Myofibrillar ATPase activity in human muscle fast-twitch subtypes

Under local anesthesia, a muscle biopsy was removed from an individual with a high percentage of type II subtypes. The muscle sample was assayed for myofibrillar ATPase activity using various acid preincubation pH values. Subtle differences were found to exist in the acid-lability of ATPase activity among the type II fibers. These observations suggest different ratios of IIA/IIB myosis and may reflect a gradual transformation between type IIA and IIB fibers.