The ?-axons supplying muscle spindle (MS) induce a particular pattern of activity in spindle Ia and II afferents on contraction. From a dynamic viewpoint, the Ia shows increasing spike rates, but II-afferents do not show any activity. For evaluation of nerve fiber activity, spike amplitude (SA) is suggested to measure it as well as its benefit of diagnosing clinical symptoms. This paper aims to find relative activity between ? axons and spindle afferents and to measure their SAs in both mechanical stimuli and background discharge (BD) for determination of their activity. For this purpose, 15 rats were anaesthetized with urethane (170 mg/kg, i.p.). Spinal dorsal and ventral roots of L3 to L5 were exposed by laminectomy and tails skin was removed. The ?-axons and spindle afferents of caudal MS were isolated from ventral and dorsal roots respectively. Then, ?-axons were examined to determine the affected caudal MS. A single mechanical pinna reflex (PR) and one dynamic stretch (DS) (1-second at 50 Hz) were used to stimulate ? and spindle afferents. The means of peak-to-peak SA computed in PR, DS, and also BD for comparison of means together. Meanwhile, 15 single ?-axons and also corresponding spindle afferents were tested in this study. During PR, the late compound Ia and II spikes elicited later than ?-spikes, as Ia-SAs (193µv±5.39SD) calculated higher than II-SAs (60 µv ± 3.87 SD), and ?-SAs (92 µv ± 4.88 SD) were lower than Ia-SAs. There were also significant differences (P<0.001) in the SAs mean values of Ia, II, and ? between PR and BD and also DS and BD, whereas in DS, the SAs of Ia (245 µv ± 5.35 SD) were strongly higher than II (65 µv ± 3.56 SD), and ?-SAs were obviously suppressed (13 µv ± 2.16 SD). There was a strong increase in SA mean values of Ia and II in DS with respect to PR, but ?-activity was inhibited in DS. In PR, the Ia and II spikes elicited later than ?-spikes but it was dependent on ?-activity.