Under G. L. c. 152, § 35B,1 when an employee suffers a recurrence of an earlier injury after returning to work, the employee is entitled to be paid compensation at the average weekly wage rate in effect at the time of the subsequent injury. Under G. L. c. 152, § 34B,2 compensation benefits are subject *9to an annual cost of living adjustment in accordance with changes in the average weekly wage. At issue in the present appeal is the date used to calculate the § 34B adjustment in cases involving recurrence of an earlier injury. The reviewing board (reviewing board) of the Department of Industrial Accidents (DIA) concluded that the § 34B adjustment should be calculated using the date of the employee’s original injury, rather than the date of the subsequent injury. The Commonwealth3 contends that the appropriate date instead should be the date of the subsequent injury, which establishes the rate of the benefits subject to the § 34B adjustment. We agree with the Commonwealth, and reverse the decision of the reviewing board.

Background.4 The employee, John M. Camara, was employed by the Commonwealth as a laborer for the highway department when, on October 23, 1987, he injured his lower back. He was paid temporary and total incapacity benefits under G. L. c. 152, § 34, at the rate of $258.37 per week, based on an average weekly wage of $387.56, from October 23, 1987, through January 15, 1992, when he returned to work. After almost four months, the employee again left work because of lower back pain, and was paid § 34 temporary and total incapacity benefits at the rate of $325.91 per week, based on an average weekly wage of $488.87, from May 8, 1992, through November 1, 1992, when he returned to work.

*10The employee continued to work until February 4, 1994, when his back pain became too great. From February 4, 1994, through March 13, 1999, the employee was paid partial incapacity benefits under G. L. c. 152, § 35, in the amount of $239.25 per week, based on an average weekly wage of $488.87 and an earning capacity of $130 per week.5 On March 13, 1999, the Commonwealth terminated payment of benefits, on the ground that the employee had received benefits for the maximum period of five years authorized under § 35. The employee filed a claim with the DIA for permanent and total incapacity benefits under G. L. c. 152, § 34A, which resulted (on January 11, 2000) in an order of payment awarding the employee § 34A benefits at the rate of $325.91 per week, based on an average weekly wage of $488.87, beginning on March 13, 1999, and continuing. The employee’s benefits were adjusted pursuant to G. L. c. 152, § 34B, in 1997, 1998, 1999, 2000, and 2001, based on a date of injury of October 23, 1987.

On or about April 10, 2002, the Commonwealth unilaterally determined that the § 34B adjustment should instead be based on the February 4, 1994, date of the employee’s subsequent injury, rather than the October 23, 1987, date of the employee’s original injury. The Commonwealth recalculated the adjusted benefit rate and advised the employee that it would thereafter pay § 34A benefits at the recalculated rate, less a weekly deduction to recoup the overpayment attributable to the previous use of the 1987 injury date for purposes of calculating the § 34B adjustment. In response, the employee filed a claim with the DIA for reinstatement of the § 34B adjustment based on the 1987 injury date and penalties pursuant to G. L. c. 152, § 8(5). An administrative judge concluded that the employee was entitled to § 34B adjustment of his benefits based on the 1987 date of injury and, on the Commonwealth’s appeal, the reviewing board affirmed (albeit on *11different grounds).6 The Commonwealth’s appeal to this court followed.7

Discussion. At issue is the interplay between G. L. c. 152, §§ 34B and 35B. Under the unambiguous terms of § 35B, the compensation rate applicable to an employee’s claim for benefits based on a subsequent injury is determined as of the date of the subsequent injury. See Taylor’s Case, 44 Mass. App. Ct. 495, 499-500 (1998). See also Don Francisco’s Case, 14 Mass. App. Ct. 456, 463 (1982) (“Under § 35B, . . . the employee’s right to compensation . . . originate] in the change in the employee’s condition subsequent to his return to work”).

Section 34B provides for an annual adjustment of benefits, on a review date of October 1 of each year, based on the change in the average weekly wage from the date of injury to the review date. The evident purpose of the cost of living adjustment under § 34B is to protect an individual’s economic position “by acting as a buffer against the erosion of inflation.” Sliski’s Case, 424 Mass. 126, 135 (1997).

In concluding that the employee’s § 34B cost of living adjustment should be based on the October 23, 1987, date of his original injury, instead of the February 4,1994, date of his subsequent injury, the reviewing board compared the rate adjusting provisions of § 35B, applicable upon the occurrence of a subsequent injury, to those of G. L. c. 152, § 51 A,8 which apply when no compensation has been paid on a claim before the date of the final decision on that claim. Noting its conclusion in a prior *12case that the cost of living adjustment under § 34B should be based on the date of injury, notwithstanding an adjustment of benefits under § 51A based on a later date of the claim’s final decision, the reviewing board stated that its analysis of § 51A is equally applicable to the rate shifting effect of § 35B, and accordingly concluded that the establishment of benefits for a subsequent injury under § 35B, based on the date of the subsequent injury, would not affect the date of injury used for purposes of the cost of living adjustment under § 34B.9

The employee also offers an alternative path to the same conclusion, rooted in the language of the statute. Section 34B provides that the cost of living adjustment prescribed thereby is to be determined by reference to the date of injury. Since § 35B involves a recurrence of a preexisting injury, rather than a new injury, the employee suggests that the “date of injury” for purposes of the § 34B cost of living adjustment must be based on the date of the original injury.10

In our view, the reviewing board’s and the employee’s interpretations violate both the letter and the spirit of the statutory scheme.

“[A] statute must be interpreted according to the intent of the Legislature ascertained from all its words construed by the ordinary and approved usage of the language, considered in connection with the cause of its enactment, the mischief or im*13perfection to be remedied and the main object to be accomplished, to the end that the purpose of its framers may be effectuated.” Scheffler’s Case, 419 Mass. 251, 255 (1994), quoting from Board of Educ. v. Assessor of Worcester, 368 Mass. 511, 513 (1975). “[T]he statutory language itself is the principal source of insight into the legislative purpose.” Schejfler’s Case, supra, quoting from Hoffman v. Howmedica, Inc., 373 Mass. 32, 37 (1977). “The language of the statute is not to be enlarged or limited by construction unless its object and plain meaning require it.” Gateley’s Case, 415 Mass. 397, 399 (1993). “The interpretation of a statute by the agency charged with primary responsibility for administering it is entitled to substantial deference.” Ibid. However, “[a]n incorrect interpretation of a statute by an administrative agency is not entitled to deference.” Kszepka’s Case, 408 Mass. 843, 847 (1990). See G. L. c. 30A, § 14(7)(c).

We have previously held, in accordance with the express terms of § 35B, that an employee who suffers an aggravation of an earlier injury two months or more after returning to work is entitled to compensation at the rate applicable on the date of his subsequent injury. See Don Francisco’s Case, 14 Mass. App. Ct. at 460-461. In so holding, we construed the term “injury” to include “the aggravation of a preexisting condition.” Id. In accordance with that holding, the employee’s benefits in the present case were determined on the basis of the average weekly wage rate as of the date of his subsequent injury. Under the terms of § 35B, then, the date of injury used for purposes of determining the compensation rate applicable to a subsequent injury is the date of the subsequent injury, rather than the original injury.

Under § 34B, the cost of living adjustment is determined by the change in the average weekly wage between the date of the injury and the applicable annual review date. As we have observed, the purpose of the adjustment is to maintain the purchasing power of the benefits, once established, against the erosive effects of inflation. That purpose is met by the application of the § 34B formula to the date on which benefits are established — in other words, to the date of the subsequent injury in cases to which § 35B applies. It would be anomalous to apply § 34B’s cost of *14living adjustment to benefits using a date other than the date on which the benefits are determined, if the purpose of the cost of living adjustment is to preserve, rather than to augment, the purchasing power of the benefits initially established.

Our conclusion, that for purposes of the § 34B cost of living adjustment to § 35B benefits the “date of injury” should be the date of the employee’s subsequent injury, is also consistent with the treatment of adjustments to benefits under G. L. c. 152, § 35C,11 in cases where there is a lengthy delay between the date of the injury and the date on which the employee becomes eligible for benefits. Under § 35C, the applicable date for purposes of the § 34B cost of living adjustment is the date on which the benefits are determined, rather than the date of the injury. The essential point is that in order to maintain the purchasing power of benefits once awarded, the cost of living adjustment under § 34B should be applied to such benefits as of the date on which they are established.12

For the reasons we have stated, the decision of the reviewing board was based on an error of law and is reversed. A new order shall issue stating that the employee’s benefits shall be adjusted under § 34B based on the change in the average weekly wage between February 4, 1994, and each applicable review *15date.13 The case is remanded to the reviewing board for consideration of any right the Commonwealth may have to recoupment of any excess payments made based on use of the October 23, 1987, date for previous § 34B adjustments.

So ordered.