Ba'al Shem Tov, Noach 66/בעל שם טוב על התורה·נח ס״ו

1At times, a person may desire to pray with enthusiasm, but cannot due to heaviness in his limbs. About this King David prayed: “Do not cast me off in the time of old age…” (Psalms 71:9). That is, when my limbs feel heavy like those of an old person.Ohr Torah, Ekev