Lahey was unable to determine whether there was any relationship between claimant's current condition and the 1981 injury. (Id. at 22.) He was also unable to attribute claimant's inability to return to heavy labor to any specific injury. (Id. at 25.) He did not disagree with Dr. Canty's opinion that absent the additional injuries claimant probably could have continued working after his 1981 injury (id. at 17) and testified: