Muscle volume as a predictor of maximum force generating ability in the plantar flexors post‐stroke

Abstract

Introduction Post-stroke muscle weakness is commonly thought to be the result of a combination of decreased voluntary activation and decreased maximum force generating ability (MFGA). We assessed the ability of muscle volumes obtained using magnetic resonance imaging (MRI) to estimate the MFGA of the plantar flexor muscle group in individuals post-stroke. Methods MRI was used to measure muscle volume for the plantar flexor muscle group for 17 individuals with post-stroke hemiparesis. A modified burst superimposition test was used to measure MVC and predict the MFGA of the plantar flexors. Results While muscle volume obtained via MRI provided information on the overall size of muscle, it overestimated the force generating ability of the paretic plantar flexors. Discussion Results suggest that MRI-derived muscle volume underestimates the functional impairment in individuals post-stroke. Interestingly, the central activation ratio had a strong relationship with the maximum volitional force of contraction.

Activation deficit correlates with weakness in chronic stroke: Evidence from evoked and voluntary EMG recordings

“…The result of a moderate correlation between the relative voluntary activation and weakness (Figure 4) further support the important role of voluntary activation deficit in weakness. This is consistent with an earlier report that the force generating ability of the paretic plantar flexors is overestimated by the size of the paretic muscle (Knarr et al, in press). Overall, these findings using M-wave and MVC EMG comparisons in this study are consistent with previous findings obtained from other techniques (MRI, ultrasound, ITT as mentioned in the Introduction section).…”

“…Peripheral factors such as muscle fiber loss, fat infiltration, altered contractile properties have also been reported (reviewed in (Gracies, 2005)). Muscle size estimated by MRI or ultrasound (Klein et al , 2010, Klein et al, 2013, Knarr et al, in press, Triandafilou and Kamper, 2012) shows small to minimal changes on the impaired side. Furthermore, these estimates do not reflect altered contractile properties of the impaired muscle.…”

“…As such, these observed changes are not sufficient to account for weakness on the impaired side. For example, the force generating ability of the paretic plantar flexors is overestimated using the muscle volume obtained from MRI (Knarr et al in press). Thus, these findings suggest an important role for central factors.…”

Skeletal muscle changes following stroke: a systematic review and comparison to healthy individuals

“…The paretic thigh muscle size was an average of 87% (range: 76–101%) and the paretic lower leg muscle size was an average of 95% (range: 80–104%) of the nonparetic muscle size. Specific muscle size values could not be extracted in three 25,27,28 of the 15 studies, two of which used MRI and one ultrasound. (Table 1).…”

Development of a neuromusculoskeletal computer model in a chondrodystrophic dog.

“…Muscle volume, fiber length, pennation angle, and specific tension were used to define the maximum isometric force for each muscle, using the following equation (Knarr et al, 2013):…”