The Importance of Knee Extension Strength for Clients with Cerebral Palsy
Spasticity, while common in clients with CP, doesn’t appear to be a cause of muscle weakness¹. And low muscle strength, not spasticity, appears to be the greatest limitation in motor function for individuals with Cerebral Palsy². So instead of focusing our attention on ways to mitigate symptoms of spasticity, we should likely prioritize strength training strategies to improve functional ability.
Research shows that the greatest difference between independent walkers and those dependent on walking aides was in the hip abductors and knee extensors at 30° (Thompson et al. 2011 PMID: 21169021). These are important muscle groups in sagittal and coronal plane walking stability. Weakness was more significant as you moved from GMFCS (Gross Motor Functioning Classification System) I to III; which correlates to ambulatory function.
While many papers look at aerobic capacity, strength discrepancies, and spasticity, a paper from Noelle Moreau looked at the importance of rate of force development (RFD) and impulse of the knee extensors and how they relate to gait. Many everyday activities, including walking and running, involve contraction times of 50-200ms, faster than it takes to contract human muscle tissue. This makes a case for not only training maximal strength qualities but also power.
Their paper (Moreau et al. 2012 PMID: 21930383) demonstrated that Vastus Lateralis muscle thickness was the primary determinant of RFD and impulse in CP. Another paper from Moreau and colleagues (Moreau et al. 2013 PMID: 23292847), compared traditional strength training (ST) and velocity training (VT). Their results showed that muscle in CP is capable of adapting to different training stimuli and that only the VT group saw increases in power and faster walking speeds.
Combining Moreau’s work, Ross’s discussion on the influence of strength vs. spasticity on function, and Thompson’s conclusions about the importance of the knee extensors at 30° gives us insight into the importance of training the quadriceps for hypertrophy, power, and potentially isometrically at specific joint angles in clients with Cerebral palsy.
In our AdaptX course, we cover lower-body variations for clients with mobility impairments; ways to set up common exercises with the addition of external stability as well as unconventional uses of equipment such as Prowler Sleds and Rowing Machines to function as modified leg presses.
The Feasibility and Efficacy of Blood Flow Restriction Training
Since our capacity to load some lower body exercises for our clients with Cerebral Palsy is often limited by the stability or setup, we've begun to look into the utility of training methods like Blood Flow Restriction (BFR) training. The thought process is that BFR has demonstrated similar hypertrophic responses with low loads as traditional resistance training has shown; provided that the proximity to failure is similar.
Our paper, Implications and Clinical Outcomes of Blood Flow Restriction Training for Adults with Cerebral Palsy, looked at whether the addition of BFR to low-load leg extensions and leg press improved hypertrophy and strength in a case series of three individuals with CP. Results were inconclusive and statistically insignificant; which is to be expected with a sample size of only 3. But we learned a lot about the logistics, feasibility, and safety of incorporating blood flow restriction training with the population.
Future projects with more participants and potentially different equipment, like the pneumatic resistance that can be applied with Keiser's line of machines, might elicit a better training response as it applies consistent resistance across the entirety of the eccentric and concentric contractions. Both of the exercises we performed in this study were predominantly concentric muscle actions; and we know that the eccentric component is important for hypertrophy.
Using Force Plates to Assess Strength and Rate of Force Development
With Moreau's work emphasizing the importance of rate of force development and inconsistencies in the results achieved through hand-held dynamometry to measure strength, we're seeking a more valid and reliable way to assess these characteristics in the population.
Using the VALD ForceDecks in our lab at AdaptX, we've constructed a set-up for our clients with Cerebral Palsy GMFCS levels 1-3 to test Isometric Mid Thigh Pull (IMTP). The data we can gather from this testing includes rate of force development, peak force output, right to left symmetry in force application, and many other metrics.
We'll share the results of this project in a later blog post.
