Abstract
This study determined the discharge characteristics of motor units from two lower limb muscles before and after fatiguing exercise in people with type 2 diabetes (T2D) with no symptoms of polyneuropathy and activity‐matched controls. Seventeen people with T2D (65.0 ± 5.6 years; 8 women) and 17 controls (63.6 ± 4.5 years; 8 women) performed: (a) intermittent, isometric contractions at 50% maximal voluntary isometric contraction (MVIC) sustained to failure with the ankle dorsiflexors, and (b) a dynamic fatiguing task (30% MVIC load) for 6 min with the knee extensors. Before and after the fatiguing tasks, motor unit characteristics (including coefficient of variation (CV) of interspike intervals (ISI)) were quantified from high‐density electromyography and muscle contractile properties were assessed via electrical stimulation. Fatigability was ~50% greater for people with T2D than controls for the dorsiflexors (time‐to‐failure: 7.3 ± 4.1 vs. 14.3 ± 9.1 min, p = .010) and knee extensors (power reduction: 56.7 ± 11.9 vs. 31.5 ± 25.5%, p < .001). The CV of ISI was greater for the T2D than control group for the tibialis anterior (23.1 ± 11.0 vs. 21.3 ± 10.7%, p < .001) and vastus lateralis (27.8 ± 20.2 vs. 24.5 ± 16.1%, p = .011), but these differences did not change after the fatiguing exercises. People with T2D had greater reductions in the electrically evoked twitch amplitude of the dorsiflexors (8.5 ± 5.1 vs. 4.0 ± 3.4%·min‐1, p = .013) and knee extensors (49.1 ± 10.0 vs. 31.8 ± 15.9%, p = .004) than controls. Although motor unit activity was more variable in people with T2D than controls, the greater fatigability of the T2D group for lower limb muscles was due to mechanisms involving disruption of contractile function of the exercising muscles rather than motor unit behavior.
Highlights
Type 2 diabetes mellitus (T2D) is characterized by chronically elevated blood glucose and is one of the most costly chronic diseases in the United States (Trogdon et al, 2015)
Reduced muscle size and impaired muscle contractile function are commonly observed in people with T2D and diabetic polyneuropathy (Allen et al, 2013; Allen, Major, et al, 2014; Ijzerman et al, 2012), we found no reductions in muscle mass in the upper or lower leg muscles or contractile function of the knee extensors or dorsiflexors in people with T2D compared to controls
The present study provides novel evidence that people with T2D without clinically detected diabetic polyneuropathy (DPN) have greater variability in both torque and motor unit characteristics compared with controls matched for age, body fat, and physical activity
Summary
Type 2 diabetes mellitus (T2D) is characterized by chronically elevated blood glucose and is one of the most costly chronic diseases in the United States (Trogdon et al, 2015). Physical activity is a cornerstone of management of T2D (Sigal, Kenny, Wasserman, and Castaneda-Sceppa (2004)), there are numerous musculoskeletal complications associated with T2D that create a significant barrier to complete regular exercise (Smith, Burnet, & McNeil, 2003) These musculoskeletal complications include reduced maximal muscle strength and power (Ijzerman et al, 2011) and greater fatigability of limb muscles (activity-induced reductions in strength or power (Gandevia, 2001; Hunter, 2018)) during exercise compared with healthy controls (Allen, Kimpinski, Doherty, & Rice, 2015; Almeida, Riddell, & Cafarelli, 2008; Bazzucchi et al, 2015; Halvatsiotis, Short, Bigelow, & Nair, 2002; Ijzerman et al, 2012; Petrofsky et al, 2005; Senefeld, Limberg, Lukaszewicz, & Hunter, 2019; Senefeld, Magill, Harkins, Harmer, & Hunter, 2018). Whether motor unit behavior differs in people with T2D who do not have clinical signs of DNP is not known
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