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nkarman
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| Location: | New York USA |
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Posted: Wed Jul 6th, 2011 12:32 pm |
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This question was posed in response to comments regarding the strength required to achieve full knee extension in mid-stance (i.e. when full knee extension is not achieved, what is the likeliest culprit.
My response at the time was related to how that "4/5 strength" is calculated. Most strength assessment (and specifically those that yield a "score" on a scale of 0-5) are based on manual muscle testing (MMT), specifically a brake test performed at mid-range. This test was developed to score strength following peripheral nerve injury (polio), and while reliable, not particularly valid when applied to the gait cycle. The knee extensor (quadriceps) strength necessary in mid-stance during gait requires several components not accounted for in the standard MMT, in which a score of 4 indicates (though rarely tests) the ability to hold against some resistance throughout the range of motion).
Firstly, to make that claim, the test would have to be performed throughout the ROM, including in the last few degrees of extension, which are necessary to achieve terminal knee extension in mid-stance. These tests are typically performed at mid-range, where the length-tension relationship is closer to optimal, and not in the last 20 or 30 degrees of knee extension, much less throughout that portion of the range, where the length-tension relationship is less ideal, and children with CP have been shown to have greater weakness(quadriceps moments increase as knee flexion increases in children with CP, Hamstring moments do not change, Ikeda, et. al. 1998). The functional portion of the range needed for mid-stance is very different than that tested in a typical MMT, and is likely to be a weaker portion of the range than what is typically tested, therefore not representative of functional strength. It is also typically tested only once, perhaps twice, but not repeatedly, and therefore does not indicate endurance, which would be necessary to take multiple steps sequentially while walking: fast onset of fatigue would limit walking distance (number of steps) to the number of repetitions able to be performed prior to onset of fatigue.
Secondly, the resistance provided during MMT is at the discretion of the evaluator, and "4/5" refers to a broad range of strength ("more resisance than gravity, but less than normal/opposite side, which in children with CP may also not be normal)... The strength "needed" in mid-stance is adequate to fully extend the knee in closed kinetic chain, against full body weight resistance (single limb stance). It is unlikely that the amount of resistance applied during a typical MMT reaches that level of resistance.
Thirdly, the MMT does not evaluate power, it only tests total force production. Considering that the normal adult cadence of gait is 1 cycle/sec (higher in young children), and stance phase is 60% of that, and midstance (for argument's sake) is halfway through stance phase, that would give the child .3 seconds to achieve adequate force production to fully extend the knee against (above-quantified) resistance. "4/5" does not speak to power (or time to peak force production) at all.
Fourthly, 4/5 strength tested in open kinetic chain does not take into consideration some of the dynamic components of walking that should help to extend the knee at midstance in normal walking, but may not be present in children with CP. Specifically, the relationship between the center of mass and the axis of rotation of the knee joint in the sagittal plane, to create a knee extension moment. If the center of mass does not fall anterior to the knee joint, the biomechanical assistance (extensor bias) will not be present, thereby increasing the force necessary to achieve full knee extension.
Considering all of this, a more functional quantification of quadriceps "strength" necessary to achieve full knee extension in mid-stance, would be the ability to fully extend the knee in single limb stance (without upper extremity support), repeatedly (how many steps/day), at the speed required for gait (power).
The challenge, then, would be to assess this clinically, and to strengthen in a functional context: we have identified this need in the adult orthopedic population (closed kinetic chain strengthening), but due to "other" (balance, etc.) deficits in the population of children with CP, we rarely have a mechanism to perform this type of strengthening within the child's ability: Due to weakness and balance deficits, more often than not, they strengthen in open kinetic chain, or with upper body loaded/UE weight bearing, and rarely in closed chain, FWB... and, rarely for large numbers of repetitions, and (often due to spasticity concerns), rarely at the speeds necessary to access that force development within the context of the gait cycle...
The floor is open, let's keep the conversation going!Last edited on Wed Jul 6th, 2011 12:58 pm by nkarman
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nkarman
Instructor
| Joined: | Sat Jul 15th, 2006 |
| Location: | New York USA |
| Posts: | 259 |
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Posted: Wed Jul 6th, 2011 12:01 pm |
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| Could you please talk more about quads strength? If they are able to hold the stand they at least have 4/5. Do you think it is weakness vs muscle contraction (hamstrings complex)?
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