Is there any point measuring static variables in clinical practice? By Trevor Prior

I would like to welcome a new guest to the Biomechanics blog – Trevor Prior. Trevor is a fantastic London based podiatrist, working at Premier Podiatry with more than 30 years clinical experience. As a result he has a wealth of knowledge which he is always keen to share, especially if you have the pleasure of sharing a beer with him. This will be the first of a couple of blog pieces from Trevor, and we will top this off with a great podcast. Discussion points here and on Twitter will be covered during the podcast. For now, over to you Trevor to discuss the importance (or not) of static static assessment …………..

Having been qualified for more years than I care to remember, I have thought more about foot function and lower limb function in general than any other aspect. It seems fundamental to everything I do whether it be foot surgery, managing complex diabetic / rheumatoid feet or sports injuries. The first 1/3 of my career was spent understanding the concepts of Podiatric Biomechanics as it was relatively new at that time. The next 1/3 were spent considering the evidence that showed these concepts to be flawed, understanding the varying theories that have been postulated since (all yet to be validated) and trying to make some practical sense. This has overlapped with the last 1/3 whereby I have tried to evaluate whether or not the function I observe is due to the underlying foot structure, factors proximal to the foot or extrinsic factors. Throughout this period, it has always struck me that my static clinical examination still determines my method of treatment and, above all, there is a need for a sensible process that allows us to teach students a reasonable method of assessment rather than leave them to gain experience and develop subjective assessment approaches.

To begin with, let us try and define what we mean by static versus dynamic assessment. Many of the definitions are provided in the computing environment but Wagner et al1 provide a nice definition which can be applied to the clinical assessment:

  1. Static: Assessment of an individual isolated variable where the subject is static /passive at the time of the measure and the examiner performs the assessment

Examples: A joint ROM / position, muscle flexibility

  1. Quasi static: Use of discrete variables to estimate function but velocity and acceleration set at 0

Examples: STJ axis location, great toe extension test, navicular drop / drift, muscle strength – subjective resistance / dynamometer, etc.), core strength assessments

  1. Dynamic: The subject is moving with no direct contact from the examiner. Strict dynamic assessment should allow assessment of joint forces, internal stresses

Examples: Single leg squat, single leg heel raise / strength testing through range / isokinetics, motion analysis

With regards foot function, Root, Orien and Weed2,3 are credited with the introduction of podiatric biomechanics, outlining a set of clinical measures (mainly static and quasi static) and purported function, much of which was geared around the foot functioning about subtalar neutral. However, it soon became apparent that the foot did not actually function around this position during stance4,5 and that there was more than one pattern of foot pronation6. Interestingly, whilst it is reported that static measures have poor repeatability, many of the papers that have evaluated foot function were able to demonstrate adequate repeatability for the use of the measures in the study. This aside, it became clear that static foot structure did not predict dynamic foot function.

Nester went on to demonstrate that the two axes concept of the midtarsal joint proposed by Root, Orien and Weed is inaccurate and referred to an ‘overall’ or resultant axis.7,8 Nester subsequently reviewed the literature relating to foot function and noted that all of the joints of the rearfoot / midfoot had triplanar motion, that the degree and direction of motion has high inter subject variability and that the amount of motion decreases with increased activity levels.9 Thus, the resultant axis of the midtarsal joint is a combination of the many individual axes occurring throughout the midfoot / forefoot. To make life more complicated, this resultant axis will vary throughout stance. Evidence is now emerging regarding the coupling between leg and foot motion.10

This video demonstrates the rearfoot inversion / eversion for a patient who has had a successful left sided subtalar fusion. Whilst motion is reduced on the left, it is greater than one might expect as motion can occur in the frontal plane at the ankle.

Clearly, structural alignment alone is a poor predictor and to try and provide a basis for assessment and management, Kirby applied physics principles and described the orientation of the subtalar axis in the transverse plane on the sole of the foot.11 He introduced the concept of rotational equilibrium whereby the intrinsic (muscle contraction, ligamentous constraint, structural, proximal drivers etc.) and extrinsic (ground reaction) forces medial and lateral to this axis position would produce pronation and supination moments and thus provide a principle for management.12,13,14 He observed that patients may demonstrate deviation of this axis and thus change the relative forces and function.

Ian Griffiths, in an excellent blog,15 describes this concept and how it may be applied. This is well worth a read for all interested.

It is fair to say that, whilst underpinned by sound physics principles, this concept has yet to be validated in clinical trials. Additionally, the effect of the axis orientation in the other planes and how the position changes through stance is unknown and the effect of the midtarsal / forefoot function on the axis position is difficult to determine. This should not detract from the principles that relate to the intrinsic / extrinsic factors so that we consider both the kinematic and kinetic factors of function, but does highlight further work is needed to improve the clinical application of this theoretical paradigm.

Similarly, Dananberg introduced a sagittal plane theory based around a functional limitation of 1st MTPJ dorsiflexion16,17,18. He outlined a cascade of events and compensations that can occur with this condition and a further treatment approach. Whilst largely kinematic in basis, further research is required.

When assessing patients dynamically, we have many tools for evaluating function both subjectively and objectively. When we observe dysfunction, we may be able to quantify a particular dysfunction but observing movement dysfunction does not explain why it happens. An easy example may be early heel lift. This may be due a forefoot equinus, OA at the ankle, a tight calf muscle, knee flexion, a leg length discrepancy amongst others. Thus, we can observe this dynamically but this does not provide the reason and thus allow management (if required). In this example (early heel lift) an assessment of ankle and knee range of motion / position, leg length and muscle flexibility / strength would be required. Heel raises to temporarily offset posterior muscle inflexibility or accommodate forefoot equinus, appropriate exercises or leg length control are some of the options that may be applied dependent upon the findings.

Excessive knee flexion can also occur with sagittal plane spinal dysfunction/imbalance whereby altered alignment of the pelvis / spine results in increased thoracic kyphosis shifting weight forwards. In some individuals, this is counteracted by excessive knee flexion and thus altered foot and ankle function.19,20

It then becomes a question of what measures are relevant. If we take the rearfoot as an example, we can consider alignment versus range. The theoretical neutral position of the subtalar joint alone is small and has not been demonstrated to have any clinical value, whereas the tibial position maybe more indicative of functional variation.If this position is considered with an evaluation of the range of the rearfoot motion available, it is possible to gain an indication of how the rearfoot might move. Thus, if we had a moderate amount of tibial / rearfoot varum and a moderate amount of rearfoot inversion / eversion, a certain amount of motion would occur – I have deliberately ignored the effect of the midfoot at this stage as the aim is to explain a concept. If we then consider another case with high tibial varum but restricted rearfoot inversion / eversion it is likely we would observe a different type of function. Thus, the tibial position and rearfoot motion provide us with an indication of the functional capability of this particular area.

We can extrapolate this to consider rearfoot dorsiflexion / plantarflexion which will primarily be around the ankle joint. A tight calf muscle will restrict the functional capability in this plane and thus alter function. How the foot / leg will compensate will depend on the variables both proximally and distally to the rearfoot. A foot with restricted motion is likely to compensate differently to a flexible foot irrespective of the direction of the motion.

Static measures (which includes alignment, strength, flexibility, control etc.) provide an indication of the functional capability. In isolation, it is like having two pieces of the jigsaw, it is impossible to form the picture. The more pieces, the more complete the picture. That one piece is not representative of the picture does not make it irrelevant but the relevance is determined by where on the scale of normal this sits and thus how it interacts with the other factors. The skill is identifying and putting the pieces together to build the picture. The practitioner that only evaluates one aspect, misses the picture.

In some instances, the case may be simple and not much is required. As the problem becomes more complex or an injury is persistent or a range of injuries have occurred, a broader assessment is required. Ignoring the functional capability determined by static measures prevents a complete assessment. With straight forward cases or experience, many of these can be performed subjectively. With less experience and more complexity, objective measures have value. However, these should be repeatable and a normal range should be available so one can determine where within the normal range that measure sits and thus how it potentially places the tissues under more or less stress.

In summary, static measures are important in determining the functional capability of an individual but dynamic assessment is required to observe the consequences and subsequently coupled with the specific activity and other extrinsic factors pertinent to the individual. Subjective assessment works well in the hands of the experienced and less complex cases. Objective, repeatable measures with a normal range have a place for the less experienced and more complex cases.


  1. Wagner et al:
  2. Root ML, Orien WP, Weed JH, Hughes RJ, Biomechanical examination of the foot, Volume 1, Clinical Biomechanics Corporation, Los Angeles, 1971.
  3. Root ML, Orien WP, Weed JH, Biomechanical examination of the foot, Volume 1, Clinical Biomechanics Corporation, Los Angeles, 1977.
  4. McPoil T, Cornwall M W, Relationship between subtalar joint neutral position and pattern of rearfoot motion during walking, Foot & Ankle, 1994, Vol. 15, No.3, pp 141-145
  5. Pierrynowski M R, Smith S B, Rearfoot inversion/eversion during gait relative to the subtalar joint neutral position, Foot & Ankle, 1996, Vol. 17, No.7, pp 406-412
  6. Cornwall MW, McPoil TG, Classification of Frontal plane rearfoot motion patterns during the stance phase of walking, JAPMA 2009, 99(5):399-405
  7. Nester CJ, Fibdlow A, Bowker P, Scientific approach to the axis of rotation at the midtarsal joint, JAPMA 2001, 91(2):68-73
  8. Nester CJ, Bowker P, Bowden P, Kinematics of the midtarsal joint during standing leg rotation, JAPMA 2002, 92(2):77-81
  9. Nester CJ, lessons from dynamic cadaver and invasive bone pin studies: do we know how the foot really moves during gait?, JFAR 2009 2:18
  10. Dubbeldam R, Nester C, Nenee AV, Hermens HJ, Buurke JH, Kinematic coupling relationships exist between non-adjacent segments of the foot and ankle of healthy subjects, Gait & Posture 2013, 37:159-164
  11. Kirby K A, Methods for determination of positional variations in the subtalar joint axis, JAPMA, 1987, 77(5): 228-234
  12. Kirby K A, Rotational equilibrium across the subtalar joint axis, JAPMA 1989, 79(1):1-14
  13. Kirby KA, Subtalar joint axis location and rotational equilibrium theory of foot function, JAPMA 2001, 91(9):465-487
  14. Kirby K A, The medial heel skive technique, improving pronation control in foot orthoses, JAPMA 1992, 82(4): 177-188
  15. Griffiths I, Putting the mechanics back into ‘biomechanics’,
  16. Dananberg H J, Functional hallux limitus and its relationship to gait efficiency, JAPMA 1986, 76(11):648-652
  17. Dananberg H J, Gait style as an etiology to chronic postural pain; Part I. Functional hallux limitus, JAPMA 1993, 83(8):433-441
  18. Dananberg H J, Gait style as an etiology to chronic postural pain; Part II. Postural compensatory process, JAPMA 1993, 83(11):615-624
  19. Johnson RD, Valore A, Villaminar A, Comisso M, balsano M, Sagittal balance and pelvic parameters – a paradigm shift, J Cl Neuro 2013, 20:191-196
  20. Gottipati P, Fatone S, Koski T, Sugrue PA, Ganju A, Crouch gait in persons with positive sagittal spine alignment resolves with surgery, Gait & Posture 2014, 39:372-377