This study addresses the need for a more rigorous mechanical basis for comparing alternative drivetrains in adaptive cycles, particularly where conventional performance measures such as output power and efficiency do not fully capture topology-dependent internal dynamics. The study investi-gates the internal mechanical energy behavior of two adaptive cycle drivetrains—a rotary crank and a reciprocating lever—under identical output-cadence constraints. A multibody dynamics model was de-veloped in SOLIDWORKS Motion, with wheel-side angular velocity prescribed at 0–500 deg/s to isolate drivetrain topology from differences in output motion. Pedal-side internal mechanical energy, compris-ing kinetic and gravitational potential energy, was evaluated using mean absolute energy, energy range, and standard deviation. The results demonstrate a clear mechanical distinction between the two configurations. The rotary crank exhibited a low and stable energy envelope, with an energy range of 0.520 J and standard deviation of 0.167 J, whereas the reciprocating lever exhibited substantially greater internal energy variation, with an energy range of 5.871 J and standard deviation of 1.440 J. The larger energy excursions were associated with reciprocating motion, variable transmission geome-try, linkage inertia, and motion reversal near dead-centre configurations. These findings identify a fun-damental trade-off between phase-dependent torque-shaping capability and internal energy smooth-ness. The principal contribution of this study is the establishment of internal mechanical energy as a topology-sensitive metric for systematically comparing adaptive-cycle drivetrains under controlled output conditions. The findings provide a physics-based foundation for future drivetrain optimization aimed at balancing torque capability, dynamic robustness, and energy smoothness in adaptive mobility and re-habilitation applications.