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Selected muscle groups in individuals with FSHD, ADLO and PF-LGS, HMSN, NC-MMD, and DMD were evaluated with isometric and isokinetic quantitative strength measurements. The isometric measurement was peak force in Newtons, and the isokinetic measurement was peak torque in Newton-meters.
Data from the NMD individuals were compared with age and gender matched non-disabled controls (CON), and disease, gender, and disease-gender interactions determined. Peak torque or force results are reported in Tables 1 and 2, and the percentage of CON values in Table 3.
There was a gender effect in CONs and in all of the slowly progressive NMDs in all of the isometric and isokinetic measurements since males were stronger than females. For isometric measurements, there was a disease-gender interaction for all measures except neck flexion and extension in FSHD, for shoulder extension in LGS, and for all measures in NC-MMD. Male NMD individuals showed greater weakness relative to male CONs than female NMD individuals demonstrated relative to female CONs. There was no disease-gender interaction in HMSN. Disease-gender interactions for isokinetic measurements were less frequent and sporadic: elbow-flexion concentric peak torque in FSHD, and concentric and eccentric peak torque of the elbow flexors.
In a previously published study, isometric peak force in the knee flexors and extensors of non-disabled individuals was highly correlated with isokinetic concentric and eccentric peak torque (Lord et al: Arch Phys Med Rehabil 73:324, 1992). While there was a moderate correlation between knee flexor and extensor isometric and isokinetic peak torque or force percent reduction from CON values in FSHD, HMSN, and DMD in this study, isometric percent force loss was significantly greater than isokinetic percent torque loss in LGS and MMD. While there was a higher percent concentric torque loss relative to CON than percent eccentric loss in all disease groups, the difference in reduction was only significant for FSHD and LGS.
With all muscle groups combined, the DMD group had the greatest average percent loss, relative to CON, in isometric (67%) and isokinetic eccentric and concentric (82%) force or torque. The greatest percent reduction in isometric force in the slowly progressive NMDs was in the LGS (50%) and MMD (43%) groups with average losses of 32% in FSHD and HMSN. Of the slowly progressive NMDs, LGS had the greatest average percent reduction in concentric and eccentric isokinetic torque relative to CON (57%) with no difference between MMD (36%), FSHD (33%) and HMSN (32%). With neck flexors and extensors, grip and pinch eliminated, the greatest average percent isometric reduction was in DMD (75%) and FSHD (61%).
Since only elbow and knee muscles were evaluated with isokinetic measurements, a comparison of upper and lower extremity muscle groups was restricted to these muscles. In general, there was no significant difference between the elbow and knee flexor and extensor muscles in percent torque reduction relative to CON. The only exception was in FSHD in which elbow flexion and extension concentric and eccentric percent isokinetic torque loss was greater than knee loss.
Evaluation of the difference between extremity proximal and distal strength was limited to the upper extremity since there were no distal lower extremity isometric measurements and no upper or lower extremity distal isokinetic measurements. In the FSHD, LGS, and DMD groups, diseases considered to have greater proximal than distal weakness, there was a significantly greater reduction in percent isometric force relative to CON in the shoulder and elbow muscles than in grip and pinch. Surprisingly, there was no difference between proximal and distal muscles in HMSN, a predominantly distal weakness NMD. In MMD, usually considered to have equal proximal and distal weakness, pinch loss of percent force was the same as percent proximal muscle loss, but grip percent force loss, relative to CON, was greater than proximal loss (difference of 17%) and pinch loss (difference of 20%). Since grip loss was greater than pinch loss, the results were probably influenced by the myotonia.
The greatest percent force loss in grip and pinch was in MMD (67% and 47%) and DMD (62% and 57%). FSHD was the only disease in which percent isometric force loss, relative to CON, was greater in the dominant extremity and only in grip. Combined with the MMT results, this is further evidence of probable "overwork weakness" in FSHD.
There were only a few significant differences between extremity flexor and extensor muscle percent isometric and isokinetic tension loss relative to CON (pinch and grip not included). Percent isometric force loss was greater in the extensor muscles than in the flexor muscles in LGS and DMD, and percent isokinetic concentric and eccentric extensor torque loss greater than flexor loss in DMD. Isokinetic measurements were not obtained for the neck muscles in any of the NMDs, nor for isometric measurements in DMD. MMD had the greatest percent neck flexor and extensor isometric tension loss, relative to CON (44%). Losses in FSHD, LGS, and HMSN were 29%, 23% and 15%. The neck flexor muscles had a greater percent force loss than the neck extensors in MMD (difference of 32%), while the neck extensors were weaker than the flexors in HMSN (difference of 20%). Differences were not significant in HMSN and LGS.
Relationship Between MMTs and Isometric Measurements (IQM)A MMT is an ordinal, partially subjective, isometric measurement while an isometric quantitative test is a cardinal, objective measurement. Strength is the common variable measured by these two techniques (Bohannon: Arch Phys Med Rehabil 67:390, 1986; Aitkens et al: Muscle and Nerve 12:173, 1989), and both have high intratester and intertester reliability in NMDs (Andres et al: Neurol Clin 5: 125, 1987; Edwards et al: Muscle Nerve 10:6, 1987; Florence et al: Phys Ther 64:41, 1984; Fowler and Gardner: Arch Phys Med Rehabil 48:629, 1967; Wiles and Karni: J Neurol Neurosurg Psychiatry 46:1006, 1983).
When IQM was predicted from MMT, there was a significant positive relationship as indicated by a high correlation between the two testing techniques (Aitkens et al: Muscle Nerve 12:173, 1989). However, correlations were not high enough to represent equivilance between the two measures since MMT does not distinguish varying degrees of strength in individuals without disease resulting in a loss of precision. As seen in this study, and as reported by other investigators, there are widely variant IQM values in individuals graded normal or near normal (MMT grades 4.5 - 5) by MMT (Aitkens et al: Muscle Nerve 12:173, 1989; Griffin et al: Phys Ther 66:32, 1986). MMTs over-estimate the extent to which the participant has full or near full strength (Bohannon: Arch Phys Med Rehabil 67:390, 1986), or under-estimate the degree of weakness (Beasley: Arch Phys Med Rehabil 42: 398, 1961). Individuals in this study with relatively high MMT grades ( 4.5) had widely varying percent IQM values relative to CON (29%-54%), substantially overlapping the IQM values (24%-84%) in participants graded as relatively weak by MMT grades ( 3.5). MMTs also under-estimate the degree of weakness since percent IQM losses occur as high as 50% in individuals with high MMT grades. Other studies have shown that MMTs fail to detect strength differences when muscle groups present almost equal strength (Saraniti et al: J Orthop Sports Phys Ther 2:15, 1980), or remain at relatively normal values in individuals who exhibit the most clinical improvement during interventions (Griffin et al: Phys Ther 66:32, 1986).
On the other hand, muscles with less than grade 3 MMT strength are difficult to evaluate with IQM. Quantitative measurements are also very expensive with questionable cost-benefit ratios, and in individuals having weakness clearly detectable by MMTs and whose course is rapidly deteriorating, quantitative testing probably does not add clinically significant information. The major use for quantitative measurements is in research studies of therapeutic interventions, especially in individuals with mild weakness or minimal changes in strength.
