Reduced rate of knee extensor torque development in older adults with knee osteoarthritis is associated with intrinsic muscle contractile deficits

Abstract

We examined the effect of knee osteoarthritis on the rate of torque development (RTD) of the knee extensors in older adults with advanced-stage knee osteoarthritis (OA; n=15) and recreationally-active controls (n=15) of similar age, sex and health status, as well as the relationship between RTD and the size and contractility of single muscle fibers. OA participants had lower RTD when expressed in absolute terms (Nm/ms). There were sex differences in peak RTD (P<0.05), with greater RTD in men, but no group by sex interaction effects for any variables. The lower RTD in OA versus controls was not explained by variation between groups in the fiber type admixture of the muscle, and was mitigated when RTD was normalized to peak torque (PT). In knee OA volunteers, we found strong correlations between the RTD expressed relative to PT and the velocity of contraction of single myosin heavy chain (MHC) I and IIA/X muscle fibers (r=0.652 and 0.862; both P<0.05) and power output of MHC I fibers (r=0.642; P<0.05). In controls, RTD relative to PT was related to fiber cross-sectional area of MHC IIA/X fibers (r=0.707; P<0.05), but not measures of single fiber contractile performance. To our knowledge, these results represent the first demonstration that variation in whole muscle contractile kinetics in patients with advanced-stage knee osteoarthritis and healthy older adults is related, in part, to the size and function of single muscle fibers.

Keywords: Single muscle fiber; Skinned fiber; Velocity.

Figure 1

RTD variables in control (open bars) and knee OA (closed bars) groups. RTD, rate of isometric torque development; RTD₂₅ and RTD₅₀, rate of torque development per unit time from the onset of torque development to 25% and 50% of peak torque, respectively. Data are mean ± SE. *, P<0.05 and **, P<0.01 group effect.

Figure 2

Relationship of RTD_rel to shortening velocity (V_max) in MHC I (n=12) and IIA/X (n=7) fibers in knee OA volunteers. Note that variation in sample size is related to the fact that not all volunteers had fibers of a specific fiber type analyzed due to simple random variation in the fibers that were selected for analysis (ie, fiber type is blinded until after analysis, when MHC isoform is determined) and because data from one male knee OA volunteer was removed because his average data were outliers for MHC IIA and IIA/X fibers (detailed in Methods).