We compared the effects of a non-weight bearing protocol (NWB) and a weight bearing (WB) protocol on energy stored, stiffness, and shock absorption in the plantar flexor muscle-tendon unit of patients managed non-operatively following an Achilles tendon rupture. Thirty-eight subjects were randomized to a WB cast fitted with a Bohler iron or a traditional non-weight-bearing cast. At a 6-month follow-up, a biomechanical assessment utilizing an isokinetic dynamometer allowed measurement of peak passive torque, energy stored, shock absorption, and stiffness. The WB group had greater peak passive torque (∼20%). Irrespective of group, peak passive torque in unaffected legs was greater (∼26%) than affected legs. Across the groups, energy stored in the NWB group was 74% of the WB group. The energy stored in affected legs was 80% of that in unaffected legs. Shock absorption was not significantly different across legs or groups. Irrespective of group, affected legs had significantly less stiffness (20-40%). While the augmentation of plaster with a Bohler iron to allow increased weight bearing had positive effects, deficits in affected compared to unaffected legs irrespective of group were notable, and should be addressed prior to participation in vigorous physical activities. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res XX:XXX-XXX, 2013.