Early initiation of electrical stimulation paired with range of motion after a volumetric muscle loss injury does not benefit muscle function

Abstract

New findings: What is the central question of this study? First, how does physical rehabilitation influence recovery from traumatic muscle injury? Second, how does physical activity impact the rehabilitation response for skeletal muscle function and whole-body metabolism? What is the main finding and its importance? The most salient findings were that rehabilitation impaired muscle function and range of motion, while restricting activity mitigated some negative effects but also impacted whole-body metabolism. These data suggest that first, work must continue to explore treatment parameters, including modality, time, type, duration and intensity, to find the best rehabilitation approaches for volumetric muscle loss injuries; and second, restricting activity acutely might enhance rehabilitation response, but whole-body co-morbidities should continue to be considered.

Abstract: Volumetric muscle loss (VML) injury occurs when a substantial volume of muscle is lost by surgical removal or trauma, resulting in an irrecoverable deficit in muscle function. Recently, it was suggested that VML impacts whole-body and muscle-specific metabolism, which might contribute to the inability of the muscle to respond to treatments such as physical rehabilitation. The aim of this work was to understand the complex relationship between physical activity and the response to rehabilitation after VML in an animal model, evaluating the rehabilitation response by measurement of muscle function and whole-body metabolism. Adult male mice (n = 24) underwent a multi-muscle, full-thickness VML injury to the gastrocnemius, soleus and plantaris muscles and were randomized into one of three groups: (1) untreated; (2) rehabilitation (i.e., combined electrical stimulation and range of motion, twice per week, beginning 72 h post-injury, for ∼8 weeks); or (3) rehabilitation and restriction of physical activity. There was a lack of positive adaption associated with electrical stimulation and range of motion intervention alone; however, maximal isometric torque of the posterior muscle group was greater in mice receiving treatment with activity restriction (P = 0.008). Physical activity and whole-body metabolism were measured ∼6 weeks post-injury; metabolic rate decreased (P = 0.001) and respiratory exchange ratio increased (P = 0.022) with activity restriction. Therefore, restricting physical activity might enhance an intervention delivered to the injured muscle group but impair whole-body metabolism. It is possible that restricting activity is important initially post-injury to protect the muscle from excess demand. A gradual increase in activity throughout the course of treatment might optimize muscle function and whole-body metabolism.

Keywords: muscle function; neuromusculoskeletal injury; range of motion; skeletal muscle.

FIGURE 1

Evaluation of range of motion about the ankle joint. (a) Representative digital images of plantar‐ and dorsi‐flexion. To determine the range of motion about the ankle, visible markers were placed on the lateral tibial plateau, lateral malleolus and distal metatarsal. Quantification of joint angles was determined at extremes of plantar‐ and dorsi‐flexion. (b) The total range of motion was most impaired in the group undergoing rehabilitation alone, compared with VML alone (P = 0.001). Additionally, the plantarflexion range of motion was significantly impaired in both rehabilitation groups (P ≤ 0.0001). There was no difference in dorsiflexion (P = 0.934). *Significantly different from total range of motion in VML alone group. †Significantly different from plantarflexion range of motion in VML alone group. Abbreviations: Restricted, restricted housing; ROM‐Estim, range of motion and electrical stimulation; VML, volumetric muscle loss

FIGURE 2

Twenty‐four‐hour metabolic rate and activity 6 weeks after VML. (a) Total ambulation over 24 h, a marker of activity, was reduced in both groups that received rehabilitation (P < 0.0001). (b) Over 24 h (P = 0.001) and during the 12 h active and inactive periods (main effect of group, P < 0.0001), average metabolic rate was reduced only when activity was restricted. As expected, metabolic rate was highest during the active period across all groups (main effect of time, P < 0.0001; interaction, P = 0.372). (c,d) Quantification of area under the curve also revealed a reduced metabolic rate only for the restricted‐activity group (P < 0.0001). *Significantly different from VML alone group. †Significantly different from VML ROM‐Estim group; all significant main effects are noted on the individual graphs. Abbreviations: Restricted, restricted housing; ROM‐Estim, range of motion and electrical stimulation; VML, volumetric muscle loss

FIGURE 3

Twenty‐four‐hour RER 6 weeks after VML. (a,b) Independent of activity, both rehabilitation groups had an increase in RER at 6 weeks (P = 0.022). (c) All groups had a higher RER during the active period than during the inactive period (main effect of time, P ≤ 0.0001), and RER was highest in the active and inactive periods when activity was restricted (main effect of group, P = 0.0001; interaction, P = 0.818). (d) The increased RER corresponded to reduced lipid oxidation for both rehabilitation groups (P = 0.001). *Significantly different from VML alone group; all significant main effects are noted on the individual graphs. Abbreviations: RER, respiratory exchange ratio; Restricted, restricted housing; ROM‐Estim, range of motion and electrical stimulation; VML, volumetric muscle loss

FIGURE 4

In vivo active and passive function after VML. (a) Loosely packed (green) and densely packed (red/orange/yellow) collagen areas as a fraction of total collagen area within the gastrocnemius muscle were similar across groups (P = 0.168). (b) Representative images of the Picrosirius Red staining using a non‐polarized lens (top) or polarized lens (bottom). Polarized images were used to calculate the area of loosely and densely packed collagen. Scale bar: 100 μm. (c) Across 40° of ankle motion, the group that received rehabilitation but was allowed to ambulate freely demonstrated the greatest passive torque (main effect of group, P = 0.019; main effect of joint angle, P < 0.0001; interaction, P < 0.0001). (d) Isometric torque normalized to body mass was impaired in mice that received rehabilitation and were able to ambulate freely, whereas mice that underwent rehabilitation with restricted activity demonstrated a similar torque to the VML alone group (P = 0.008). For comparison, data from previously published age‐ and sex‐matched control mice (Dalske et al., 2021) are indicated by the grey band to highlight the VML‐induced impact on function. *Significantly different from VML alone group; all significant main effects are noted on the individual graphs. Abbreviations: Restricted, restricted housing; ROM‐Estim, range of motion and electrical stimulation; VML, volumetric muscle loss

FIGURE 5

Local muscle metabolic adaptability. (a) Evaluation of the distribution of myofibre cross‐sectional area revealed a higher proportion of smaller fibres in mice receiving rehabilitation that was most evident with activity restriction (P < 0.0001). (b) The measurements shown in panel (a) were made using Masson's Trichrome staining of the gastrocnemius muscle. (c) The Oil Red‐O staining of the gastrocnemius muscle revealed a similar percentage area of neutral lipid deposition across groups (P = 0.849). Scale bar: 100 μm. Significant main effects are noted on the individual graphs. Abbreviations: Restricted, restricted housing; ROM‐Estim, range of motion and electrical stimulation; VML, volumetric muscle loss