Scoring below the target suggests that your effort during the workout was lower than intended. Physiologically, this means your heart rate remained under the range designed to drive adaptation.
Over time, repeated under-target scores may result in slower progress and reduced improvements in cardiovascular fitness.
As Lionheart is not a medical device, if you feel like you are unable to push harder yet are still getting low scores and you have confirmed your in-app biometrics are correct, please consult a doctor to understand this further.