Although genetics affect things like the colour of your hair, eye colour, height and build etc. With the right amount of hard work and dedication in the gym, nutritional diet and healthy lifestyle you can see the results that your looking for! Genes can even affect your ability to want to work out it can affect things like your emotions, motivation to train etc. The stamina of an individual has a big part to play as someone with a lot of stamina would find it easier to exercise for harder and longer rather than someone with low stamina who can’t last as long.
Muscle fibres have two different types, fast and slow twitch muscle fibres. Fast twitch muscle fibres fatigue quicker but have more power and explosion than slow twitch muscle fibres, e.g a weightlifter who needs short burst of power. Slow twitch muscle fibres are more endurance for people like marathon runners and endurance sports. Although you can’t grow the amount of muscle fibres you have you can improve and work on them with the correct training type.