Well, if you only do situps and barbell curls, then I'm not sure what you were expecting?
Strength is the application of force by an entity onto another object/entity: from a human perspective, strength is derived from:
- The brain's ability to produce a movement pattern (numerous names for this: motor pattern is my preferred)... the "map" of how water flows through a system.
- The capacity of the central nervous system to stimulate your muscles (i.e. the volume of "central drive" - both in terms of the extent [how many times] and pitch [how big] - available to deploy)... the pressure and amount of water held back by the dam.
- The ability of your nervous system to relay those stimulating signals (i.e. the quality and volume of signal transmitted from your CNS to your muscles) ... the size of the rivers and pipes leading to your muscles.
- The physiological makeup of your muscles (Type 1, 2a or 2b fibres)... type 1 is slow twitch; type 2b is fast twitch.
So strength is a simple concept that has a lot of variables involved with it, the balance/influence of which changes according to your training history.
Isolation exercises (such as barbell curls) are repeated exercises that don't require much central drive, innervate tiny muscles and are not an explosive movement. This means that you won't increase your central drive capacity or the ability of your nervous system to carry those signals. If you only do them once a week, the body won't really respond to the stressors because it knows its only going to get hit once a week and therefore does not need to divert energy/nutrition to cope with: i.e. you won't get stronger.
The age at which you start training can change
some genetic triggers, but a bigger factor is your inherited genes: if your parents were athletic and stacked, there's a chance you have that latent potential, too.
Take my training history: I have lifted weights for the past seven years in - generally - low rep compound/high intensity schemes. This means I have spent 7 years:
- increasing the capacity of my central nervous system (lifting heavy stuff for 3-5 reps)
- increasing the ability of my CNS to drive my muscles
- adapting the physiological makeup of my muscles (increasing the drive for Type 2 expression)
The fact that I've basically spent five of those years doing heavy compounds relating to olympic weightlifting means I'm very good at expressing explosive force in those movements. Most people would class me as relatively strong as a result. Do I have massive arms? No. Are *big* muscles necessary for big strength? Not necessarily (there is some correlation and some relationship, but it's not linear).
However, the nature of my training means I cannot readily cope with repeated bouts of exercise, or high repetition training: I've recently taken up road cycling and I am - physiologically - poorly adapted for it. Does that make me unfit? No, becaues I can recover quickly from short/explosive bouts of exercise. Does it make me a low W/kg cyclist? Yes.
As a runner, the OP will have established motor patterns and muscular adaptations suited to running - the frequency of strength training is not sufficient to change any of the aforementioned characteristics, hence no significant change is noted over time. The most effective (generally speaking) method to change this is to use a scheme such as Stronglifts that demands compound lifts (systemic CNS stimulation) three times a week (enough periodicity to force the body to adapt to the new stressors).
Does it take commitment and consistency? Yes - just like anything (cycling, running, football, etc.); does it require progressive overload? Yes - boundaries must be pushed to find out what lies beyond them.
