How muscle imbalances can lead to contractures

Healthy muscles will contract and relax normally, but hypertonic muscles hold on to too much tension.  The excessive tension can cause muscle fibrosis overtime causing a muscle contrature. The muscle becomes more shorter and rigid and will not stretch out.

Contractures are the permanent shortening of the muscle tissue , and often develop from extreme long term hypertonic muscles. This is why a well learned exercise program will always have a stretching program to prevent hypertonic muscles (muscles that hold too much tension while at rest). 

If a muscle balance is not corrected or prevented, then the relationship between the agonist and antagonist is altered. we get altered reciprocal inhibition, which will cause a movement dysfunction. Imbalances can be remedied with corrective exercises, but some imbalances can become so extreme (if left for a long time) that the hypertonic muscle turns into a contracture. This will eventually become a disability. 

A muscle imbalance between an agonist and antagonist muscle can occur due to a neurological disorder, spinal cord injury, and our lifestyle/postural habits.

 A decrease in muscle tone leads to continuous disuse and eventually muscular atrophy. The constant contraction of the agonist muscle with minimal resistance can result in a contracture. Farmer, S.E; M. James (2001). "Contractures in orthopaedic and neurological conditions: a review of causes and treatment". Disability and Rehabilitation. 23 (13): 549–558

In other words a imbalance between opposing muscles can cause contractures, this is what happened to me. 

All muscles should be strength trained in balance too, if you keep doing bench press and no back exercises. You will get and imbalance eventually which can alter you posture and cause issues. It is vital to keep all muscles strong and supple, with a well implemented program. 

Contracture by incorrect training can be avoided, but there are many other ways of getting them. Some are by inactivity due to illness or accident (serious injury). Many diseases which affect the nervous system and muscle tissue lead to contractures. Contractures are debilitating and life changing so do not get there by incorrect training.

Dry Needling and contractures (extracts from the Gunn approach to the treatment of chronic pain)

Long-standing muscle tension eventually leads to fibrosis and contracture formation. When the needle enters the dense fibrotic contracture the patient experience a cramp, this gradually diminishes as the needle grasp is limited. The dense contracture is an important and crucial clinical find which is invisible to radiology, CAT or MRI techniques and may be labelled as the “invisible lesion”.