The use of vibration as an exercise intervention offers new possibilities for coaches, clinicians, and strength and conditioning practitioners. It appears that muscle activation by means of vibration may induce improvements in strength and power similar to those observed with conventional strength training. Furthermore, vibration is thought to affect the neuromuscular functions that regulate muscle stiffness. There has been preponderance in the literature to assume that the stiffness properties of the muscle affect muscle function, which in turn affects functional performance. Improving functional performance is a major goal for recreational to elite athletes. However, improving the performance of well-trained athletes, especially whilst engaged in in-season training, is challenging, as the training status of such athletes is near optimal. Furthermore, training and playing schedules usually do not allow for adequate training frequencies. Therefore, the application of in-season vibration training may offer a means by which substantial loading can be placed upon the athlete with minimal interference to the weekly schedule. Consequently, the aims of this thesis were to: 1) determine the reliability of a new method to measure the stiffness of the knee extensor muscles; 2) investigate the relationship between knee extensor stiffness, strength, power, and speed; and 3) examine the effect six weeks of squat training with and without whole body vibration (WBV) has on knee extensor stiffness, strength, power, and speed. The reliability of a new method to measure the stiffness of the knee extensors was assessed. The within trial variation (coefficient of variation [CV] – 5.41-7.45%) for all five loads (30, 40, 50, 60, and 70% one-repetition-maximum [1RM]) and test-retest reliability (intra-class correlation coefficients [ICC] – 0.92-0.96) were deemed acceptable. Thirty semi-elite male rugby union players were assessed on sprint speed, squat strength, countermovement-jump (CMJ), and drop-jump (DJ) performance to investigate their relationships to knee extensor stiffness. Knee extensor stiffness was found to have no significant relationship to any of the functional performance measures (r = -0.16-0.17). On the basis of these findings it was suggested that the relationship of the knee extensor stiffness to functional performance was not significant (P>0.05). It was suggested that either the postural requirements of the test were inappropriate, or the knee extensor muscles were not the most suitable muscle group to measure. Immediately after the testing session described above, subjects began a six-week intervention of squat training with or without WBV. The percentage change over the six-week intervention for the stiffness and performance data were calculated and compared to determine if differences between training interventions were significant (P<0.05). The mean percentage change in knee extensor stiffness (average load of 30, 40, 50, 60, and 70% 1RM) for the squat only and squat vibration group were -4.18% and -6.59%, respectively. Non-significant mean percentage changes in squat strength (1.32 and -0.50%), CMJ height (-7.92 and -1.47%), DJ reactivity co-efficient (1.25 and -3.54%), and sprint times over 5, 10, and 20 m (range = -0.86 to -1.16% and 0.54 to -1.88%) were observed for the squat only and squat vibration groups, respectively. Squat training with and without WBV was capable of maintaining, but not improving knee extensor stiffness, strength, power, and speed during a six-week in-season training cycle.