and inside the body. Communication links must be optimized with fade mitigation techniques to achieve high quality-of-service requirements for biomedical systems. The results provide insight into how the propagation channel affects performance and allow evaluating technical feasibility for different scenarios.