First strengthen your thigh muscles Do some strengthening exercises rather than stretching like for example squeezing something soft like a pillow or ball between your legs ,squats etc

Are you doing stretching exercises for thigh? How many seconds you hold and how many repetitions?

Hi If your thighs are shaking,it means they are weak. Along with stretching you also need to strengthen your muscles. Very important. Even after that, if you face this problem, consult a Physiotherapist for detailed assessment.

Since how many days you are doing exercises, how many times of each set and what exercises you are doing ??? It happens when you start it after a ling time or the first time, there will be pains fir 2 - 3 days. if you have this issue for more than 3 days then do share above asked details or consult in person with physiotherapist near you.

Tones

It is because of muscle weakness

It means ur muscles are weak,along with stretching u have to start doing strengthing exercises to quadriceps also. For proper assessment and treatment pls visit nearby physiotherapy clinic.

May be faulty stretching exercise or  tight muscles.