‘I don’t care if it hurts, I want to have control. I want a perfect body, I want a perfect soul.’                        Radiohead: Creep
The body’s joints should be correctly aligned so that the forces applied during everyday functional movement (lifting, carrying, pushing and pulling, twisting and turning, etc) won’t create inappropriate stressors through the joints that eventually cause injury. This concept is vital when undertaking specific training to create a physiological adaptation. For example, runners increase the volume, or intensity, of their daily runs to go further and faster. Weight-trainers increase their training poundages and the sets or repetitions of an exercise. Or the speed of lifting, relative to the poundages, when developing power; especially with sports that require sprinting ability.
Weight trainers usually perform anything from 1-3 sets of up to 20-25 reps of an exercise with multiple sets (1-5 being most common). So although squatting, for instance, can impart heavy loads on the hips, knees and back; it does not create the same type of impact stress as running distances where the athlete shock loads his bodyweight on to one foot. However, regardless of exercise mode injuries can occur if the skeletal frame, via joint alignment, isn’t in balance. 
Inexperienced gym users frequently develop ‘chest mirror muscles’ by excessive bench-pressing. This creates an imbalance because of the over development of prime-mover muscles (agonists): Pectoralis Major and Minor and the under-development of opposing (antagonistic) muscles of the back: Rhomboids and the mid/lower fibres of the Trapezius. This causes impingement of the shoulder (Gleno-Humeral) joint as the scapulae are pulled forward out of alignment.
Also many runners have elongated hamstrings and a poor knee lift, due to a forward tilted pelvis. Their stride length and ability to ‘toe-off’ whilst fully extending their rear leg is compromised by tight hip-flexors. This can cause nerve the impulses that initiate hamstring contraction to lose their correct recruitment patterns on hip extension. This causes the hamstring muscles to contract in a weak, dysfunctional pattern (reciprocal inhibition) increasing injury risk. Additionally, this misalignment causes runners to develop tight lower backs and, or, an inefficient forward lean. These imbalances cause pain and wear and tear around the hip and knee joints, potentially shortening an athletic career.
Robustness describes the ability to sustain high performance levels over an extended period. Some performers exhibit physical strength and power through training, but still move inefficiently because of joint restrictions. Therefore, despite producing high levels of performance, they haven’t utilised their full movement capabilities. Consequently, they may fail to fully apply their acquired strength or power optimally during sporting performance. Those athletes have more strength and power than they can functionally apply. Basically, they’re ‘over-engineered’ strength and power-wise for the limited ranges of movement their bodies can accommodate. This indicates that inappropriate training methods, with over-emphasis on strength and conditioning exercises that are sport specific, are being used too early in the athlete's programme. Rather than building a solid base of balanced exercises for the whole body first.
Although individuals may survive those imbalances when they are younger and their body ‘copes,’ eventually injuries will occur with incorrect repetitive movements during training.  That’s when the physio asks the inevitable ‘when did you first notice the pain?’ Which implies a single sporting or training incident was responsible. Yet frequently it is the cumulative effect of muscle imbalances, and the associated inefficient movement patterns they produce, that lead to the single significant time-point of injury. Janda described how injuries changed an individual’s movement patterns which subsequently created changes in the nervous system. This causes faulty motor programming - messages from the nervous system dictating how we move – to dominate. This leads to inappropriately altered joint forces and proprioception. Proprioception is the gathering of stimuli - mechanical stimuli like a change of joint position- from the periphery of the body by specialised receptors. Those stimuli are transmitted via neural (nerve) pathways to the brain and spinal cord for integration and final processing.  So the brain senses the joint’s position. This altered proprioception may feel normal despite the joints being biomechanically unbalanced and subject to an uneven distribution of forces.
A trainer can identify- and eventually correct - muscular imbalances and joint dysfunction by carrying out a postural assessment. The plumb-line method is frequently used for this purpose.  Kendall described how in Muscles Testing and Function: the subject stands behind a plumb- line facing forwards. This splits the body in half down the midline so the following can be evaluated: knock knees (genu valgum) or bowlegs (varum); foot pronation through comparisons of one foot against the other relative to the plumb-line; pelvic rotation and rib cage misalignment. This process is repeated with the plumb-line behind the subject and then from the side with the plumb-line lining up: the bodies of the cervical vertebrae, the shoulder joint centre, midway through the trunk, through the greater trochanter (outer hip), middle/slightly anterior aspect of the knee joint and slightly anterior to the lateral malleolus (bony bit to the outside of the ankle joint). Deviations from those anatomical landmarks, relative to the reference point of the plumb-line, indicates muscular imbalance. These indicators assists in gauging which muscle groups are unbalanced. Active hands on testing procedures can also be used to confirm those findings. For example, the Thomas Test for hip flexor tightness or laxity.
Additionally, a subject’s movement patterns can be tested to check if muscle imbalances are restricting joint movements, and creating a potential injury situation. This movement screen also confirms the findings of the previous postural analysis. Gray Cook has developed such a functional screen to evaluate the quality of an individual’s movement. He uses twenty six initial screening movements so trainers can ‘get their eye in’ by assessing client movement. This evolves into a five gross movement pattern screen which consists of: a deep squat, an in-line lunge, a hurdle step, a lying down single straight leg rise and a seated spinal rotation. Cook’s excellent Athletic Body in Balance describes these procedures in depth.
This is a summary of the basic tools that can be used to ensure that novice or experienced sports-persons identify potential injury causing imbalances. For a runner inefficient movement means wasted energy which translates to him/her finishing further down the field. For example, a forward knee lift is going to be much harder as a tight hamstring restricts this movement. And a weight trainer may struggle to press (and position) a barbell safely overhead if they have a shoulder imbalance due to maladapted or tight rotator cuff muscles (muscles that stabilise the shoulder joint). Or scapulae (shoulder blades) that are incorrectly positioned on the rib cage; perhaps because the upper portion of the spine (thoracic region) is excessively curved (long kyphosis). Patience is the key. Individuals should address muscle imbalances before increasing training loads or intensities. Failure to do so may result in more debilitating injuries and lost training time. Further, screening can be regularly repeated to measure progress. Also remedial exercises that arise from this process can be incorporated into an individual’s training schedule, perhaps as part of the warm-up. Ultimately, it is better to slowly build layers of increasingly difficult physical conditioning; which are dictated by positive feedback from remedial postural training, and eventually reach your highest peak of fitness. Than never realise your full potential because injuries prevent true progress.