The writer of Hebrews is presenting growth in Christ the same as physical conditioning. He is talking about exercising the human faculties of sensing and perception. As you experience God's Spirit ministering through His Word, there is a process that is taking place. Your senses are being exercised. You are becoming more in tune and more sensitive to the Spirit of God and what God's Word is saying. The more we hear the Word of God, the more we become in condition of hearing His word. And we are able to understand His Word better as we keep hearing, and hearing, and hearing and studying. It is putting us into a spiritual condition of being able to hear the Word.