The effects of grade and speed on leg muscle activations during walking
Highlights
► Leg muscle activities during level, uphill, and downhill walking at various speeds. ► Hip, knee, and ankle extensor muscle activities increase with steeper uphill grade. ► Only knee extensor muscle activities increase with steeper downhill grade. ► Changes in muscle activity with grade are more pronounced at faster walking speeds
Introduction
The ability to appropriately recruit leg muscles in response to changes in the environment is fundamental to the control of human locomotion. For example, many of our daily activities routinely include walking up and down hills which involve very different biomechanics [1], [2]. Electromyographic (EMG) recordings can provide insight into how the neuromuscular system makes these adjustments. A handful of studies have investigated muscle activations during uphill or downhill walking and have provided important insight into the muscle recruitment strategies employed [3], [4], [5], [6], [7]. However, most have done so at a single, self-selected walking speed, and recent evidence also suggests that faster walking speeds elicit greater increases in thigh muscle activations with steeper uphill grade [8]. We sought to build upon these studies to obtain a more comprehensive understanding of how leg muscle activations change when walking up and downhill over a range of walking speeds.
The major leg muscles are most active during the stance phase of walking [9], when they perform work on the center of mass and support body weight. Compared to level walking, additional muscle actions are required to raise and lower the center of mass during uphill and downhill walking, respectively. Lay et al. [4] recorded leg muscle EMG signals for walking at extreme uphill and downhill grades (21.3°). They surmised that the leg extensor muscles in particular meet these demands presumably through greater concentric activity to walk uphill and greater eccentric activity to walk downhill (though see [10]). More specifically, Lay et al. [4] reported that when walking at the same speed, hip, knee, and ankle (plantarflexor) extensor muscle activations increased during uphill walking, but only the knee extensor muscle activations increased during downhill walking. However, it remains unclear whether such muscle recruitment strategies occur generally over a range of walking speeds, and/or at more moderate and typical grades.
When walking faster over level ground, the normalized temporal patterns of leg muscle activity remain fairly stable but with increasing amplitudes [11], [12], [13], [14], [15]. Is this also the case when walking up or downhill? Recently, Wall-Scheffler et al. [8] found that stance phase quadriceps muscle activity increased during uphill walking to a greater extent when walking faster. This finding demonstrates an interaction between grade and walking speed for muscles of the thigh when walking uphill, but it is not known if the same trend generally occurs in other leg muscles during both uphill and downhill walking. A more comprehensive understanding of how leg muscle activations change with grade and speed could help guide the development of therapies aimed at improving the quality of life and maintaining the independence of people with walking ability limitations due to injury, disease, or aging (e.g., [16]). A first step in this direction is to establish these muscle activation patterns for healthy young adults.
The purpose of our study was to quantify leg extensor muscle activations during the stance phase of level, uphill, and downhill walking at various speeds. We hypothesized that over a range of walking speeds, compared to level walking, (1) hip, knee, and ankle extensor muscle activations would increase with steeper uphill grade, but (2) only knee extensor muscle activations would increase with steeper downhill grade. We also hypothesized that, (3) the changes in muscle activations with grade would become more pronounced at faster walking speeds. To test these hypotheses, we recorded leg extensor muscle activations while subjects walked on a treadmill on the level, at a range of uphill and downhill grades, and at various speeds.
Section snippets
Subjects
10 young adults (5F/5M) volunteered for this study (mean ± standard deviation, age: 25.3 ± 3.9 years; height: 1.73 ± 0.10 m; mass: 69.2 ± 13.1 kg). All subjects were experienced treadmill users and had no known neuromuscular, cardiovascular, or orthopedic diseases. Each subject gave written informed consent before participating as per the University of Colorado Institutional Review Board.
Experimental protocol
Subjects walked on a classic motorized treadmill (model 18-60, Quinton Instruments, Seattle, WA) set to 1.25 m s−1 and
Results
As expected, stance phase muscle activity amplitudes generally increased with faster walking speed for all grades tested (Fig. 1, Fig. 2, Fig. 3 and Table S1). Also, as walking speed increased, subjects took progressively faster strides, and spent less time in stance (Table 1, p < 0.0063). Fig. 1, Fig. 2, Fig. 3 display the mean EMG values and example EMG profiles across all conditions for muscles acting to extend the hip, knee, and ankle, respectively. Table S1 summarizes these values. All
Discussion
This study quantified leg extensor muscle activations during the stance phase of level, uphill, and downhill walking over a range of speeds. Compared to level walking, hip, knee, and ankle extensor muscle activations increased to walk uphill but only knee extensor muscle activations increased to walk downhill. This study expands upon our understanding of muscle activities during uphill and downhill walking to show that these muscle recruitment strategies occur generally over a range of walking
Acknowledgements
A grant from NIH (5T32AG000279) and a student Grant-in-Aid Award from the American Society of Biomechanics awarded to Jason R. Franz supported this study.
Conflict of interest
The authors have no conflicts of interest to disclose.
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