Neuromuscular determinants of maximum walking speed in well-functioning older adults

Abstract

Maximum walking speed may offer an advantage over usual walking speed for clinical assessment of age-related declines in mobility function that are due to neuromuscular impairment. The objective of this study was to determine the extent to which maximum walking speed is affected by neuromuscular function of the lower extremities in older adults. We recruited two groups of healthy, well functioning older adults who differed primarily on maximum walking speed. We hypothesized that individuals with slower maximum walking speed would exhibit reduced lower extremity muscle size and impaired plantarflexion force production and neuromuscular activation during a rapid contraction of the triceps surae muscle group (soleus (SO) and gastrocnemius (MG)). All participants were required to have usual 10-meter walking speed of >1.0m/s. If the difference between usual and maximum 10m walking speed was <0.6m/s, the individual was assigned to the "Slower" group (n=8). If the difference between usual and maximum 10-meter walking speed was >0.6m/s, the individual was assigned to the "Faster" group (n=12). Peak rate of force development (RFD) and rate of neuromuscular activation (rate of EMG rise) of the triceps surae muscle group were assessed during a rapid plantarflexion movement. Muscle cross sectional area of the right triceps surae, quadriceps and hamstrings muscle groups was determined by magnetic resonance imaging. Across participants, the difference between usual and maximal walking speed was predominantly dictated by maximum walking speed (r=.85). We therefore report maximum walking speed (1.76 and 2.17m/s in Slower and Faster, p<.001) rather than the difference between usual and maximal. Plantarflexion RFD was 38% lower (p=.002) in Slower compared to Faster. MG rate of EMG rise was 34% lower (p=.01) in Slower than Faster, but SO rate of EMG rise did not differ between groups (p=.73). Contrary to our hypothesis, muscle CSA was not lower in Slower than Faster for the muscle groups tested, which included triceps surae (p=.44), quadriceps (p=.76) and hamstrings (p=.98). MG rate of EMG rise was positively associated with RFD and maximum 10m walking speed, but not the usual 10m walking speed. These findings support the conclusion that maximum walking speed is limited by impaired neuromuscular force and activation of the triceps surae muscle group. Future research should further evaluate the utility of maximum walking speed for use in clinical assessment to detect and monitor age-related functional decline.

Figure 1. Representative force and EMG data

Data from a representative heel-rise trial are shown for one participant. The top figure shows vertical force produced by the left (black) and right (gray) sides. Peak force occurs at point “b”. Rate of force production was calculated between points “a” and “b”. The lower set of figures shows unprocessed EMG data from the soleus (SO) and medial gastrocnemius (MG) muscles.

Figure 2. EMG processing for rate of EMG rise

The light gray line is EMG after rectification. The solid black line is the rectified signal after being smoothed with a low pass filter (10 Hz). The dashed black line is the derivative of the smoothed, rectified signal. The peak rate of rise is indicated by the arrow. These data are shown for illustrative purposes and are not drawn to scale.

Figure 3

Flow chart of participant screening and assessment.