The purpose of this study was to review the literature with respect to the outcome measures of muscle strength available in physiotherapy using Achilles tendon rupture patients as an example. Then to, review the prediction methods available for isoinertial one repetition maximum (1RM) testing as a possible alternative clinical measure of muscle strength. Secondly a pilot study was undertaken to compare the accuracy of eight 1RM prediction methods in persons following injury to the plantar flexor unit including patients following an Achilles tendon rupture and to then utilise the most accurate 1RM prediction method in an attempt to predict return to sport in this sample. Achilles tendon rupture is a significant injury. Ongoing plantar flexor muscle weakness has been reported in the literature many years post injury. This weakness may be one factor that limits a person’s return to sport at their pre-injury level. To accurately prescribe a strengthening regime using traditional isoinertial equipment, knowledge of a person’s 1RM is necessary. Prediction methods for 1RM are also available in the literature. These methods have been reviewed with respect to their accuracy with different loads, repetition numbers, training states, exercise modes, gender, age and anthropometric parameters. No previous study has assessed their accuracy following injury. This was the primary aim of this study. Twenty subjects completed the testing procedures which included a familiarisation session, an actual 1RM testing session and a predicted 1RM session. The accuracy of predicted 1RM was determined by establishing the actual 1RM and predicted 1RM for a unilateral calf raise. The unaffected limb was used as a control. Differences between 1RM values (predicted 1RM – actual 1RM) were computed using a paired t-test, Bland and Altman analysis, intraclass correlation coefficients and obtaining measures of typical error. A comparison of relative strength between limbs was also made. A logistical regression analysis was then used in an attempt to predict which subjects had returned to sport. The results showed that all equations displayed high accuracy and low error with respect to both limbs. There was no significant differences (p>0.05) across either limb for any of the prediction methods. Overall the Lander equation displayed the greatest predictive accuracy for the injured limb (bias = 0.09kg (SD 3.66), ICC = 0.985 & typical error = 2%). A significant mean 11% deficit in strength was measured across injured limbs in comparison to the control limb (1RM injured = 131.46kg (SD 19.53); 1RM control = 149.61kg (SD 28.16) p<0.05), with a range of differences between limbs from 6.6% stronger to a 49.8% deficit. Logistical regression suggested that subjects with a less than 15% strength deficit across limbs had a greater chance of returning to sport at their pre-injury level in comparison to those who had a greater deficit (odds ratio 35, CI 3-465). This variable explained 39% of the variability in the regression model. When 1RM was standardised to body mass a ratio of 1.59 provided a 100% accurate cut off for those who had returned to sport. In conclusion the Lander equation appears an accurate method of 1RM prediction for the unilateral calf raise following a lower limb injury.